Condensation control

Warm air holds more water vapour than cold air. Ordinary living puts a great deal of moisture into that air — cooking, washing, drying clothes indoors, and simply breathing. When that air touches a surface cold enough, it can no longer hold the vapour and it condenses out as liquid water on the cold surface.

So condensation needs three things: moisture being produced, insufficient ventilation to remove it, and a surface cold enough to reach dew point. Change any one of the three and the problem improves. This is why it is seasonal, why it clusters in corners and behind wardrobes where air does not circulate and the wall is coldest, and why it got noticeably worse in a lot of houses after double glazing and draught-proofing removed the accidental ventilation the building used to rely on.

Condensation on a ground-floor external wall produces a damp wall that reads high on a conductance meter. If a surveyor arrives with the meter and the assumption, the reading appears to confirm rising damp and a quote for injection and replastering follows.

The tells that distinguish them are usually plain once you look for them. Condensation produces black spotted mould, which rising damp does not. It appears on cold surfaces and in unventilated corners rather than at a consistent height. It is markedly seasonal. And it is frequently worst in rooms where moisture is generated rather than on the wettest external wall.

It is worth saying plainly: a homeowner quoted several thousand pounds for a damp proof course, whose actual problem is a bathroom extractor that does not work and a tumble dryer venting into the kitchen, is not a rare case.

Start with moisture production, because it is free. Vent the tumble dryer outside or replace it with a condensing model. Lids on pans. Extract fans actually used and actually working — many run so weakly they move no meaningful air. Dry washing outside or in a ventilated room with the door shut.

Then ventilation. Adequate extract in bathrooms and kitchens, ideally humidity-sensing so it runs when it is needed rather than when someone remembers. Trickle vents left open. Where the whole dwelling is under-ventilated, a positive input ventilation unit — typically a loft-mounted unit gently introducing filtered air to dilute the humid internal air and push it out through the fabric — is often an effective and low-running-cost answer for the whole house at once.

Then the cold surfaces. Thermal bridges at lintels, reveals and wall–floor junctions produce cold spots where mould appears first, and insulation or detailing improvements at those points can remove the condensation risk. Heating patterns matter too: intermittently heating a cold house produces more condensation than keeping it at a low steady temperature.