Condensation in the Loft: Causes and Fixes

Condensation or a leak?

The two look alike from inside, so check the pattern before you call anyone. A leak usually shows as a stain or a drip below one spot, often beside a chimney, valley or window, and it gets worse during or just after rain. Condensation shows across a wide area, tends to be worst on cold mornings, and appears even when it has not rained for days. You may see beads of water on the nail points that come through the roof, a damp underside to the felt, dark spotting on the rafters, or a white fur on the timber. Insulation that is wet on its upper face while the tiles above are sound is another sign.

Condensation on its own is a nuisance. Left year after year, it soaks timber, encourages mould, and can lead to rot in the ends of the rafters where they meet the wall. That is why it is worth sorting out early. Our guide on why a roof leaks when nothing looks damaged helps when you are unsure which problem you have.

Why it forms

Everyday living puts moisture in the air: cooking, showers, drying washing indoors and even breathing. That warm air rises through the ceiling, finds the gaps and reaches the loft. If the roof space is cold and the surfaces are colder than the dew point of that air, the vapour turns to liquid on contact. Modern houses are more airtight than older ones, and a lot of insulation sits on the loft floor, which keeps the roof space colder than it used to be. That is good for heating bills and harder on the roof structure, so air movement in the loft matters more.

Four causes come up again and again. The first is blocked eaves, where loft insulation has been stuffed right into the eaves and shuts off the gap that lets fresh air in. The second is a bathroom or kitchen extractor fan whose duct ends in the loft rather than outdoors. The third is gaps in the ceiling below: round the loft hatch, recessed downlights, pipes and cables. The fourth is an old roof covering with no underlay or with a traditional bitumen felt that does not let moisture pass, which traps vapour against the timber.

What fixes it

Start with the source. Have every extractor fan ducted to the outside, with the duct sloping down and out so condensation does not run back. Seal the ceiling gaps and fit a draught-proofed, insulated hatch. Moving air out of the house is cheaper than treating the loft afterwards.

Then let the roof space breathe. Pull the insulation back from the eaves and fit plastic eaves trays, sometimes called baffles, which hold the insulation clear of the opening and keep an air path open above it. Where the soffit (the board under the roof edge) has no vents, adding them gives the loft fresh air; our page on soffit ventilation shows how that is done. Ridge or tile vents help air flow through, and the ridge ventilation guide explains the options.

Do not stuff a loft with extra insulation and block the air path as a cure. A thicker layer is sensible, but only when the ventilation and the vapour barrier below it are right. Where condensation has already caused mould or soft timber, treat it, dry the area, and have the damaged wood inspected. A roof survey will find the cause on the day.

When the roof itself is the cause

Sometimes the covering contributes. Old houses with no underlay at all can let wind-driven rain and dust in, which confuses the picture. Roofs with spray foam applied to the underside are a special case, because the foam changes how the roof breathes and can hide damp timber; see our page on whether spray foam causes damp. Flat roofs are different again, because warm-roof and cold-roof builds have separate ventilation rules.

Our survey is free, and the photographs are taken in your own loft, so you can see exactly where the moisture is forming and which surface it settles on first. Often the evidence is plain once you look at the underside of the felt and the nail tips. If the cure is a fan duct and a set of eaves trays, we say so. You can go back to the Beacon Basingstoke Roofing homepage for the rest of what we cover, or ring 01256 213956 to book a visit.