Should you replace your old timber floor with a concrete floor?

Traditional ground floors have consisted of little more than a few flagstones or bricks placed directly over the soil. The Victorian era saw the widespread introduction in mass housing with suspended timber floors, alongside rudimentary solid floors in hard-wearing areas such as kitchens and hallways. This combination persisted well into the 1930s, with solid concrete predominating throughout the second half of the 20th century, until the advent of the modern ‘beam and block’ suspended concrete floor in the late 1990s.

The suspended timber ground floor has, over time, given rise to a number of potential problems. This has been largely due to the change in our life styles. We have tended to seal up our homes trapping moisture in and reducing natural ventilation. We have blocked up our chimneys. We have painted our walls (inside and out) with impervious paints. We have re-plastered our houses with dense sand and cement renders.  We have put down patios and concrete footpaths which raise up the ground levels and trap moisture beneath. We have filled our cavity walls with retro-fill insulation. We have allowed our air bricks to block up. We have covered the timber floors with impervious flooring.

treaditional floor

In light of the above, it’s worth taking a moment to reflect on how these floors were built. A typical example might comprise joists supported or resting on timber wall plates over brick ‘sleeper’ walls to support the sleeper plates and joists.  To reduce the risk of damp and timber decay, a good flow of air under the ground floor is important, so short brick sleeper walls were built in with a ‘honeycomb’ pattern; the gaps allow air to circulate. The air enters via small vents or airbricks sited in the lower walls. Suspended timber floors to kitchens and bathrooms can be particularly at risk from hidden plumbing leaks and condensation behind fitted units, too. Other sources of damp include defective water supply pipes run in from the street under the house.

If the oversite appears to be wet or if the flooring timbers are in the ‘at risk’ range of moisture content, the walls are more likely to be significantly damp in places. This is because moisture-laden air in the floor void may rise and condensate under the floorboards or behind skirting’s, giving a false impression of a rising damp problem. This phenomenon is more accurately termed ‘rising damp in the air’.

There are choices when it comes to timber floor repairs. You can replace like for like and this has several advantages: It is much easier to repair or adapt a timber floor and a timber floor will make the routing of service pipes and cables much more safe, economical, efficient and straightforward to install. One additional benefit is that it is much easier to supervise a suspended timber floor installation as its various components are more exposed and visible for inspection than a concrete floor would be.

Rotten wood should be cut out and remaining timbers treated. Once dried out, new treated timbers can be fitted. Joist ends in walls should be protected with a DPC (damp-proof course) or new joists hung from steel hangers.  Clear any blocked airbricks, replace damaged vents, or fit additional terracotta or plastic airbricks. Where an extension has blocked off old airbricks, it may be possible to improve airflow by fitting ‘periscope’ vents channelled to the exterior.

If insulating the floor then use breathable insulation (as this allows the whole of the floor to breathe) and also a breather membrane (this allows you to create an airtight seal to reduce draughts) and also for any moisture to pass through and dissipate naturally. Care needs to be taken when fitting the membrane as it needs to be taped together to form a consistent barrier and also to be attached to the underfloor walls using a long lasting sealant. Look for products like Orcon F rather than relying on silicon sealers.

Concrete replacement floor

The trend now is to replace these traditional timber floors with concrete floors with a damp-proof membrane (DPM). The membrane is supposed to link to a damp-proof course in the walls which in practice consists of just turning the DPM up the face of the wall and not tucking it into the DPC course in the wall as is the practice with a new build. This could create a moisture pathway between the wall and the concrete floor slab.

In any case, the DPM will stop moisture evaporation through the floor in an historic building, forcing any moisture to travel under the non- porous floor until it reaches the walls. If the moisture cannot escape through the walls because they have a cement render or a waterproof coating on them, it will accumulate in the wall, causing deterioration. External concrete paving will only compound the problem. Water will always find a way to escape and if the only option is up an internal wall, so be it.

concrete floorThe installation of a solid floor can also block airflow to other rooms. It is common for one room to have its floor replaced. This then can cut off the underfloor ventilation to other rooms. This is turn can cause more problems for the other rooms. At the risk of repeating myself, installing a concrete floor inside a building to replace an older breathable floor means ground moisture is forced out to the walls, increasing the damp within the walls. After all, a large previously ventilated void has been filled by a solid, dense and obstructive material.


Limecrete replacement Floors

If you really have to remove your timber floor and replace it with a solid ground floor then one option is to use a slab based on lime (‘limecrete’) which is breathable, rather than Ordinary Portland Cement, which is virtually impermeable. A limecrete floor can be designed to meet modern insulation requirements and can incorporate under-floor heating (UFH). It may be possible in some cases to re-lay the original surface on top of the new slab if desired, although this can be difficult to achieve successfully and requires a methodical approach if the character of the floor is not to be altered. The greatest danger associated with limecrete floors is the potential need to excavate to a greater depth than the foundations. This can destabilise the building, so great care should be taken to establish how deep the foundations are before any new floor is considered. Two or three test pits by the walls could be dug to allow a plan to be formulated.

Having decided on the Limecrete floor, preventing ground water penetration is a major consideration and requires some form of capillary break. This can be achieved using recycled foam glass or expanded clay aggregate insulation as loose-lay insulation. Being made from re-cycled glass or clay, these are more durable and widely perceived to be more environmentally friendly than an insulation based on petro-chemicals. Furthermore, due mainly to the open pore structure of these aggregates, they have low capillary attraction to moisture.

limecrete floor


The installation is as follows: the ground is first excavated and levelled – the greater the depth available, the better the insulation. A breathable geotextile membrane is laid and the loose-lay insulation is added followed by another layer of geotextile membrane (a slightly more expensive option than using expanded clay aggregate as loose-lay is foamed glass which is around 20 per cent more thermally efficient and more structurally stable as it requires compaction). The limecrete slab is then cast using a mix containing expanded clay aggregate or sand, and natural hydraulic lime. This is usually left for around three weeks before any Underfloor Heating Pipes (UFH) pipes are fitted and covered with a lime mortar screed. Having created a breathable insulated floor, it’s important to avoid an impermeable covering if possible e.g. glazed ceramic tiles, cementitious adhesives and grouting, rubber carpet underlay.
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Insulating a solid wall internally – the issues!

Old buildings were traditionally constructed with technologies handed down through generations which allowed the building to breathe naturally. The building fabric was constructed in natural materials, typically with solid walls providing good permeability and flexibility. External surfaces were designed to deflect the rain, penetrating and rising damp was absorbed by the structure that allowed the moisture to evaporate away naturally through the porous surfaces. We get problems when we try and apply modern construction methods when upgrading or renovation older properties, in particular pre 1919.

There are quite a few systems on the market for internally insulating walls of domestic properties. The most commonly used are polystyrene or PIR backed plasterboard. People also use timber studwork, infilled between with mineral wool or PIR, covered with a vapour control layer (often polythene) and plasterboard. These should never be used to upgrade an existing wall in a ‘heritage’ property for reasons I will try and explain later in this Blog.

Before outlining issues associated with insulating a solid wall internally, there are 3 main concepts to bear in mind if the effect of water on buildings is to be fully comprehended. These are:

  • Vapour permeability
  • Hygroscopicity
  • Capillarity

Briefly, in this instance we can take capillarity to mean the ability of the plaster or wall to absorb a liquid in the manner of a candle wick, whereas hygroscopicity is the capacity of the plaster or wall to absorb or releasing water vapour from the air. Permeability refers to the ability of a material to allow water vapour to pass or diffuse through it.


Transporting Moisture Vapour

There is a big difference between air transported moisture and vapour diffusion. Proponents of vapour barrier systems tend to confuse these two transport mechanisms.

Vapour diffusion is the process of moisture passing through breathable or permeable building materials, like drywall and insulation. Vapour barriers are there to prevent that from happening.

Air leakage is due to air pressure differences between indoors and out, which forces air through any holes in your air barrier.

The confusion arises because air often holds a great deal of moisture in the vapour form. When this air moves from location to location due to an air pressure difference, the vapour moves with it. This is a type of migration of water vapour. In the strictest sense air barriers are also vapour barriers when they control the transport of moisture-laden air. The function of a vapour barrier is to retard the migration of water vapour. Vapour barriers are not typically intended to retard the migration of air. That is the function of air barriers.

air and vapour barrier


Solid internal insulation with vapour impermeable barrier or insulation.


blocked moisture path


As the name suggests, these barriers are designed to block moisture vapour. Vapour Barriers can be foil-backed PIR Insulation (such as Celotex PL4000) or Cementitous Tanking or dense sand and cement render with a waterproofing additive which are generally applied to a damp wall by Timber and Damp Companies.       Cement: sand renders are relatively impermeable – to the extent that at 38mm it can act as a vapour barrier.

