Damp Problems in Houses FAQ

With thanks to the denizens of uk.d-i-y for editorial input.

Disclaimer; This page is for advice only and no warranty of any sort is implied.

Damp is a problem in some houses. It can result in the growth of moulds and fungi, damage to the interior (and exterior) decor, wood rot, and ultimately damage to the structure of the building.

Damp problems are normally from one of three causes;

  1. Condensation; This can be due to poor insulation, poor ventilation, overcrowded accommodation, or poor control of water vapour generation.
  2. Rising damp; This is water rising by capillary action, and is due to a Damp Proof Course (DPC) which has either been bridged or breached.
  3. Water Ingress; This is where water is entering the building from above the DPC. A faulty roof, brickwork defects, faulty rainwater goods, or leaky plumbing could all be responsible.
The first phase of any remedial work is to assess where the damp is coming from. Condensation should be obvious, and often the problems associated with it are widespread. Rising damp normally only affects the building up to about 4' from ground level, while water ingress tends to affect the upper parts of a building.

The second phase is to eliminate the problem. Sometimes the solution is simple and inexpensive, other times it is the Job From Hell.

Condensation;

Poor insulation can be remedied by foam injection into the wall cavity, or there is a foam coating for walls which is applied like wallpaper, and then decorated over. Unfortunately, it is prone to denting, so if you have small children, this may not be an option. Condensation onto single-glazed windows is to be expected during cold weather.

Poor ventilation is often caused by air-bricks being papered over internally, or worse still being bricked up. An external inspection will reveal which rooms should have an airbrick, and approximately where. You can also try going around each room tapping or prodding the wall firmly at about 9" below the ceiling every 6" on the external walls. If you find a soft patch try pushing e.g. a screwdriver into it firmly. If it penetrates, you have found the airbrick. Another technique is to draw the curtains on a bright day, shut the door and allow your eyes to acclimatize, you may see a patch of light shining through the paper. Cut the paper clear, and fit a decorative grille. You may have to strip the wallpaper and plaster to find the bricking- up, or if you can get a metal rod into the airbrick from the outside and hit it hard, this may break the obstruction through. Another avenue of approach, particularly for bathrooms and kitchens is the fitting of an extractor fan.

Overcrowding causes damp problems because the amount of water vapour generated in a house is roughly proportional to the number of people in it. The only practical approach, other than kicking the in-laws out, is to invest in a dehumidifier. The trouble with these is that according to my back-of-an-envelope calculations, every litre of water condensed uses just over 1 kWh of electricity assuming 60% efficiency. They may also need frequent emptying, although the water, being distillate, can be used in steam-irons and for watering lime-hating plants.

Controlling excess water vapour generation is a matter of common sense. Do not leave the shower running for 5 minutes before you use it, and simmer saucepans rather than using a rolling boil. Unvented fuel heating (e.g. paraffin heaters or portable gas heaters) produces a huge amount of water vapour. Approximately for every pint of paraffin you burn, you produce a pint of water in the form of vapour. Roof condensation is normally because the gap at the eaves has been blocked. The solution is obvious.

Rising Damp;

A bridged DPC is usually the result of earth being built up so that the moisture can get into the brickwork above the DPC. The remedy is simple; dig the earth away. Poorly executed extensions can also result in the DPC being bridged. In this case consider legal action.

A breached DPC is not a DIY project but is very rare. There is an army of consultants (i.e. salesmen) who will insist you need your walls silicone injected, but then, they would say that because they are on commission. Specialist contractors drill a series of large (1" dia.) holes in the wall after ripping off all the decor and plaster and inject silicone fluid into the wall under high pressure to make the brickwork water repellent. Then it is made good. Normally there is a 10 year guarantee with the work, but if the company goes bust......
Caveat emptor.

The industry is now so concerned about its image, caused by numerous cases where customers were knowingly sold unnecessary treatment that it has formed a professional body, the BWPDA. Not only does this body deal with damp treatment contractors, but wood preservation specialists.  It is clearly a good idea to use one of their member companies.  There is another professional body the NRPPS 0500 223505, which also has an insurance-backed guarantee for members. They do not, as I write, have a website.
 

