Causes of Damp in Houses

There are many potential causes of damp in houses which could be causing problems to the structure. Broadly speaking the causes of damp are often separated into three categories – Condensation, Rising Damp and Penetrating Damp. 

This article will go through these three main causes of damp in houses in detail. 

damp houses damage to wall


Condensation is one of the most common casuses of damp in houses and throughout the winter season we find a large number of calls to Damp Genius will be about this issue.

What Causes Condensation in Houses?

Condensation is caused when the air in a room can no longer hold the water suspended in it with the result that the water ‘condenses’ out onto cold walls and windows.

This process is possible to understand and control using a combination of heating and ventilation. Some houses will breathe less than others and these will be more prone than others. When carrying out a survey on a property at risk of condensation the key indicator we work with is Relative Humidity, this tells the damp surveyor exactly how much water is being held in the air compared to the maximum the water can hold. This subject can be tricky to get a grip of but just remember that 100% Relative Humidity means that the air is holding the maximum amount of water it can ever hold but it is still air, albeit it will feel quite uncomfortable and sweaty to be in. A deeper discussion of this topic can be found here Relative Humidity.  

For now, let’s look at the causes of condensation in houses and how to solve it.

Too Much Moisture

Excess moisture in the air is the number one cause of condensation damp in homes. The difficulty is that excess moisture comes from so many sources:

  • Showering and leaving the bathroom door open
  • Breathing overnight – An adult breathes out 1 litre of water into the atmosphere while sleeping
  • Cooking
  • Drying clothes on radiators or on racks indoors


These daily activites can release 4 litres of water into the atmosphere per person per day! Modern homes with double glazing and cement rendered walls are not designed to breathe and so the moisture has no where to go except on the walls and windows as water which the mould will love.

You can learn more about reducing condensation on windows in our article here.

Not Enough Ventilation

Poor ventilation levels within a home allows the moisture to build up which in turn leads to condensation and mould forming. With good ventialtion and breathability most homes can handle the moisture vapour by letting it escape from the house or by pulling in fresh air to dilute the moisture content.

  • Modern homes are not built to breathe; they are waterproof and may need mechanical ventilation to help change the air
  • New double glazing units can further seal in the modern waterproof homes creating additional problems
  • Removal of open fireplaces decreases the natural ventilation
  • Innapropriate cement renders and waterproof paintscan reduce the breathability of natural products used in older buildings

Not Enough Heating

The properties of air mean that the higher its temperature the more water it can hold in suspension. This fact is the reason that really cold places, like the Alps in winter, have such dry air. If you have been skiing you will no doubt notice that chapped lips are pretty common and this is due to the air being so cold it holds no water at all… all the water is on the slopes as snow!

In a poorly heated house this means that water is far more likely to form condensation and damp spots on the walls.

What Does Condensation Look Like?

Condensation typically causes the following tell tale signs:

  • Water running down windows
  • Mould around window frames
  • Darker damp patches on walls, particularly in cold corners
  • Mould growths in corners of rooms or behind furniture
  • Peeling wallpaper
  • Puddles on windowsills
  • Musty smells in rooms
causes of damp in houses
causes of damp in houses
causes of damp in houses
causes of damp in houses

How to Stop Condensation in a House

Working out the correct way to stop condensation in your hosue can take a bit of time and thought. The first thing we always advise people is to ensure you have covered the basics:

  • Closing bathroom doors after showering/bathing
  • Using kitchen extractor fans
  • Keeping trickle vents open in windows especially ones in your bedrooms overnight
  • Open bedroom windows in the morning for an air change


Once the basics are being routinely carried out then you can start to investigate other options. Key to this is increasing ventilation and heating. When heating your home you must ensure that it is heated evenly, i.e. you have heating on for a long time at a low heat. This will get warmth into the walls and is especially important if your house is a thick walled older property.

Ventilation can also be increased by installing extract ventilation fans or passive input ventilation (PIV) fans into the building. These will work to force an air change either by pulling out the humid air or by pumping in drier air. 


Rising Damp

Now we have discussed the most common cause of damp in houses let’s look at what we consider to be the least understood. Rising damp occurs when excess moisture at the base of a wall is pulled up the wall by capillary action – i.e. it is sucked up a bit like when you dip a sheet of kitchen roll into water. The trouble with rising damp, and indeed, the reason that it causes so much confusion seems to come from it being mis-diagnosed over the years by surveyors whose job it is to sell damp proof course injections.

What Causes Rising Damp in Buildings

Modern buildings, constructed since around 1900, typically have a damp proof course (DPC) installed at the base of the walls to form a physical barrier which stops moisture rising up the wall. This system should work well but unfortunately we find that over the years damp proof courses are messed with in some way and this can lead to problems.

It is also worth mentioning at this point that buildings pre 1900 did not always have damp proof courses incorporated into them. Some did, but most didn’t and it depends where you are in the country as to the construction methods used at the time. These older buildings did not have damp proof courses but that does not mean that rising damp caused them problems as it was managed in a different way through evaporation and breatheable materials. This subject deserves a whole post to itself so when that it is written I will include a link to it here.

No Damp Proof Course Installed

As mentioned pre 1900 this was typical and is not necessarily a concern. However, what causes more issues is when a damp proof course was not included in a building built since 1900. Sometimes this happens when an original wall has been taken down and new extension added by an ‘amateur’ builder who forgets the damp proof course, this should be very rare but sadly I have seen it too many times. Internal walls seem to be particularly prone to this and I believe that some builders mistakenly think damp ground does not reach internal walls.