Penetrations punctured through the membrane (for example when sockets have been fitted which break the vapour barrier) will allow vapour to form as condensation on the wall and make the wall damp.

There can also be issues with interstitial condensation which can create significant problems for buildings and occupants alike, including poor air quality, mould and mildew, and even structural damage. In this situation rising damp and rainwater penetration will only be dispersed slowly to the outside.

Vapour impermeable barriers can also increase the risk of surface condensation as it prevents the wall acting as a ‘moisture sink’ (which takes up moisture from the air and releases it when the temperature rises or ventilation increases).

This schematic vertical cross section is showing condensation of moisture on a vapour barrier externallyvapour-retarding material placed on the external cold side of a wall assembly (interior on the left and exterior on the right). This may be the case where there has been a perfectly adequate permeable wall allowing moisture to diffuse through it. A non-permeable coat of paint or render has then been applied to the external face. Moisture gets trapped in the wall and condensates behind the coating.

moist air gap

Schematic vertical cross section showing leakage of air-transported moisture (red arrow) from the interior to the exterior, causing moisture to become trapped within a wall assembly due to the presence of a two vapour barriers (interior on the left and exterior on the right).  The two impermeable barriers may be PIR Insulation or Cementitous Tanking or dense sand and cement render internally and dense sand and cement render or an impervious coating of paint externally.

The key factor here, is the amount of vapour molecules that will pass through a moisture permeable barrier is insignificant compared to the air-transported moisture that will pass through a non-permeable barrier if you cut just one small hole in it and there was an air pressure difference between the inside and the outside.

 Moisture transport over one heating season is illustrated below. Air transported moisture through a 2cm x 2cm hole in a 1m2 air barrier which is NOT vapour permeable would allow 30 litres of moisture through whereas in a 1m2 vapour permeable surface it would be 1/3rd of a litre.



                   Solid internal insulation with permeable hygroscopic insulation.

wall with woodwool

In a wall that has permeable hygroscopic insulation (such as  Woodwool Insulation), vapour that penetrates through the punctured insulation say via sockets, will be held hygroscopically (due to absorbing moisture from the air) and dispersed either inwards or outwards through the wall depending on the Vapour Pressure (Vp) and the porosity of the wall as it seeks to achieve equilibrium. Generally, the Vp is higher internally due to the internal air having a higher moisture content (from washing, cooking and breathing etc.) than the outside air.

In addition, moisture vapour will naturally move from the warm side of a wall to the cooler side. If the temperature is high inside the building and lower outside the building, vapour will be directed outward (and vice versa). In other words, the movement of moisture via diffusion is a result of differences in vapour pressure which is related to the temperature and moisture content of the air.

When there is a difference in concentration of the moisture in the air (or Vp) between the inside and the outside, there will be a corresponding difference in the partial pressure between the internal air and the external air. This will cause a flow of moist air from the point of higher concentration (inside) to the lower (outside). The moist air will diffuse through the wall until the partial pressures of the moist air both inside and outside are equalised. The rate of diffusion will be determined by the partial pressure difference, the width of the wall, and the permeability to the moist air of the wall structure.

One of the problems in the building industry today is that it have a spreading obsession about lining everything with polyethylene or Vapour Control Layers (VCL). The proponents of this view believe the answer to all moisture problems to be the installation of a polyethylene vapour barrier on the inside of buildings. This obsession is responsible for many more building failures than building successes when it is applied to old buildings which were built to be moisture permeable.

English Heritage strongly warns against the use of modern synthetic insulation materials, as the natural materials in the walls are designed to “breathe”, or allow the exchange or moisture vapour between outside and in. Vapour barriers and other impermeable materials are to be avoided as they may trap and hold moisture in the wall.

The answer? If you are using a timber stud wall to provide an inner skin to a half-brick wall by way of upgrading the internal wall insulation or wish to achieve a low U-value  then use a moisture permeable membrane which is also air tight such as ‘breathable’ roofing felt or an intelligent membrane.  These membranes have a low vapour resistance, they are strong, flexible and airtight. Their design permits vapour to pass through the wall and permits diffusion, allowing any condensation that may occur to escape toward the inside and the outside of the wall.

One way is to fit 25 or 40mm woodwool slabs over this vapour permeable air-barrier and finish with a lime plaster and a breathable emulsion. This finish will be hygroscopic where it can act as a ‘moisture sink’ in times of high humidity and release it again when the room is ventilated. Another method involves battening out the wall and uses sheep’s wood as the insulation. This gives a lower (and more desirable) U-value.

Information follows regarding Internal Wall Insulating (IWI) applied directly to a solid wall.

                                                          Internal Wall Insulating (IWI)

Information on ‘Building Regulations & Internal Wall Insulation’ is available here from Lime Green’s website for those of you who feel it applies to you. There are various other optional methods and materials you could use for IWI but I will outline one below involving fixing wood wool boards to the internal face of a solid permeable wall where we used Lime Green Solo one coat lime plaster.

Wall Preparation

Externally check that the pointing, paint or render is in fair condition. If not repair with suitable breathable materials (e.g. Natural Hydraulic Lime render, lime putty, etc.). Remove any impervious paints or materials which are sealing the wall or acting as a vapour barrier externally (this does include dense render). Internally, remove vinyl wall finishes and sealed plasters (e.g. sand and cement renders, gypsum plaster)

The Masonry must be flat, level and porous to allow for moisture diffusion. Old lime plasters can be left and be ripped open with a scouring float. Bare brick or stone must be plastered with a coat of lime render to flatten the wall as it’s important that the wood wool slabs fit flush to the wall surface.

Pipes, cables etc. should be chased and run in the masonry or plaster before fitting the wood wool boards.


Apply Lime Green Solo one coat lime plaster (or similar) in 2 passes to a combined thickness of 8 to 12mm. (I use Lime Green Solo as I find it good to work with and it has a fast set)

The first pass is applied approx. 5-6mm thick – the 454 glass fibre mesh is pushed in to the plaster immediately while it is still tacky. Overlap joins in the mesh by 100mm. Lime green 454 glass fibre mesh is doubled up around windows and doors, with a diagonal layer applied. The diagonal piece should be at least 200mm x 400mm.

The second pass (about 5mm deep) is applied over the top usually within 4 hours to give a total thickness approx. 8 to 12mm thick. Make sure the mesh is fully covered.  Level the second pass with a straight edge or ‘Darby’. Do not over-work the surface as this may lead to “fire cracking”.

Allow at least 2 weeks drying time, during which time the plaster should be protected from rapid or forced drying. Apply in temperatures above 5°C and below 30°C.


The system is vapour-open and functions by allowing moisture to pass freely. It is therefore important that only vapour-permeable paints are used for decoration.

schematic of woodwool wall


Photos of  work we undertook to deal with a condensation issue

Wood Wool boards in the process of being fixed to the wall

Wood Wool boards in the process of being fixed to the wall


Laying the fiberglass mesh into the first pass coat

Laying the fiberglass mesh into the first pass coat


First pass coat layer on

First pass coat layer on

second pass coat finished

Second pass coat finished


Lime versus Cement

Sand and Cement pointing trapping moisture in wall

All buildings constructed before the 20th century will almost certainly have been built using lime, because cement was only invented in 1824, by Joseph Aspdin, and did not begin to be used extensively for another 100 years.  Lime has been used as a binder for stones and brick, and as a plaster or render, for thousands of years. The knowledge of its properties and how to use it has only been lost to current practice in the UK in the last 100 years and there is now a huge ignorance about lime and its properties. European countries still use lime extensively within construction.

Ignorance and lack of teaching about Lime has led to major problems in construction, as architects and managers specify the use of cement, a modern material whose properties and failings over the long term are only just being recognised. Problems of damp and durability associated with the use of cement may not become apparent for 50 years or more from the time of build. English Heritage and Historic Scotland have banned the use of cement on all historic buildings because it encourages damp and can actually destroy buildings that have stood for hundreds of years.

