Common causes of damp

What are the main causes of damp?

  • Condensation
  • Penetrating damp
  • Rising damp


Air can contain water in its gas form, aka water vapour. This increases when cooking, boiling kettles, taking baths or showers and is exacerbated further by poor levels of ventilation. This warm moisture-laden air from a kitchen or bathroom can then obviously travel to other parts of the house which might be cooler. When warm humid air comes in contact with cold surfaces the water vapour will condense and revert back to water. It may be very obvious and form drops of water like we see on window condensation, but could also be invisible and just cause the wall plaster to be damp in places. Sooner or later though this may present as visible wet patches and show signs of mould or smell musty.

The temperature of the surface at which water condenses is call the dew point. It provides a measure of the actual amount of water vapour in the air – so the higher the dew point, the more moisture in the air.


  • Replace single glazed widows with double glazing, or add secondary double glazing. This means the inside of the windows will not be as cold and so less condensation. Stretching cling film across windows is cheap and easy (temporary) solution.
  • Ensure the house is well ventilated. Check whether extractor fans are working properly and are powerful enough.
  • Look for cold spots on walls, although in general the lowest part of the wall is likely to be quite cold (Underfloor heating can help here). If you do have a condensation problem these cold spots may be obvious due to discolouration or even mould. Houses that were retrofitted with some types of cavity insulation can often have cold spots. This either due to poor insulation in the first pace leaving areas that did not get insulated, or else the insulation itself has degraded and collapsed in places. This is particularly an issue with urea formaldehyde. This type of insulation can be removed but the process is more expensive than installing it. Note that degraded cavity insulation can also cause problems with penetrating damp, see below.

More ventilation? But I’ve just gone round fitting draft excluders!

Drafts vs ventilation

Drafts are basically a form of uncontrolled ventilation happening where you don’t want it. We often go to great lengths fitting draft excluders to doors, replacing drafty old windows only to find that the house is no longer adequately ventilated. This can be a confusing problem because having sorted out the drafts to prevent the house from being so cold, you then find you need more ventilation to deal with condensation. The ideal solution, is to get heat recovery ventilation which uses the heat from the extracted air to warm the incoming fresh air. To install such a system can be very costly, but it is possible to install individual room heat recovery extractors such as these.


Penetrating damp

  • This can be caused by water leaking or overflowing from the roof, gutters or downpipes onto the walls (although this should be fixed as part of general house maintenance on any property. This can happen when gutters or downpipes are full of debris, or  joints between them have failed.
  • Cracks or damage to walls. Before repairing it is worth finding the cause of cracks, it could mean something very serious such as movement in the ground or foundations.
  • Damp proof course (DPC) that has been bridged by rendering or pointing, creating a path for dampness into the wall above.
  • DPC that has been bridged by rising the level of an adjacent path, patio or border.
  • Failed damp course. This seems to be less common than the above. Many people talk about rising damp, but this rarely happens although it’s possible for houses to have been built right on top of an underground watercourse.
  • Wall ties that are dirty or corroded. Metal wall ties are fixed at intervals between the outer and inner skins  brickwork. They are designed so that any moisture will flow away from the inside wall down into the cavity below the damp course.. However if they are corroded or if a careless bricklayer allowed motor to fall on them (a.k.a. snots) then moisture can travel, across them and cause a damp spot on the wall.
  • As with condensation issues, badly installed retrofit cavity insulation (especially urea formaldehyde) can exacerbate penetrating damp issues as it can act like a sponge, soak up moisture from the outside wall skin and transfer it to the inside. Other forms of injected cavity insulation such as polystyrene beads may not have this issue as they don’t soak up water (the sponge effect) and have a certain amount of air around them.


Repairing and maintaining rainwater goods is a crucial part of house ownership. If you are renting inform you landlord of their obligations as it could be that their negligence is causing your hardship and potentially affecting your health.

Inspect the damp course. In older houses this is often a course of slate or lexible bitumen felt between two courses of bricks.

Damp proof course

When houses get repointed it’s not uncommon for these to be “pointed over” for cosmetic reasons, however this forms a bridge for dampness to travel from below the DPC to above:

Pointed over DPC
Note the tell tale wider pointing

A note about retrofitted cavity insulation

As this can affect both types of damage mentioned above, perhaps it should have a whole section to itself see here: cavity wall insulation removal. We all tend to think that cavity insulation is a good way to be energy efficient so what can go wrong?

As mentioned certain types of retrofitted insulation (such as the notorious urea formaldehyde) can cause issues:

  • When such insulation is injected, it will completely fill the cavity and stop air circulation. If it gets damp it can act exactly like a sponge and as there is no or little air circulation, once it gets damp, it can stay damp. This can be a worse problem for  houses on hills and in coastal areas due to an excessive amount of wind blown rain  – see below
  • If the insulation itself loses its integrity it can turn into powder, drop down and cause a blockage around the damp course. You often get this in the corners of rooms with external walls.
  • When this happens the gaps in the insulation can mean cold spots in the wall – attracting condensation.

So the big question is should you remove it of you have this problem? In the case study here, removal did appear to solve the issue. After removal you could either have a better form of insulation or leave the cavity empty. As one architect mentioned: “it’s better to have dry air in the city than damp insulation.” The best energy saving solution may either be re-installion of better cavity insulation or else internal wall insulation.

Wind blown rain

A bit of debris in the cavity or just a couple of failed wall ties or may not be an issue by themselves, but in a Southern or western coastal area exposed to prevailing winds from the Atlantic – especially if you live in a house at the top of a hill or otherwise in an exposed location – then you may be subject to more wind blown rain than normal and so these problems may be exacerbated.

Wind blown rain mapEven without any damp course or blocked cavity issues, when there is excessive wind blown rain, the idea of the “old breathing houses” getting wet and drying out doesn’t work quite so well.

… and cavity walls

The above is especially true when there is cavity insulation that has been retrofitted to older properties with cavities that were built before anyone thought of insulating them.  Houses these days are built with wider cavities so that there was air next to the insulation. When cavity insulation is retrofitted, it is forced into the existing cavity through small holes drilled in the outside skin of bricks so that it fills the cavity with no room for air to circulate.

Is there a solution?

There are some “breathable” damp sealers such as Emperor Masonry Creme which can solve this by sealing the outside of a wall while still allowing the damp to escape outwards. This claim has been verified by the Energy Savings Trust). There is a very good chance that this will solve the issues. However if damp is retained within the walls for other reasons such as:

  • internal lime plaster being replaced or skimmed with gypsum
  • damp due to insulation having been poorly installed

then you may want to remove the insulation first (not as difficult as it sounds), apply the Masonry Creme and allow the cavity to dry before re-insulating by a reputable company.

Why nothing about rising damp then?

Rising damp seems to be something that is extremely rare Once the main causes of damp caused by condensation and various forms of water ingress mentioned above, typically damp problems are solved. Yet the waterproofing companies my tell you otherwise and recommend a new chemical damp course and “tanking.” which basically seals the house on the inside of the walls.

See more:

Damp floors

Even though most older houses generally have suspended floors, there are still many with all or some solid (concrete) downstairs floors.

A short term solution if damp is not actually causing smell or construction issues is to use a dehumidifier. People who have done this have reported immediately noticing warmer rooms.

If you do have this older type of house with a solid floor at the back, you may even have damp issues there without knowing it. A very useful device is this simple humidity and temperature monitor such as this inexpensive one from Govee which gives you a readout on an app which show not only current level but how the levels rise and fall over time.

See our Damp case studies


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