Faulty Damp Proof Course

Damp proof courses are pretty tough things being made from bitumen, plastic or slates sandwiched underneath a few tonnes of masonry so out and out failure due to cracking or snapping is extremely unlikely although still possible.  What it far more common to see is that changes to the house and surrounds have rendered the damp proof course useless in a specific area of the house.

  • Bridged damp proof course by render covering it and touching the ground
  • Bridged damp proof course by new pointing going over it
  • External ground levels being too high and covering the damp proof course
  • Heavy vegetation, creepers etc, against the wall bridging the damp proof course
  • Solid floor internally bridging the damp proof course
  • Cavity wall becoming filled with debris or wet insulation
  • Penetrating damp from above running down cavity to sit on the damp proof course

How to Identify Rising Damp

Rising damp has some key characteristics which you should look for when diagnosing. Please keep in mind that these do not often mean the damp proof course has failed but more often mean it is faulty due to the reasons noted above.

  • Crumbling and flaking plaster or paint
  • Salts visible on the walls -often these look like white deposits and can be mistake for mould
  • Bubbling appearance to the paintwork
  • Damage does not go above 1m from the external ground level
  • Floorboards touching the wall show insect attack or decay
causes of damp in houses
causes of damp in houses
causes of damp in houses

A slate damp proof course visible at the base of a wall


Damage caused by rising damp on brick wall in exeter

How to Fix Rising Damp

If you own or are buying a property which has suspected rising damp then the best advice for fixing rising damp is to first have a comprehensive and impartial damp survey carried. True rising damp caused by a damp proof course breaking down is very rare as explained by Stephen Boniface, former chair of the RICS, and we therefore approach any suspected rising damp with caution. Before any diagnosis is made a thorough survey needs to be carried out to look at all other possible causes following methodolgy outlined in BRE245.  

More than likely the existing damp proof course has been bridged by something or penetrating damp is causing the issues. If you need help working out the cause then Contact Us to request a survey and we can identify the cause for you.

Having said all of that…. If it is found that no damp proof course is present and the property should have one (i.e. it was built in an age where it should have one) then it is possible to drill and inject a siliconate damp proof course, also called a chemical DPC.

Should the property be built before 1900 then please do not install a damp proof course and please please please above all else do not believe anyone who tells you to replaster internally using cement. This is totally wrong and will cause more damage to the building. Contact us if you need advice on this and we can help you solve the problem without ruining the building. 

Penetrating Damp

Penetrating damp is the second most common cause of damp in houses and is responsible for damaging internal plaster, causing wet and dry rot in timber, and sometimes even causing rising damp style symptoms.

What Causes Penetrating Damp in Houses?

Penetrating damp can be caused by an almost limitless source of external building defects. The list below gives a flavour of the most common causes we have seen over the years:

  • Cracks in external render
  • Blocked and overflowing guttering
  • External ground levels too high compared to internal ground levels
  • Missing roof tiles
  • Damaged chimney flaunches and flashing
  • Broken or missing ridge tiles
  • Damaged flashing in roof valleys or parapet walls
  • Gaps around windows and doors
  • Waterproof masonry paint cracking
  • Creeper plants gowing up walls

How to Identify Penetrating Damp

Penetrating damp can be identified by looking at the location of the dampness together with a visual assessment of the damp pattern. Penetrating damp very often forms on upper floor walls around windows or at the ceiling/roof junction. Some key things to watch for when identifying penetrating damp include:

  • Mould may or may not be present
  • Any white salts present may look extra fluffy like cotton wool
  • True rising damp will only rise to 1m above the external ground level so above that you may be looking at penetrating damp
  • Any sections of wall which are below ground will be prone to penetrating damp
  • Look closely outside for signs of damage to property
  • Eliminate the possibilty of condensation
causes of damp in houses
causes of damp in houses
causes of damp in houses

How to Treat Penetrating Damp

Penetrating damp will involve fixing the external problem which is letting water enter the property. There is no single treatment to fix the problem of penetrating so please do not believe the marketing employed by companies selling water repellent paints and coatings. If your property is cement rendered or cement pointed then there is no reason that water will penetrate it provided that the fabric is maintained in good condition

Sadly numerous problems do occur in Victorian/Edwardian brick properties which have repointed by well meaning builders using cement. This can have the affect of forcing water to evaporate from the walls through the softer bricks instead of the pointing and this leads to brick spalling and damage. At this point it may seem a good idea to use a breatheable water repellent on the walls to prevent the ‘porous’ bricks absorbing more water. This may work, however, the better solution would be to remove the cement pointing and redo it with lime mortar. This softer, lime mortar will let the wall breathe as intended and the bricks can then dry out. If you have or intend to buy a period property then it is strongly recommend to take advice from someone who understands dampness in older buildings.

How Long Does it Take For Penetrating Damp to Dry?

As a rule of thumb a wall will dry out roughly 1″ or 30mm depth per month. Therefore a typical 9″ brick wall will take 9 months and stone walls can take far longer. A thick stone wall can easily take 2 years to dry out fully.

Clearly you must keep this in mind when carrying out any external repairs – the full resolution of the problem will not happen quickly.

Damp Genius - Impartial Damp Experts

There is alot to consider when trying to resolve a damp issue in a house. If you are not sure where to begin then try downloading our ‘Diagnose your Damp’ guide below or to request a survey then get in touch on 01803 262064 or fill in the contact form here.

Table of Contents

Contact Us

If you need a solution to a damp problem or require a specialist survey then get in touch using this form. We are very friendly and happy to help. 

download how to diagnose your damp free guide

Click the button below

for instant download of your free guide