The short answer
Wet loft insulation means water is getting into the roof space, either from a leak in the covering or from condensation. Find the source first. Light, lightly damp insulation can often be dried out and reused, but anything sodden, compacted, mouldy or stained should be replaced once the roof is sound. Replacing it before the cause is cured simply lets the new layer get wet too.
Rain getting in, or damp air settling on cold timber?
The two look alike from the loft hatch, but they leave different clues. A leak is local. The insulation is wet in one patch, usually beneath a particular slate, valley, chimney or roof window, and the timbers around it are dark and damp. Water stains tend to follow the line of a rafter or batten down to the lowest point. Condensation is spread out. It shows as beads of moisture hanging from the nail points and the felt above the rafters, a general dampness across the whole slope, and often a musty smell, particularly after cold nights.
Warm, moist air from kitchens, bathrooms and drying laundry rises into the loft and meets cold roof surfaces. If the loft is poorly ventilated, that moisture turns to water and drips onto the insulation below. Our page on telling condensation from a leak goes through the signs in more detail.
Timing helps too. A leak shows soon after rain, even when the weather turned warm. Condensation follows cold, still weather and bath nights, and can build up through a week in which not a drop of rain fell. Noting what the weather did in the days before you noticed the damp is a simple way to narrow the cause down.
Tracing the entry point from the loft
Start inside, with a torch, on a dry day. Look for patches of insulation that are darker, flattened or clumped together, and for stained timber above them. Follow any staining uphill, because water runs along rafters and the underlay before it drops. The real entry point is usually higher and to one side of the wet patch.
Common culprits are slipped or cracked tiles, a worn lead flashing at a chimney or wall, a blocked valley, a failed roof window seal and a gap at the ridge. After a heavy downpour the problem often becomes plain, which is why our page on roof leaks in heavy rain is worth reading alongside this one.
You will not always find it by looking from inside. Old roofs with felt underlay can hide a defect for a long stretch, and bitumen or sealant patches put on by someone else can send water somewhere new. A roofer looks at the outside as well, and photographs what they find, which is how we trace it at survey.
Dry it out or replace it?
The answer depends on the type of insulation and how wet it has been. Mineral wool laid between and over joists loses most of its value once it is soaked or compacted. If it was lightly damp for a short time, it can often be lifted, left to dry in a ventilated space and put back. If it is heavy with water, flattened, stained or showing mould, replace it.
Blown fibre and loose-fill insulation behave differently, and once they have been wet they tend to clump and settle, so they usually need to be taken out. Rigid boards fitted between rafters can sometimes be dried and kept, provided the face is intact and there is no rot behind it.
Look at the timber at the same time. Wet insulation holds water against wood, so rafters, joists and wall plates under it deserve a check for softness. A probe such as a screwdriver will tell you if wood has gone spongy. Soft, crumbling timber needs specialist advice before anything new goes over it.
Do not spray foam onto a roof as a quick answer to a leak. It hides the underside of the timbers, so rot can run on unseen, and it does not cure the fault in the covering.
Fixing the cause before the insulation
Whatever you decide about the insulation, the roof comes first. If water is entering through the covering, the fix could be as modest as re-seating loose tiles, replacing a worn flashing, or cleaning out and relining a valley. If condensation is the cause, the cure lies in ventilation and moisture control: eaves vents kept clear, extractor fans that vent outside the building, and insulation that does not block the airflow at the eaves.
If one fault explains the damp, one fault is what we put right, rather than pricing a wholesale strip. Sealant has its place for a minor gap, but it is a short-term measure for most faults; our page on roof sealant repairs sets out when it holds and when it does not.
When the covering has been mended and the timber has dried, fresh insulation can go down. That is a sensible moment to bring the depth up to a level that suits the house, because the floor is already cleared and the old material is out of the way. Leave a clear gap at the eaves so that air can still move across the roof space, since blocked eaves vents are one of the reasons condensation returns.
Renting out, or living in a let property
Wet insulation in a rented home is a matter to report at once, in writing, with photographs. A landlord is generally expected to deal with leaks and damp that come from the roof, and a tenant will want a dated record of when the problem began. If you manage a let property, our guide to landlord roof repairs covers how we deal with access, tenants and the paperwork.
Whoever owns the house, stay off the slopes yourself. Inside the loft, tread only on boarding or the joists themselves, as plasterboard between them will not take your weight. Photographs of the damp patch and the ceiling below it are enough for us to give you an early view of the likely cause before anyone visits.
Our roof repairs service starts with a survey and a written quote, and we explain what is wrong in plain terms, with pictures from your own roof, before you commit to any work.