A loft that drips in January, has damp patches on the underside of the felt, or shows black mould on the rafters near the eaves, almost never has a leak. It has warm moist air from the house arriving in a cold space and condensing on the coldest surface it finds, which is the underside of the roof covering.
The sequence that causes condensation in loft space
Moist air rises through the house and finds its way into the loft through the loft hatch, gaps around pipes and cables, downlighters in the ceiling below and any unsealed service penetration. In the loft it meets the cold underside of the roof. Insulating the loft floor made the space above colder, which makes it worse, so a house that was fine for decades can start dripping the winter after the insulation was topped up. That is not a fault in the insulation, it is the ventilation that never got looked at.
Roof condensation solutions: ventilation is the fix
A cold loft needs cross ventilation so that moist air is carried away before it condenses, and the guidance expects a ventilation path at the eaves on opposite sides, supplemented at the ridge on some roofs. The most common defect is insulation stuffed into the eaves, blocking the very path it depends on. Eaves ventilation trays keep that path open while letting the insulation run right out to the wall plate, which is where it has to reach to avoid a cold bridge.
Stop the air getting up there
Ventilation deals with what arrives; sealing reduces what arrives in the first place. An insulated and draughtproofed loft hatch, sealed service penetrations, and hoods over recessed downlighters together make a large difference and cost very little. Bathroom and kitchen extract ducts that terminate in the loft rather than outside are a serious and common fault: they deliver a continuous plume of warm wet air into the coldest space in the house.
Breathable membranes and sealed roofs
Older roofs were felted with a bitumen sheet that holds moisture in, and rely entirely on eaves ventilation. Modern breathable membranes allow some vapour through, which is why some re roofed properties need less added ventilation. Spray foam applied to the underside of the roof is the opposite: it can seal the space, trap moisture against the timbers and make the structure impossible to inspect, which is why lenders and surveyors treat some installations as a defect.
Questions people ask about loft condensation
Is loft condensation a leak?
Usually not. A leak follows rain and appears in one place. Condensation appears in cold weather across the underside of the roof and near the eaves, and it is worst after a still cold night.
Will more loft insulation make it worse?
It makes the loft colder, so on a poorly ventilated roof it can. The answer is to keep the insulation and fix the ventilation and the air leakage, not to take insulation out.
Can I just fit a few tile vents?
Sometimes that is exactly right, and it works only if there is a path for air to enter as well as leave. Ventilation needs both, which is why blocked eaves are the first thing to check.
Does a bathroom fan venting into the loft matter?
A great deal. It is one of the worst single causes of a wet loft and it is common. The duct should be insulated and terminate outside through a tile vent or the soffit.