This is the question asked most often after a leak, and the answer people least like to hear. Insulation that has got wet almost never dries out completely in a roof build-up. Not because the material holds on to the water, but because in a closed structure there is no airflow to carry the moisture away.
And wet insulation material no longer does what it is there for. Water conducts heat about twenty times better than air, so as soon as the air in an insulation layer has been replaced by water, the layer barely insulates any more. You are then heating through a roof that is insulated on paper.
What is still possible per material
Mineral wool, glass wool and stone wool. Slumps when it gets wet and does not regain its shape. Slumped wool leaves an air gap above it, so even if it dries, the insulation value is gone. With soaked wool, replacement is the only serious option.
Rigid plastic boards such as PIR and EPS. Absorb little water themselves. If the water was between the boards and on the underlay, the layer can sometimes stay after drying. If the water is in the joints or under the boards on a saturated substrate, it has to come out.
Wood fibre and other bio-based insulation. Can buffer a limited amount of moisture and release it again, but only in a build-up that can dry out. When soaked, the risk of mould and decay is too great.
Insulation in a flat roof. Sits between vapour-tight layers and can hardly dry at all. If the roof feels spongy when you walk on it, there is water in the layer and sealing a seam will not help. See
why a flat roof leaks.
“A repair over wet insulation locks the water in instead of out. That is the most expensive way of sealing a roof.”
Fouad, owner of Astrodak Service
How to tell whether the insulation is wet
On a pitched roof insulated from the inside, it is often visible: stains on the plasterboard, a sagging ceiling, or insulation you can see and feel when you take off a board. If the attic smells musty while the roof slope is closed, that is also a sign.
On a flat roof it is less visible and you are more likely to feel it: the roof gives when you walk on it and leaves an imprint. Blisters can often be seen on the top layer. In doubtful cases, a test hole is made or moisture is measured, because the difference determines whether you need a repair or a new build-up.
Mineral wool wet
replace
insulation value gone
Rigid board damp
sometimes salvageable
depending on where
Not sure about the right approach?
I would rather come and look at your roof with my colleagues first than come up with a solution from a distance. During the free inspection you hear what is really needed, even if that is less than you thought.
The consequences if you leave it
Higher heating costs. You pay for insulation that does not work, every month, without seeing it.
Decay of the timber. Roof boarding, rafters and wall plates in contact with wet material stay damp and eventually go soft. See
replacing roof boarding.
The good news
If the roof has to be opened anyway to replace the insulation, that is also the moment to do it better than before. A thicker layer, a continuous layer without cold bridges, a vapour control layer that is neatly sealed at the junctions. Often that results in a roof that is a class better in energy terms than before the leak.
How to establish how far the moisture has got
On a pitched roof insulated from the inside, part of the finish is removed at the suspect spot and the insulation is assessed. If it feels and smells damp, we check how far it extends by opening up further in a few places.
On a flat roof the same is done with a test hole or a moisture measurement. That is an hour's work and it determines whether a repair will do or whether a new layer has to go in. That outcome is a considerable difference, and it is not a question to answer on gut feeling.
Frequently asked questions about wet insulation