A solid brick or stone wall, built with lime mortar, needs to breathe. It loses its moisture content through the mortar joints. If this permeability is blocked, through the use of cement, the wall immediately starts to get wet. Water is trapped, and the only way it can get out is via the brick or stone. In winter, the damp brickwork then freezes, and the familiar rotting and spalling bricks or stone start to appear.

Cement pointing is responsible for damage to thousands of walls all over the country. At the same time as trapping water, it forces water into the wall – where it emerges inside – blowing plasterwork, and creating the usual symptoms of ‘rising damp’. If you have cement pointing to an old wall – the simple solution to any damp problems is to get rid of the cement.

Lime mortars and plasters are:

  1. Permeable. This means that vapour can pass through them at an almost imperceptible level, which is a healthier option for inhabited buildings as it regulates humidity.
  2. Flexible. Stone or brick laid with lime can move as the earth moves through changing seasons, without cracking the structure or causing instability. There is no need for expansion joints.
  3. Soft. Plasters and mortars should not be harder/stronger than the backing surface to which they are applied.
  4. Weatherproof. Not waterproof, thus protecting the building without sealing it.
  5. Not susceptible to frost. They do not freeze as they are not ‘wet’, therefore do not require foundations to be below the frost line.
  6. Do not attract moisture. They are not a ‘wet’ material, and so don’t need to be covered with a waterproof barrier in order to protect other materials around them.
  7. Deal with moisture effectively. They can hold excess moisture from the atmosphere in humid conditions e.g. in a shower, without becoming ‘wet’ and then release it slowly back as humidity drops.
  8. Proven over centuries. The Romans used lime very effectively for many applications including major engineering projects such as bridges, domes, suspended floors and heated floor slabs. The earliest known use of lime is 4000 years ago.
  9. Reduces greenhouse gas effect. Over its lifetime, due to the cycle of lime changing from limestone to quicklime and back to limestone again, most of the CO2 released during the manufacturing process is re-absorbed during the lifetime of the plaster, thus being close to carbon neutral.

On the other hand, cement is

  1. Not permeable. Creates a sealed surface that does not allow vapour passage.
  2. Rigid. Requires expansion joints to allow for natural earth movement without cracking.
  3. Hard. A great property in the right place but often cement is too strong for the materials it is used with.
  4. Waterproof. Completely seals mortar joints or walls.
  5. Susceptible to frost. Will crack in very cold conditions and therefore if used in foundations, needs to be in deep trenches that make contact with the warmth of the earth to avoid problems associated with frost heave.
  6. Attracts moisture. Other materials around cement need to be protected from it as it holds water and can cause rot to develop.
  7. Does not deal with moisture. Can cause condensation problems in bathrooms, kitchens, bedrooms etc. as it does not regulate moisture.
  8. Does not have a long history. It was invented in the mid 19th century and began to be used extensively from 1930 onward. We are seeing some major damp and durability problems now, caused by the inappropriate use of cement from 50 or so years ago e.g. collapse of cob walls re-plastered with cement, the need to reinforce some motorway bridges, excessive cracking in town houses leading to difficulty in re-selling.
  9. Causes greenhouse gas effect. The manufacture of cement is one of the major causes of the greenhouse gas effect globally as it releases tonnes of carbon dioxide into the atmosphere, none of which is re-absorbed by cement plaster.

From the above list of properties it becomes clear why lime would be used in buildings in preference to cement. Building breathable buildings is a healthier option than building sealed buildings, and causes fewer damp problems. Often the non-permeability of cement, coupled with its rigidity and hardness, can cause damp and erosion problems.

In a traditional stone or brick wall laid with lime mortar, the wall works as a weatherproof surface because the stone keeps the rain out, and the lime absorbs water whilst it is raining, and then releases it when it stops. Any moisture that enters the wall either through the mortar or from inside moving outwards will leave the wall once the cause of the moisture stops.

If you replace the lime with cement, you are relying on the cement to make a permanent tight join with the stones or brick to keep water out. In practice this doesn’t happen because of the rigidity and hardness of the cement, which causes tiny cracks to develop as the wall moves that allow moisture into the wall. This trickles down inside the wall and then cannot escape because the cement is not permeable, thus creating a damp problem inside the base of the wall. Also, water collects at the join between the stone/brick and cement and begins to erode the stone/brick, because the stone/brick is softer than the cement. Many old houses that have been re-pointed with cement show signs of erosion of the stone many years later and suffer from damp problems.

What is Rising Damp?

Rising Damp can be defined as “moisture that is soaking up (typically) a wall or floor from below ground, i.e. finished external ground level, or internal oversite level”,  Ralph Burkinshaw and Michael J Parrett, ‘Diagnosing Damp’, RICS.   The legal requirement for a damp course was introduced in 1875 so if your house is younger then it most probable has a DPC. Slate, engineering brick or bitumen mixed with mortar were in earlier damp courses. Bitumen felt damp courses are the most common type and were used mainly between the 1920’s and 1980’s. Plastic strip damp courses were used from the 1970’s and are now the most common type of damp course in new construction.

‘Rising Damp’ must be the most argued over type of damp ever. This is because it is used to extort millions of pounds out of trusting clients for the benefit of the Chemical Companies. An unwitting accomplice in this is the RICS as they promote unqualified people to present themselves as ‘qualified damp surveyors’.  Pay a visit to

These unqualified people are those whose only qualification is a CSRT, CSTDB or CSDB having attended a three day in-house course put on by the Property Care Association (PCA). There is no requirement for someone to have a basic recognised academic construction qualification or to have a required level of experience in order to be accepted on the course. They only need the course fee of £650 + VAT. The majority of RICS Valuation Surveyors are being told they MUST recommend these surveys.

More often than not it is as a result of mortgage lenders who have a Home Survey Report from a Surveyor who actually recommends ‘damp proofing’ work and which the mortgage lender then demands as a condition of providing the mortgage.  This has nothing whatsoever to do with finding the correct remedy to the problem – merely having guaranteed work (effective or not) which has been recommended by a Surveyor who has the all-important Professional Indemnity Insurance.  It’s all about risk and reducing liability, not fixing the issues and certainly nothing at all to do with achieving the best for the home owner.

The Sheffield based Institute of Specialist Surveyors and Engineers has found that products used in the damp proofing industry are, according to British Board of Agrement who have tested the products, not fit for purpose and that the CEO of the Property Care Association, and now a director of the flagship Consumer protection body TrustMark, wrote back to BBA in October to advise that this information must be kept from the public and PCA contractor members. More on this here

The PCA are now calling Rising Damp ‘Dampness from the Ground‘ but their sub-heading is still Rising Damp. I think their members were finding too much ‘rising damp’ and taking thousands off people for treating it when it didn’t solve the damp problem. It attracted bad publicity as ‘rising damp’ is a rare source of dampness in the home despite the PCA saying that “Rising damp is a commonly encountered problem in some types of building, however it is often misdiagnosed. It is important that the investigations into dampness are undertaken by a trained and competent surveyor who can recognise and understand the problem. We would always recommend that the surveyor who undertakes investigations has been awarded the CSRT qualification“.

Now they are setting themselves up to be a ‘learned body’ capable of ‘awarding’ qualifications! They are a trade association made up of  Chemical Companies! The Federation of Awarding Bodies is the trade association for professional and technical awarding organisations. The PCA are not members. Check the membership here:

I spent 3 years getting my BSc (Hons) degree. Think of the time I could have saved getting to be a ‘trained and competent surveyor‘ with a ‘qualification‘ which is ‘widely recognised‘ if I had spend just one day learning the “Causes, diagnostics and treatment options” of Condensation and Dampness! (Well, take out tea and lunch breaks and its less than one day).  The second day deals with “Legal aspects, survey methodology and report writing“. The third day is doing your Health and Safety via the e-learning module. Have a look at the Syllabus here. You are interested in Module 3.

Back to ‘Rising Damp’! It’s very rarely due to a ‘failed DPC’ or a ‘broken down’ DPC however much the National Timber and Damp Companies would have you believe. Timberwise say on their website that “Damp can occur for a number of reasons but is typically as a result of a failing damp proof course (DPC), poor ventilation or poor property maintenance”. The LEAST cause for dampness in the home is a ‘failing’ DPC.  They then go on to say “However, the most common causes of damp are usually developed by inappropriate building work that stopped moisture escaping”.  One of the best ways to stop moisture escaping is to seal the moisture into the wall with dense sand and cement render containing waterproof additive when treating walls for ‘rising damp’. There’s a bit of a dichotomy going on there! Timberwise seems to have a default position that the major cause of damp problems is a failed DPC when they say “While some damp problems can be easily fixed than others, the more common fixes are the installation of a damp proof course“.

