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;

This can be due to poor insulation, poor ventilation, overcrowded accommodation, or poor control of water vapour generation.

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.
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.

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. 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.

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.