Why lofts get condensation, and how to stop it

Where the water comes from

An ordinary household puts litres of water vapour into the air every day from cooking, washing, drying clothes and breathing. Warm air rises and carries that vapour with it. It reaches the loft through the hatch, gaps around pipes and cables, the holes cut for downlights, and extractor fans that end in the loft. When it meets a surface colder than its dew point, such as roofing felt on a winter night, it turns back to water.

Why insulating can make it worse

A newly insulated loft is colder, because heat from the house no longer reaches it. That is good for your bills, but it means any moist air that gets up there is more likely to condense. That is why ventilation and sealing are part of a proper insulation job.

How to stop it

1. Keep moist air out of the loft. Seal and insulate the hatch, seal gaps where pipes and cables pass through the ceiling, and use fire-rated covers or sealed fittings for downlights.
2. Vent fans outside. Bathroom and kitchen extractors should be ducted through the roof or wall to the outside, never into the loft.
3. Keep the eaves clear. Insulation pushed into the eaves blocks the air path. Eaves trays or vents hold it back.
4. Add ventilation if needed. Tile, slate or ridge vents let moist air out at the top. See loft ventilation.
5. Reduce moisture in the house. Lids on pans, extractor fans used properly, and clothes dried outside or in a vented dryer all help.

Roof type matters

Older roofs with no felt, or with traditional bitumen felt, behave differently from newer roofs with a breathable membrane. A breathable membrane lets vapour out through the roof covering itself, but still relies on some ventilation in many designs. We will look at which you have.