Let’s have a look at how ‘rising damp’ is treated by the majority of the National Timber and Damp Companies. They find damp. They say you have a ‘failed DPC course’ but luckily they can fix it for you (This privilege will probably cost you £130 per meter run of wall).  It will mean that your house will be severely disrupted while the plaster on the walls are being hacked off (You will need an electrician to blank off any sockets and maybe a plumber to remove the radiators. Your skirting’s may need to be replaced as they can get damaged when taking them off. Then when the work is done everything has to go back and the walls will have to be redecorated when they are dry).

Having hacked off the old plaster they will immediately inject a DPC cream. This cream will most probably be a thixotropic paste comprising at least one silane compound, at least one siloxane compound and a thickening agent, all in a water base.  The cream is injected into holes drilled at regular intervals along a mortar course.  Ask to see a copy of the British Board of Agrement Certificate for that particular cream as there are creams out there with no Agrement Certificate but an ‘opinion’ which means there is no test for it. AVOID! The active ingredient of these creams vary greatly.

The walls will be then replastered with a couple of coats of dense sand/cement mix incorporating a ‘waterproofer’ or ‘salt inhibitor’.   You pay over the money and you are told it’s going to take months to dry etc. Common practice is that sufficient time will not be given to allow the wall to dry out. This usually occurs by evaporation. (It can take between 6 months and a year for a 230mm thick wall to dry).

The Rate of Evaporation is one of the factor in controlling Rising Damp. Anything which restricts the rate of evaporation or prevents evaporation is contributing to the saturation level of the wall in question and to the height of the Rising Damp. Coatings designed to seal the surface of masonry walls (and so ‘protect’ them) trap moisture behind the coating and cause a damp problem elsewhere, such as on the other side of the wall or force the moisture further up the wall.

Another important reason to allow the wall to dry out before injecting a DPC cream is related to the findings of a scientific study on the effectiveness of injection creams.

Evaluation of Spreading and Effectiveness of Injection Products against Rising Damp in Mortar/Brick Combinations Anke Hacqueborda *, Barbara Lubellia,b, Rob van Heesa,b,Timo Nijlanda

Extract: ” With respect to the spreading and effectiveness of cream products, it can be concluded that in 100% saturated substrates no spreading can take place because the products does not become liquid, while in dry substrates, the spreading is limited. Spreading improves in water-saturated specimens, which have the possibility to dry. The drying process enhances the spreading: the presence of water favours dilution of the product in water and transport through the pores during drying. Reaction takes place in empty pores.”

The full article can be downloaded here in PDF format –

So, the proper procedure would have been to firstly determine that there was an actual rising damp problem in the first place. I am assuming that as you have invited the damp company into your house to undertake the works and to hand over hundreds of pounds that you have seen the ‘moisture grid’ readings taken by the surveyor which would have highlighted the ‘Damp Zones’. You would also have seen the Carbide Moisture Meter readings which gave a percentage reading of the actual moisture readings in the ‘damp zones’. You would have see the exposed ‘failed’ DPC (or evidence of a non-existent DPC) and understood why moisture could by-pass it.

Think of it as buying a car. You would not hand over the money without checking that it wasn’t stolen, that it had an engine,  its service history etc. so why would you hand over hundreds of pounds without checking the basis for the damp diagnosis? It is not acceptable to arrive at a ‘rising damp’ diagnosis based solely on moisture meter readings. British Standard BS6576 states that electrical moisture meter readings on their own cannot be used to diagnose dampness problems.  All other more common causes for the damp would have been ruled out such as blocked cavities, high ground levels, burst pipes, penetrating damp, condensation etc.

Having established that the cause of the damp is a damaged or missing DPC (which is very rare), injecting the chemical DPC happens after the old plaster is hacked off the walls.  However, according to the published research above, the cream will not be effective if injected into a ‘saturated’ wall and then rendered immediately. “Spreading improves in water-saturated specimens, which have the possibility to dry”.  When this is done, the DPC cream should now be left to spread and for the wall to dry out.

When the wall has reached between 3 and 5% (use a Carbide Moisture Test) then it will be a dry enough wall and be ready to replaster with Lime plaster if at all possible (if no Hygroscopic salts present) or with Cavity Drain Membrane!  If the wall hasn’t dried out then it needs to be revisited before hiding it behind a coat of dense sand/cement render!

Some contractors will wish to ‘cover themselves’ and reduce ‘callbacks’ so they may coat the wall with a cementitious compound  to seal the moisture in. This is stuff you would use in basements to protect against damp ingress (for a while anyway). They may suggest that you get your own plasterer in to re-plaster to their specification. It may be cheaper but its a wonderful get out for them for a later callback for a damp recurrence as the cause of the failure will be ‘due to the plaster not been applied as per the specification provided’. 

The PCA themselves specify in their information sheet “...Installation of Remedial
Damp Proof Courses in Masonry Walls : One function of the new plaster is to hold back the hygroscopic salts present in the wall structure due to rising damp, and to prevent them from migrating through to the surface of the new plaster. In all cases, replastering should be delayed as long as possible after the insertion of the dpc and maximum ventilation applied throughout the treated areas of the building during this period. Plasters should be suitable for use in damp situations (e.g. cement-based) but should not be a vapour barrier: Digest 245 – Rising Damp in Walls.  How often does this happen in practice????

My advice would be to get a proper independent damp survey carried out by someone other than the National Timber and Damp Companies or PCA surveyors with their in-house ‘qualifications’ alone. If Rising Damp has been identified and a chemical DPC course is recommended then do it yourself and save hundreds of pounds. Treating Rising Damp by injecting a DPC is not rocket science….identifying Rising Damp is a lot more difficult!




What is condensation?

When warm, moist air comes into contact with cooler surfaces, the excess moisture in the air condenses. That’s because the cooled air next to the cool surface can’t hold as much moisture as the warmer surrounding air.  In other words, as moisture-laden air gets close to the cold surface it starts to get cooled and so the relative humidity increases; the greater it is cooled the higher the relative humidity and the more it reaches saturation.

Years ago, before energy efficiency became a concern, homes were not built to be weathertight. Insulation concepts were very basic. Walls and ceilings were made from much more porous materials. Thus, water vapour could easily flow in and out of walls. Today’s homes are much less ‘permeable’ (they don’t let moisture escape through the walls). Windows and doors are built to reduce air leakage, and weatherstripping, modern insulation, vapour barriers and new construction techniques can help keep cold air out and lock moisture inside. As a result, moisture created by bathrooms, kitchens, laundries, plants and occupants can result in higher interior relative humidity. In the worst conditions this can build up to excessive, even harmful, moisture levels.

On new builds a Condensation Risk Analysis can predict the risk of interstitial or surface condensation by analysing the components of the building’s elements, the order in which they are assembled, the use of the building and its geographical location.

The effects on condensation on a wall

 Example 1.


The wall in the photo above was diagnosed as ‘rising damp’ by a nationwide damp proofing company due to a ‘failed’ damp proof course. The client was advised to hack off the lime plaster to a height of 1.2m and have a chemical DPC injected. The walls were then to have a dense waterproof sand and cement render applied to ‘keep the salts from damaging future decoration’.

When I was asked to investigate I found that the existing DPC had not ‘failed’ or suffered any kind of ‘breakdown’.  I lifted an already cut floorboard (which was kindly provided by a past electrician or plumber) and discovered water pooling in the floor void. Following further investigations this was discovered to be from a leaking water pipe joint.

Water pooling

The leaking water pipe was causing a build-up of moisture in the subfloor void that transferred to the room above.  This had been going on for years (pipe leakage may be the result of a burst pie due to freezing, corrosion, joint failure, accidental damage or being overloaded).

In this case the water vapour condensates on the wall above the DPC (which can be seen in the photo above) and behind the skirting. It then wicked up the plaster and evaporated out over the skirting leaving salt behind.

As water vapour, moisture moves from a centre of high concentration to one of low concentration. For example, if there is 85% relative humidity under a suspended floor and 50% relative humidity in the habitable space directly above, then moisture will move into the habitable space. This is not dependent on air flow.

s.OMoore already treated wall

This house had already been ‘treated’ for a ‘failed DPC’ before which was a waste of time as that wasn’t the cause of the dampness. The ‘treatment’ was making it worst by trapping the moisture into the wall.

