In short
Loft condensation happens when warm, moist air from the house gets into a cold roof space and cannot get out again. It condenses on the underside of the underlay and drips onto the insulation, so it looks like a leak but appears in dry, cold weather. The fix is to stop the air getting up there and give the roof space a clear route to ventilate at the eaves and ridge.
This is a job for our Roof survey & report service.
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How to tell condensation from a leak
A leak follows rain. Condensation follows temperature. If the drips appear on a clear, frosty morning, if the underlay is beaded with water across a wide area rather than wet in one line, and if the timber is dark and furry rather than stained in a streak, it is condensation. We see it most in houses around King's Lynn and Wisbech that have had extra loft insulation laid without anything done to the ventilation.
Why more insulation makes it worse
Thick insulation keeps the heat in the rooms, which is the point, but it also makes the loft colder. Cold air holds less moisture, so the same amount of vapour coming up through the ceiling now condenses where it used to stay as vapour. BS 5250, the British Standard for managing moisture in buildings, puts the emphasis on keeping moist air out of the roof void in the first place: sealed ceilings, continuous insulation and no gaps at the eaves and around openings. Marley's summary of the 2021 revision makes the same point.
Where the moist air is coming from
- Loft hatches with no seal, and downlighters cut through the ceiling.
- Bathroom extractor fans that vent into the loft instead of through the roof or wall.
- Insulation pushed hard into the eaves, blocking the air path from the soffit.
- A non-breathable underlay with no ridge or tile vents to let the air out at the top.
What a properly ventilated roof looks like
LABC Warranty's guidance for pitched roofs describes the target as a continuous opening at the eaves equivalent to a 25mm slot along the whole length, and a high-level opening equivalent to a 5mm slot along the ridge, in line with BS 5250. Those are new-build requirements, but they are a useful yardstick for an existing roof: if your soffits are solid timber with no vents and the ridge is bedded in mortar, the loft has no way to breathe.
| What we find | What we do |
|---|---|
| Fan venting into loft | Duct it through a tile vent or the soffit |
| Insulation blocking eaves | Fit eaves trays and pull the insulation back |
| No high-level ventilation | Add tile vents near the ridge, or a dry ridge system with a vented roll |
| Unsealed hatch | Fit a draught strip and insulate the hatch |
When it is worth a survey
If the timber has gone black or soft, we want to see it before winter. Condensation left for a few years rots battens and rafter feet from the inside, and that turns a ventilation job into a re-roof. A survey tells you which one you have.
