Can spray foam cause roof timber rot?
Yes — particularly closed-cell foam, which can trap moisture against rafters and accelerate decay.
The mechanism is straightforward: warm internal air carries moisture into the roof structure where, without ventilation, it condenses and lingers against the timbers. Over time this leads to wet rot, fungal growth and weakening of the structural members.
Removal often reveals timbers that look sound but are damp; treatment is sometimes required afterwards.
Frequently asked questions
Is rot guaranteed under spray foam?＋
No — many lofts come up clean. But the risk is materially higher than with traditional insulation.