It was suggested that another chemical DPC be injected and new sand and cement render be applied as the 1st one ‘hadn’t dealt with the problem’.  In my experience, a ‘failed DPC’ is one of the rarest causes of damp problems.



What didn’t help in this case was the fact that all the air vents had been blocked with ‘brushes’ placed in the air brick vents to prevent retro-fit cavity wall insulation from entering the floor void.


Example 2

Let’s look at a sand and cement rendered internal wall. Cold moist air does the opposite of warm air and drops down the wall because it’s heavier. The condensation will therefore be more severe at the bottom of the wall. The vapour pressure indoors is generally higher than outdoors so moist air in the house will diffuse into the wall as it tries to reach equilibrium with the outdoor air (Moist air will always go from the higher vapour pressure to the lower vapour pressure). This moisture stays in the wall and builds up because the dense sand and cement render traps it in the wall. Because of this, condensation can sometimes be mis-diagnosed as penetrating or rising damp.

When a ‘damp surveyor’ come in with a ‘moisture meter’ and puts it on this damp wall he will more often than not instantly diagnoses ‘rising damp’ .  He will recommend that another damp course be injected (why, I don’t know as there will normally be a perfectly good one there already!) and that the wall be possible ‘tanked’ and rendered to a certain height to ‘keep the salts’ from coming through.  This will cause even more damp problems in the wall.

The photo below from one of our projects clearly shows how this dense render traps moisture in the wall. You can see where the wall is less saturated where it was lime plastered. The lime plaster is permeable and allows any moisture to evaporate out once the humidity levels in the air have dropped.

The reason why the wall in the photo was wet in the first place was due to water vapour condensing on the colder sand and cement render and to inadequate underfloor ventilation. The moist air under the floor condensates on the wall behind the skirting. It then diffused up the plaster and into the wall (it could not evaporate through the dense cement render which had waterproof additive).

rendered damp wall

Damp wall after removing sand and cement render


Interstitial Condensation

A much more serious problem is when condensation occurs within the fabric of the building. This is called interstitial condensation.  It’s where the dew point is reached within the wall structure and where the moisture vapour turns into a liquid within the wall.

The amount of moisture migrating with diffusion through a structure is determined by the vapour pressure difference and the vapour permeability of the structure. Usually, the vapour pressure is larger inside a building than it is outside. As a consequence, diffusion usually moves moisture out from inside the building.

 ‘Summer’ condensation

A particular form of condensation sometimes referred to as summer time condensation, can be difficult to diagnose and subsequently eliminate. This form of condensation can arise with internally insulated solid walling where moisture evaporating from the solid masonry is driven inwards as a result of temperature and vapour pressure differences between outside and inside.

A very common treatment to solid external walls is to line them internally with insulation and plasterboard “laminated plasterboard”. This treatment has drawbacks. By doing this you are reducing the heat loss through the wall. This reduces the temperature of the brickwork thereby reducing the temperature gradient down to meet the dew-point a few centimetres in from its inner face. Condensation at this point is particularly insidious as it is eating away at the structure completely hidden from view.  In addition, just as with brickwork and plaster, many types of insulated when they get wet provide no insulation at all.

solid wall dew point

If the wall is an insulated timber stud wall, moisture within this air can condense on the outer face of the internal vapour control layer which is typically a polyethylene membrane. The initial manifestation of this summer condensation will be damp staining or mould growth directly above the skirting board, similar in fact to rising damp. Condensed moisture running down the back of the polyethylene sheet wets the baton or ground at floor level which provides the primary fixing for the skirting board.


Air barriers and Vapour Control Layers (VCL’s).

At this stage I should mention about the difference between Air barriers and Vapour Control Layers.

The amount of vapour diffusion that occurs in a building is determined by two things: the driving force that pushes it (known as the “vapour pressure differential”), and the permeability of the material the vapour is passing through.

The job of a VCL is to prevent vapour diffusion, and the job of an air barrier is to stop air leakage through differences in air pressure. A vapour barrier can act as a very effective air barrier, but an air barrier does not (and should not) always stop vapour from diffusing.

A wool sweater for example, is insulation and will keep you warm when there is no air movement, but will allow the wind to howl right through it. A wool sweater with a raincoat will keep you warm but hold moisture inside and soak your insulation. A wool sweater with a windbreaker will keep you warm, stop the wind from stealing your heat, yet allow moisture to diffuse through it. So think of a windbreaker as an air barrier, and a raincoat as a VCL.

How water vapour travels:

There are two main ways moisture will pass through your walls that you should be concerned about — air leakage and vapour diffusion. These are two completely different things, with two completely separate solutions.

Vapour diffusion is the process of moisture passing through breathable building materials, like drywall, bricks and insulation. Vapour Control Layers are there to prevent that from happening.

Air leakage is due to air pressure differences between indoors and out, which forces air through any holes in your air barrier.

To explain this further, Gypsum board (drywall) is vapour permeable, but stops air flow. This means water vapour can diffuse through it, but air cannot pass through it. So if you were to have a home with no windows and no VCL but simply a sealed gypsum board box all around, you would have an airtight seal with no moisture carried through by air transport.

The key factor here, is that the amount of vapour molecules that will pass through that gypsum board box is insignificant compared to the moisture that will pass through if you cut just one small hole in it and had an air pressure difference.



The need for a proper air seals in timber frame homes or in timber studwork or ceilings is grossly underestimated, and too much faith and focus is put on the VCL.

If you think of how a polyethylene vapour barrier is installed in a timber stud wall or a ceiling, it will be cut, stapled and taped, then have nails and screws put through it to install strapping and drywall, along with breaches due to electrical wires and boxes. In most cases, the vapour barrier will be perforated hundreds of times during the building process.

A proper intact air barrier is one of the most important elements of a successful timber frame building enclosure, and one of the most overlooked. Given the amount of heat loss due to air transmission and the potential moisture damage from air leaks, air barriers should be getting a lot more attention than they are along with adequate ventilation.

For roofing felts we now use low water vapour resistance (type LR) underlay that has a water vapour resistance not more than 0.25 MNs/g, which allows the transfer of water vapour. These LR underlays are sometimes referred to as vapour permeable or breather underlays. It would only take a small change in working practice to specify these vapour permeable barriers in place of VCL and focus more on preventing Air Leakage. This would allow moist water vapour to diffuse through the structure which should reduce Condensation Risks.

Can Dampness and Mould affect my health?

Some people are more sensitive to Damp and Mould than others, including:

  • babies and children
  • elderly people
  • those with existing skin problems, such as eczema
  • those with respiratory problems, such as allergies and asthma
  • those with a weakened immune system

I’ll start by explaining a bit about the different microbes and their spores that are present in the home. These are too small to be seen with the naked eye and it’s when they grow out of control that they may become a health hazard.

Moist building materials may support the growth of several microbes that are normally not present in indoor air. The growth of microbes depends mainly on the level of the moisture present and temperature conditions. It also depends on how nutritious the material is that the microbe is present in or on.  However, in buildings, moisture is typically the only limiting factor. The presence of some of the microbes, especially mould fungi, have been associated with different health effects.

Indoor and outdoor air always contain several different microbes and their spores. In a normal building, the species present in the indoor air are the same as outdoors. However, in a moisture damaged building the range of microbes is different. Some species, referred to as ‘indicator microbes’, are typically found in moisture damaged buildings.


Hydrophilic microbes (microbes that have a special affinity for water) may grow only in very moist conditions, while xerophile microbes may grow in drier conditions. As a consequence, at an early stage of a moisture damage, the damaged material may be occupied by xerophile microbes who can grow and reproduce in conditions with a low availability of water. If the damage is prolonged, they are gradually pushed out and replaced by microbes that require moister conditions until only hydrophilic microbes remain. This phenomenon is known as succession.

Microbe growth in buildings may manifest itself in several different ways. Even if the growth is not visible, it may be recognized from a mouldy or cellar-like smell or from the symptoms of the people using the building. Microbe-related health effects may be caused by several factors including microbial volatile organic compounds (MVOCs), mycotoxins, allergens, and airborne microbe spores and fungal particles.