Water ingress;

If this is through the roof, then get a specialist roofer, but first check that it is not simply roof condensation (see above).

Brickwork defects; Movement cracks can allow water in through the outer skin of the wall, and if the wall ties are correctly fitted they will not allow the water to get to the second skin. Of course they may not be correctly fitted. Repair the cracks with a soft mortar. Pointing which is old may also allow water into the outer skin, and should be repointed. If the problem is porous bricks, then a silicone waterproofer will provide a medium-term repair, but regular reapplication will be necessary. Another cause is that the bricklayers who built the cavity wall were careless and dropped sufficient mortar into the cavity to bridge it.

For plumbing problems, fix the leak. Don't forget that if there is a fall on the pipe, the water may be running along the outside of the pipe some distance, and the fitting which is dripping may be OK, and one "uphill" from the drips may be responsible.

The third phase is to repair the damage. Normally damp is evidenced by the growth of black mould. For many years it was thought that this was invariably Aspergillus Niger, but modern research would indicate that this is not the case. There are a large number of black moulds and they all look about the same. Inhaling mould spores can result in Aspergillosis, a lung disease. Black's Medical Dictionary implies that there is no risk to healthy lungs, and other sources suggest that only people who are already in poor health and/or malnourished are at risk. However, I recommend a dust mask while removing it. Scrub it off with thin bleach, (the thickener in thick bleach may provide a medium for mould growth later) allow to dry out thoroughly, and redecorate. If the mould does not receive any more water it will not return. IF THE MOULD GETS ACCESS TO WATER IT WILL BE BACK. Wood rot is best dealt with by a specialist.

Another (subtle) cause of damp problems is an old building. I am talking about buildings over about 200 years old. In those days buildings were made differently. For example, they did not have a DPC. The normal problem in these cases is that the building is no longer being used in the same way that the design (evolved over hundreds of years) expects;

  1. The walls were painted with limewash. This is either slaked lime (calcium hydroxide) or whiting (powdered chalk - calcium carbonate) bound with animal glue, and possibly a suitable pigment. Slaked lime absorbs carbon dioxide from the atmosphere to become calcium carbonate, so the end result is essentially the same. Limewash is far more vapour permeable than either wallpaper or polymeric (i.e. modern) paints, so the damp which is present in the walls can escape to the atmosphere. The upshot of this is that you cannot use modern decorating products in old buildings with any degree of confidence. Regard it as a "character feature" of your home.
  2. The stone flag or quarry tile floor was also rather more water permeable than modern floors and if you put down lino or foam- backed carpet, or attempt to change the nature of the floor expect trouble at a later date. These floors often had rush matting laid down, which is a second "character feature".
  3. The doors and windows were "draughty". This was because the fire in the fireplace (What do you mean you converted it into a wardrobe?) drew air into the house via these gaps, as it was expelling hot air out of the chimney. The continual air flow carried away the water vapour emanating from the floor and walls. The fire was burning year round, as it was also used for cooking and hot water. While it may at first seem like a good idea installing central heating and double glazing, this in itself may cause a damp problem. More "character".
This does not mean that I dislike old cottages, the reason people like steam trains, vintage cars, and old warplanes is exactly because they are quirky and unpredictable, require a greater degree of skill to use, and there is a challenge in sourcing spares and consumables. A lot of modern things are so reliable that they are boring, and while your Mondeo can be expected to start reliably, a Model T IMHO is much more fun.

Another point to bear in mind is that if you have just bought a property, and it was not inhabited for any time before you move in, there is likely to be some damp evident. Don't panic and call in Mr Silicone-Injector, if the property was uninhabited, it was unheated, and some damp will have made its way in. Heat the place, live in it, and reassess the problem in a couple of months. Don't you just love solving a problem by procrastinating?

Finally damp cellars. Generally people want to convert these to a workroom of some sort. The cost of damp-proofing a cellar is astronomical, it is generally cheaper to move. Instead fit some racks and lay down some fine wines. Cheers!

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John Schmitt <john49@mdx.ac.uk>

Last updated 19/12/00