This is occurs when there are surfaces in the house which are cooler than the warm damp air inside. You can control it by reducing the amount of moisture in the air (by ventilation and by reducing steamy activities) or by making the surfaces less cold.

It is a nuisance when the amount of moisture generated by activities in the house exceeds the ventilation flow that gets rid of it. Moisture held in the (warm, damp) air will condense out when it reaches a cooler surface. This is very obvious on the outside of a drinking glass with an iced drink in it, but also happens on outside walls and windows.

Double-glazing makes the inner surface of glass less cold, but it also usually reduces the amount of ventilation so that there is more moisture in the air.

• Wet washing lying about the house or draped over radiators (this is a dreadful source of damp)
• Using a tumble-drier without venting it outside (this includes condenser driers which release quite a lot of steam)
• Using baths and showers without running the extractor long enough to dry out the room during and after
• Cooking and washing up without running extractor.
• Damp towels
• Wet outdoor coats hanging inside

• Improve ventilation - if you have replacement windows, open the trickle vents.
• Open bedroom windows in morning
• Open upstairs windows during the day unless it is very cold.
• Use your extractor fans, especially in the bathroom. Some people have a mental aversion to ventilation, you can defeat this by having the fans fitted to the lighting circuit so that they come on whenever anyone turns on the bathroom light.
• Verify you have plenty of air bricks to ventilate the under-floor void, and they are not blocked with dust, rubble or cobwebs.

If you want to experiment, tape a piece of clear polythene tightly to one of the damp walls, see if you get more water forming on the wall side or the room side of the polythene.

There are other sources of moisture. Look for any signs of water getting in, especially small roofs over bay windows, or leaking radiators, downpipes or other plumbing. Sometimes it is caused by wet under the floors, perhaps due to waterlogged ground in low-lying areas or near a river, sometimes by a leaking watermain.

If you get condensation in the loft, it is probably due to warm air leaking up from the house (especially the bathroom) but it can be due to a plumbing/CH fault putting hot water into the loft tanks. Lofts should be well-ventilated to avoid the risk of rot in the timber. Sometimes loft insulation blocks the air gaps at the eaves, so it should be pulled back a few inches.

If you get condensation of the ceiling (especially noticeable in bathrooms when you get black spots where the cold nail-heads are under the surface of the plaster) then the insulation above is probably gappy, or very thin, or absent. This can easily be corrected.

JohnD 20070124