MVOCs are chemical compounds that are released when some microbes grow. In fact, they are typically the same compounds as the VOCs of chemical origin. MVOCs also cause the typical smell of mould. Mycotoxins are toxic compounds produced by some microbes.  Stachybotrus, Fusarium, and Aspergillus versicolor are toxigenic. In addition, some microbes include proteins that are allergens, i.e. compounds that have the ability to cause allergy. Microbe spores and fungal particles may both cause symptoms themselves and transport toxins in the indoor air.


Aspergillosis is the name of a group of conditions caused by a mould called aspergillus mentioned above. This family of moulds usually affects the respiratory system (windpipe, sinuses and lungs), but can spread to anywhere in the body. Symptoms can vary from mild wheezing to coughing up blood and people with weakened immune systems are at a greater risk of being more severely affected. You can get more information from The National Aspergillosis Centre. They set out to better understand and diagnose and treat illnesses arising from damp homes.

Black Mould

Black mould is a generic name used to describe the Stachybotrys Chartarum species of mould. 

Each type of microbe has unique preferences for the growth conditions. The growth of xerophile microbes may begin when the relative humidity of a material is 65–70 %. On the other hand, the probability of microbe growth on building materials seems to increase considerably when relative humidity exceeds 80 % . The relative humidity of 75 % seems to be a sensible critical moisture condition for microbe growth in buildings materials.  For example, a typical xerophile microbe Aspergillus versicolor (which affects the respiratory system) has been reported to require a relative humidity of approximately 75 % for growth on a nutritious material at 20 ℃.

Dust mites especially love warm temperatures (23-27 degrees C) and high humidity levels of 70-80%. One study has found that mite populations stop growing and die out when relative humidity levels drop below 60%. Dust mites don’t drink as they get their moisture from the air. When the Humidity drops the dust mites dry out and die.

Maintaining relative humidity between 30% and 60% is a comfortable range for us humans with the temperature around 18-25 degrees Celsius.  If you have a Humidity switched Extractor fan in your kitchen and bathroom set it for 60% RH.

Some other commonly reported mould or moisture related health effects are for example:

  1. Irritative and general symptoms such as rhinitis, sore throat, hoarseness, cough, phlegm, shortness of breath, eye irritation, eczema, tiredness, headache, nausea, difficulties in concentration, and fever.
  2. Infections such as common cold, otitis, maxillary sinusitis, and bronchitis
  3. Allergic diseases such as allergy, asthma, and alveolitis.

The irritative and general symptoms in the first group do not cause permanent health hazards. The symptoms typically disappear within a few weeks after the end of the exposure. This is the same for repeated infections, but possibly not until after several months. However, a prolonged moisture damage may also lead to allergy or allergic hypersensitivity.

Reviewed studies show that there is a significant association between respiratory symptoms, especially cough and wheeze, and the presence of damp and mould.

Among other sources of moisture, one of the most common  is condensation. Condensation forms when the air indoors can’t hold any more moisture. Cooking, showering, drying clothes indoors and breathing without adequate ventilation can all cause excess moisture. Droplets can form on indoor surfaces such as mirrors, windowsills and on walls, particularly when they’re cold.

You can help prevent the build-up of condensation by:

  • putting lids on saucepans, drying washing outside and avoiding using paraffin or bottled gas heaters
  • opening the bedroom window for 15 minutes each morning
  • making sure your home is well insulated
  • heating your home a little more
  • ventilating rooms regularly and leaving doors open to allow air to circulate, unless you’re cooking or showering
  • if you’re cooking, showering or bathing – opening the window, putting the fan on and closing the door of the room you’re in

For those of you interested in the effects of dampness and mould on your quality of life and living conditions I would suggest you download ‘WHO guidelines for indoor air quality : dampness and mould.

What is Humidity?

Humidity is simply the moisture in the air. In normal room air there is typically about     1 % water vapour, but it is widely present in greater or lesser amounts. The Environmental Protection Agency cites the ASHRAE Standard 55-1992 Thermal Environmental Conditions for Human Occupancy, which recommends keeping relative humidity between 30% and 60%, with below 50% preferred to control dust mites. At high humidity sweating is less effective so we feel hotter. In the winter, heating cold outdoor air can decrease indoor relative humidity levels to below 30%, leading to discomfort such as dry skin and excessive thirst.

How we measure or represent Humidity depends on what we are looking for and what we are trying to measure. There are many different ways to represent the amount of moisture that is dissolved in the air. Relative humidity, specific humidity, humidity ratio, dew point, percent by volume, and grams per cubic meter are all used to express a measure of the amount of water vapour that is mixed with Air. In dealing with Dampness in your home we are mostly interested in Relative Humidity (RH) and Absolute Humidity.

Water vapour is normally invisible, and behaves like a gas, except when it condenses to form water or ice. Even without condensing, water vapour can react with surfaces and penetrate materials. The capacity of the air to hold water vapour depends on its temperature: the higher the temperature, the more water vapour it can contain.  100% humidity is the point where liquid water and water vapour are in equilibrium, which means the water is evaporating into vapour and the vapour is condensing into liquid.

The term we are most familiar with is Relative Humidity (RH). This is the amount of water contained in the air at any given temperature as compared to the maximum amount of moisture the air can hold at that temperature when saturated. For example, at 21°C, 1kg of dry air can hold up to 15.8g of moisture. If 1kg of air at 21°C contains 15.8g of moisture, it is said to be at 100% relative Humidity. If that same quantity of air contains 7.9g of moisture at 21°C, this is compared to the amount of moisture that the air can hold when saturated at this temperature. Therefore this air is at 50% RH (Relative Humidity).

relative humidity photo

Air temperature is a key measurement alongside relative humidity.  This is because the “relative” aspect is effectively “relative to temperature” (how saturated the air is at its current temperature). For a given air sample, a rise in temperature means a fall in relative humidity.  The amount of water that air can hold changes with its temperature. While 1kg of dry air at 21°C can hold up to 15.8g, the same quantity of air at -18°C can hold only 0.92g of moisture.

When we test for Relative Humidity during a survey we can only get a ‘snapshot’ of the environment. It could be that the Relative Humidity is higher in the mornings when people are showering or in the evening when people are cooking. However, it does give us an idea as to the conditions prevailing at the time of the survey.

Measuring Humidity  is essential in undertaking a damp survey as it can give us the ‘Dew Point’, which is the temperature at which the moisture in the air will ‘Condensate’ on the walls and the materials of your home. The ‘Permeability’ of the material bears a part in the effects of ‘Condensation’ also.  Moisture vapour can also ‘Condensate’ within the walls and this is called ‘Interstitial Condensation’. There is another sort of Condensation which we call ‘Summer Condensation’. (More on these in another Blog!)

We need to be careful when measuring RH. For example if we get a RH of 75% in the Kitchen and 60% in the Lounge we could be forgiven for coming to the conclusion that the Kitchen was damp but we also need to note the air temperature. If the air temperature is the same in both rooms then we could say the Kitchen is damp.

Another measurement of Humidity is Absolute Humidity. This is measured in grams per cubic meter.  Absolute Humidity is the mass of water vapour in a unit volume of air and is sometimes thought of as the density of the water vapour. It is a measure of the actual water vapour content of the air. To give you an example, for 25 degrees centigrade and 60% relative humidity, one cubic meter of moist air contains about 14 grams of water.

With this example we begin to get an idea of the difference between a relative moisture scale and an absolute moisture scale. The relative humidity changes when the temperature changes, but the % water vapour (or vapour pressure) by volume does not change with temperature. It changes only when water vapour is added or removed from the air.

measuring humidity



Some of you may be interested in finding out how much actual moisture is present in the room and what the dew point may be (when condensation will occur).  The Absolute Humidity is measured in grams per cubic meters (g/m3).  You could buy a simple Room Hygrometer and work these out using the table below.

                                                          Climate/humidity table

The table shows the “absolute humidity” in g/m3 (upper line) and the “dew point temperature” of the air in°C (lower line) for certain air temperatures as a function of “relative humidity”.

Example: At an air temperature of 50°C and a relative humidity of 70%, the absolute humidity is 58.1 g/m3 and the dew point temperature is 43°C.

