Home › Attic moisture

Attic moisture: condensation, frost, wet insulation and mold

A wet attic sends most people up onto the roof looking for a hole. In a large share of cases there is no hole: the water came from inside the house, and the roof is blameless.

Most wet attics are not leaking

This is the section that changes how people think about their roof, so it is worth stating plainly: a substantial share of wet attics have nothing to do with the roof covering. Water is being generated inside the house — showers, cooking, laundry, breathing, a damp crawl space or basement below — it rises as vapour into the attic, meets a surface that is colder than the dew point, and turns back into liquid there.

The evidence looks alarming and is very recognizable once you know it. Frost on the underside of the roof sheathing or on the shanks of nails poking through, on a cold morning, in a roof with no leak at all. Dark, damp staining on the plywood that is spread evenly across a wide area rather than concentrated around one penetration — a leak is local, condensation is general, and that distinction does most of the diagnostic work. Insulation that is wet on top rather than wet in a stripe. Then, in spring, the frost melts all at once and soaks the ceiling below, which is why so many people report a sudden leak on the first warm day after a freeze.

The sources are usually mundane and usually fixable. A bathroom extractor fan venting into the attic instead of out through the roof or a wall is the classic, and it is astonishingly common. A dryer duct doing the same. Gaps around light fittings, loft hatches and pipe penetrations that let a continuous plume of warm indoor air straight up into a cold space. Once the moisture is arriving, it is a question of whether the attic can get rid of it, which is where intake and exhaust ventilation comes in.

Where there is sustained moisture on wood there is often growth, and this is where we stop short on purpose. We will tell you where to look, what a mold-prone area looks like, how to dry the source so it does not come back, and who is qualified to sample and identify it. We will not tell you that a dark patch is mold from a description or a photograph — nobody honestly can — and we do not write about health effects at all. That is a medical question and it belongs with medical professionals, not with a roofing site.

Attic moisture guides

Sources

What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.