RH 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
+50 8.3 16.6 24.9 33.2 41.5 49.8 58.1 66.4 74.7 83.0
+8 +19 +26 +32 +36 +40 +43 +45 +48 +50
+45 6.5 13.1 19.6 26.2 32.7 39.3 45.8 52.4 58.9 65.4
+4 +15 +22 +27 +32 +36 +38 +41 +43 +45
+40 5.1 10.2 15.3 20.5 25.6 30.7 35.8 40.9 46.0 51.1
+1 +11 +18 +23 +27 +30 +33 +36 +38 +40
+35 4.0 7.9 11.9 15.8 19.8 23.8 27.7 31.7 35.6 39.6
-2 +8 +14 +18 +21 +25 +28 +31 +33 +35
+30 3.0 6.1 9.1 12.1 15.2 18.2 21.3 24.3 27.3 30.4
-6 +3 +10 +14 +18 +21 +24 +26 +28 +30
+25 2.3 4.6 6.9 9.2 11.5 13.8 16.1 18.4 20.7 23.0
-8 0 +5 +10 +13 +16 +19 +21 +23 +25
+20 1.7 3.5 5.2 6.9 8.7 10.4 12.1 13.8 15.6 17.3
-12 -4 +1 +5 +9 +12 +14 +16 +18 +20
+15 1.3 2.6 3.9 5.1 6.4 7.7 9.0 10.3 11.5 12.8
-16 -7 -3 +1 +4 +7 +9 +11 +13 +15
+10 0.9 1.9 2.8 3.8 4.7 5.6 6.6 7.5 8.5 9.4
-19 -11 -7 -3 0 +1 +4 +6 +8 +10
+5 0.7 1.4 2.0 2.7 3.4 4.1 4.8 5.4 6.1 6.8
-23 -15 -11 -7 -5 -2 0 +2 +3 +5
0 0.5 1.0 1.5 1.9 2.4 2.9 3.4 3.9 4.4 4.8
-26 -19 -14 -11 -8 -6 -4 -3 -2 0
-5 0.3 0.7 1.0 1.4 1.7 2.1 2.4 2.7 3.1 3.4
-29 -22 -18 -15 -13 -11 -8 -7 -6 -5
-10 0.2 0.5 0.7 0.9 1.2 1.4 1.6 1.9 2.1 2.3
-34 -26 -22 -19 -17 -15 -13 -11 -11 -10
-15 0.2 0.3 0.5 0.6 0.8 1.0 1.1 1.3 1.5 1.6
-37 -30 -26 -23 -21 -19 -17 -16 -15 -15
-20 0.1 0.2 0.3 0.4 0.4 0.5 0.6 0.7 0.8 0.9
-42 -35 -32 -29 -27 -25 -24 -22 -21 -20
-25 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.6
-45 -40 -36 -34 -32 -30 -29 -27 -26 -25


Where does moisture in the home come from?

It is important to understand that not all homes are created equal in that old and new buildings are different:

Old buildings are designed in such a way that they must be allowed to breathe

New buildings work on a system of defensive barriers that keep moisture out


This is one of the main causes of moisture damage in the home. Any water vapour produced within the home will freely move around and may condense on surfaces or within the fabric of colder building materials or components. Condensing moisture can result in localised mould growth and the rapid deterioration of some building materials, for example, timber. The resultant staining or damage caused by extreme cases of condensation is often confused with moisture penetrating through walls or from the ground. The remedial actions for condensation and penetrating moisture are quite different and tests should be carried out to identify the correct source before deciding on remedial actions.

We need to understand the interactions of the moisture in the home along with the environment in the home. It all interacts together.  It’s like having a mini weather system in your home! There are various sources and manifestations of moisture within buildings and the most common sources of moisture are from the ‘life-style’ – just normal everyday living (see table below).

The amount of water produced from normal household activities can be quite considerable. Certain other activities such as using bottled gas and paraffin heaters add significant amounts of water to the air, water being a by-product of burning these fuels. Drying clothes over radiators will also significantly add water vapour. Also consider that the surface area of your lungs is in excess of 75 square metres and warm air is passing over this wet surface as we breathe 15-20 times per minute; this is being breathed back into the environment! Indeed, it is reported that a large dog can give off even more water vapour than the average adult! The table below illustrates the amount of water that could be generated in an average house during a 24 hour period.

Water vapour source in an ‘average’ house per day Approximate water generated (in litres)
4/5 people asleep 1.5
2 people active 1.6
Cooking 2.6
Washing up 1.0
Washing clothes 4.0
Drying clothes 4.5
Bathing/washing 0.5
Approximate total 15.7 litres

Water Penetration

Another source of moisture in the home may be from water penetration. Water penetration is normally associated with walls and roofs and is indicative of either: Missing roof tile, slate, flashing, damaged render or cladding, leaking or undersized Gutters.

Gutters and Downpipes:  When gutters leak, there are two ways water can go. The first one is run directly down the outside of the wall for a short distance. The other option is splatter on the ground and window cills below. Gutters and rainwater pipes can get blocked with falling leaves and other debris and should be cleaned out and checked for defects from time to time.

If you have old cast-iron downpipes, you might get damp without being aware. Downpipes that contain sharp bends are also prone to blockage from debris washed in from the gutters. Check if there are any holes in the back of the downpipes, and whether there is water tracking from the downpipe along brackets.

Cement Rendering:  Usually, this type of coating is used on the exterior of walls. However, the problem with cement coating is that it can develop cracks over the surface. The size will depend on the quality of the render and how skilled your contractor was. However, the cracks will happen, and they are impossible to prevent with cement rendering and these cracks will allow water into the wall.  Also the walls may not be robust enough for the particular exposure of the building. Water penetration can also affect apparently sound walls.

Retro-fill Insulation:  There is a risk when fitting retro-fill cavity wall insulation in highly exposed areas that moisture may be transferred across the cavity. Read ‘Post Installation Performance of Cavity Wall / External Wall Insulation.  This was produced by the British Research Establishment (BRE).

Rising ground moisture:   This is normally associated with missing or ineffective damp-proof courses or damp-proof membranes. It can also result from bridging of the DPC or damp-proof membranes by high ground levels. As a guide, ground levels externally should be a minimum of 15cm below the DPC.  It can also be caused by leaking water pipes or drains, debris bridging the cavity wall and so on. In my experience, it is rare to find a defective or ‘broken-down’ damp proof course. When I do its usually as a result of poor workmanship which damages the DPC while carrying out renovation works.

Lots of common defects in buildings can result in internal dampness, including poor or defective mortar courses in external walls, defective sealant around windows and doors, poorly maintained wooden windows and doors, and broken, cracked or defective stone window sills.


All the materials which go to make up your home contains some level of moisture.  Water in construction materials is bound in several different ways. Chemically bound water is bound so tight (intermolecular attraction within cell walls) that it typically is not taken into account when assessing moisture. Physically bound water, on the other hand, is the evaporable water typically known as moisture. Physically bound water is bound to the pores of the material both hygroscopically and capillarity.

A porous material has the ability to absorb moisture from the atmosphere and to discharge it to the atmosphere.  This property is known as hygroscopicity. Hygroscopic is any material that absorbs moisture into the cell of the material causing a dimensional
change. This is different from hydrophilic absorption which is absorption of moisture between the cells,usually NOT causing a dimensional change.

Some Hygroscopic salts may be present in plaster or deposited on a wall surface when they are left behind when moisture evaporates. When these salts absorb moisture from the air it can saturate the wall and give the appearance of ‘rising damp’. This is why it is so important to test the amount of moisture which might be present due to Hygroscopic Salts (HMC) both in the plaster and in the mortar bed or bricks. The Total Moisture Content (TMC) should be similarly tested. This can be measured using the Gravimetric method. When you subtract the HMC from the TMC it will give you the Capillary Moisture Content (CMC) which is the Free Water present. This free water must have come from somewhere else so if present, it requires further investigation.

Porous material can be referred to as a ‘moisture sinks’. These absorb moisture where there are high levels of moisture and they release the moisture when the moisture levels in the surrounding air drop. This is how bricks and plaster were originally designed to work. The moisture in the air is trying to reach Equilibrium with the moisture in the walls or timber or visa versa. Equilibrium moisture content (EMC) is the moisture level where the wall, timber or plaster etc. neither gains nor loses moisture since it is at equilibrium with the relative humidity of the surrounding environment.   The equilibrium is dynamic in nature because of the changing relative humidity and temperature.

Temperature and air pressure affect the relative humidity of the air. The amount of water vapor in a volume unit of air is referred to as the absolute humidity of the air, usually expressed as grams in a cubic meter of air. There are other terms such as Saturated Humidity which is the maximum value of Absolute Humidity.  In Damp Surveying we would be interested in the Relative Humidity (RH) and the Absolute Humidity. Relative humidity (RH) can be defined as the actual amount of water vapour in the air expressed as a percentage of the maximum amount of water vapour that could be held at the same temperature. This changes minute by minute as the environment in the home changes.   I will explain these terms and how they affect your home in another blog.

Example of a Survey from a ‘leading nationwide Timber and Damp company’

I was called in by the householder to give a second opinion on a Damp Survey carried out by a ‘leading nationwide Timber and Damp company’. What follows is an extract from the report prepared by one of their ‘surveyors’ who has CSRT CSSW qualifications.


An external inspection of the property showed evidence of an original damp proof course. Some areas have a heavy dense render applied to the wall surface internally; this would indicate previous damp proofing has taken place.

Inspection of the internal wall surfaces of the property was completed using an electronic moisture meter where high moisture meter readings were observed to several areas.

Some decorative spoiling was noted within the kitchen, kick boards were removed and testing of all elevations revealed high moisture content to the rear area. External examination revealed no external defects that could contribute to the moisture levels.

Areas of the front reception room appear to have been treated before with heavy dense render within the bay window area. Moderate to high moisture meter readings were still observed to this area. External inspection revealed no external defects that could contribute to the moisture content of the masonry. High moisture meter readings were also observed to the right hand party wall.”

Moisture Meter Timberwise report

The above photos were included in the report to ‘prove’ the rising damp finding.

And his conclusion and recommendation based on the above was:

After consideration of the instrumental readings, soluble salt contamination and other relevant factors, it is assessed that external masonry elevations within the property are affected by rising and penetrating dampness and the installation of a chemical damp proof course is recommended to the areas indicated on the enclosed sketch plan”

His instrument was the moisture meter. No samples were removed from either the plaster or the wall to test for the presence of ‘soluble salt contamination’. No ‘other relevant factors’ were mentioned.

The owner would have allowed samples to be taken.  The Surveyor could have taken plaster / wall samples where the skirting board was missing and carried out a Carbide Meter moisture test. When I took a sample from one on the ‘damp’ walls it was dry.

He found that the front reception room at the bay window showed ‘high’ meter readings as did the party wall abutting the bay window wall. The solution for him was not to investigate further but to seal in the dampness. Moisture will always find a way out unless the cause is treated.

I tested the skirting board at the junction of the bay window and the party wall and it gave a reading of 21% moisture in the skirting. I went outside and investigated the bay window wall. This is what I found.

Raised Plinth          Raised Step

The slate DPC course was lower than the rendered plinth. This is common in older buildings so don’t assume that the DPC is at a rendered Plinth level. The Path and the step had been built up and bridged the DPC.

I examined the cavity (where you see the disturbed bricks in the photo) using a Borescope and found some bridging of the cavity in this area. This corresponds to the damp area internally. I removed the bricks and cleared out the debris.

blocked cavity

I checked the air bricks next. The two airbricks were blocked as spiders find the small openings of the traditional air bricks to be good for building webs to catch dinner! I replaced the existing air bricks with plastic louvered ones to improved ventilation. This will provide lower humidity levels and help to purge excess moisture from the sub-floor void.

The airways in those terracotta airbricks typically account for no more than 8.25% of the cross sectional area of the brick, so they are pretty feeble vents even when clean. The free air opening (when clear) amounts to 1,152mm2 per air brick. The new louvered plastic air bricks have a free area of 7,750mm².   So, in effect, by replacing the old traditional (and mostly clogged) terracotta air bricks with new plastic air bricks you can increase the effective free ventilation area of each air brick opening by 700%.  Your timber floor will love you for it!


DSC01552The next issue was the external walls. They had been painted with plastic paint and it had also been re-pointed in sand and cement mortar. This had trapped the moisture in the wall. I removed some bricks to check the condition of the wall ties as sand and cement repointing on top of an impervious coating can accelerate wall tie rusting due to the moisture retention. The bricks and mortar beds were saturated. On removing the bricks to check the condition of the wall ties I found they did need replacing.

erode inner wall

  In addition, the trapped moisture was eroding the mortar beds.

The image below is a thermal image of the rear of the house and it illustrates how impervious coatings can inhibit the evaporation of moisture from the wall. The ‘blue’ area is showing up colder than the ‘Turquoise’ area next door on the left because of trapped moisture in the wall. Moisture needs heat to evaporate hence the colder wall.

thermal image

Overall, the ideal solution for the house would be to remove the plastic paint from the external walls. Hack out the dense cement pointing and re-point the walls in Lime Mortar. This will allow the walls to ‘breath’ and for the trapped moisture to evaporate.

Another factor may be ‘interstitial condensation’ due to the excessive damp outer leaf of the wall. In this case moisture may condensate on the outer face of the inner blockwork within the cavity and reduce the ability of the inner wall to take up moisture making the inner wall surface more liable to condensation. This is ‘Equilibrium Relative Humidity’ (ERH). More on condensation maybe later!

So, in my opinion, the solution to the ‘damp problem’ would be to reduce the ground levels to within 150mm of the DPC; remove the front step and rebuild with a drained cavity between it and the house; clean out the debris bridging the cavity and increase the air flow under the floors to allow better ventilation to remove moist air and remove the rendered plinth.  This would cost about £600.00.   It certainly would not have been to hack off all the internal walls to 1.2m and apply a dense sand and cement render coat at a total cost of just under £4,000.00.  This does not include the cost of any electrical, plumbing, decorating works or fixing new skirting’s.

Timberwise kind of gives it away on their website when they say “ While some damp problems can be easily fixed than others, the more common fixes are the installation of a damp proof course“.

Lesson: Follow the moisture to the source and deal with it and don’t settle for very expensive ‘fixes’!


What is a Moisture Meter?

This is a much abused and misunderstood but useful instrument. The first thing to understand is that any reference by a resistance meter to the percentage of moisture content in materials (other than timber) is incorrect.  It should be ignored. It is calibrated for timber.

The precise makeup of your house is unique to your house. The plaster, mortar beds, bricks that go to make up your walls are different to other houses. Any changes in the mortar mix and / or plaster mix changes the properties of the wall. It changes the density of the wall along with changing the resistance and absorption qualities of the wall. A heavy, dense material has a much lower moisture content than a light material which has the same amount of water in it. As a result, lime mortar is dry at 5%, yet cement mortar at 5% is wet. Some bricks such as a Fletton Brick would be thought just ‘damp’ at 2%, while others, such as a dense engineering brick, would be ‘wet’ at 2% moisture content.

(The % moisture content of a material is the amount of water in it divided by its weight, when dry:     (wet weight – dry weight) ÷ (dryweight) X 100 =  % moisture content).

Find a dry spot on the wall. This may be high up on the wall. Test this with your moisture meter. If it indicates a very low reading then this would indicate that the wall is free of moisture, free of carbonaceous materials, free of metallic material; in fact free of any material that could trigger the instrument to give a high or falsely high reading. This is how to find your ‘baseline’ reading which is the ‘dry’ reading for your house. So, you use the resistance meter to identify areas that give a higher reading than your  ‘baseline’ reading which you have obtained. Any readings you get are ‘relative readings’ which you compare to the ‘baseline’ reading for your room or house. This will highlight any ‘damp zones’.   (If your moisture meter has a ‘capacitance pad’ then use this to get readings beneath the wall surface if the wall surface is damp).

Next, work to a pattern in each room or on a particular wall or space. The Construction Industry Research and Information Association (CIRIA) advises that measurement should be taken at one meter intervals on skirting boards and more frequently if there appears to be a damp problem.  When you find a ‘damp zone’ lay a measuring tape on the ground along the base of the wall in question. Mark a timber lath say every 250mm. Lay the lath upright against the wall.  Work your way along the wall taking readings laterally say every 300mm and vertically up the height of the wall using the marked lath and plot these on some graph paper.

By looking at the data you should be able to identify those measurement which are in excess of the ‘baseline’ reading. The plot should show you how wide the damp area is, how high it goes and its shape. Further tests should then be carried out to confirm the cause of the damp problem. Manufacturers recommend ‘triangulation’ in dampness investigations where you bring together different pieces of evidence to make a meaningful diagnosis. This is a book in itself but I wanted to just give you an outline on the effective use of the ‘moisture meter’.