Does Spray Foam Insulation Cause Timber Rot in UK Roofs?
Why the UK climate is the real culprit
The British weather pushes warm humid air upward through the home all year round, and roof voids in older UK housing stock rely on continuous low-level ventilation to flush that moisture out. When sprayed foam closes off both the airflow and the visible timber, you create the exact conditions a wood-destroying fungus needs: damp, dark, and undisturbed.
How decay starts under closed-cell foam
Closed-cell polyurethane has a vapour resistance high enough to behave as an internal vapour barrier. Moisture from below condenses on the rafter face that is now sandwiched between insulation and felt. Without airflow, that moisture sits in the timber. Over five to fifteen years, surface fibres soften, fungal hyphae spread, and structural strength reduces.
Open-cell behaves differently but is not innocent
Open-cell foam allows some vapour through, which is why some installers still defend it. In practice open-cell installations frequently bridge eaves vents and trap dust and debris that hold moisture against the timber. Decay under open-cell is less common than under closed-cell, but it is by no means rare.
What we find on real UK strip-outs
Across our network, roughly one in three closed-cell removals reveals at least localised decay on the rafter underside. About one in ten requires sister-rafter replacement or borate-based timber treatment. Open-cell removals reveal localised damage in roughly one in eight cases. The remainder show stained but structurally sound timber.
Visible warning signs from the loft hatch
Before removal, look for dark linear staining following the rafter line, a musty 'mushroom' smell when the hatch opens, soft or spongy timber where it meets the wall plate, blocked eaves, and any sagging between rafters. Any one of these warrants a professional inspection.
Why DIY moisture testing is unreliable
Pin moisture meters only read a few millimetres into the surface. Foam-coated rafters can be wet on the unseen face while reading dry on the loft side. Resistance probe meters used by surveyors penetrate further but still cannot replace removal and visual inspection.
What treatment actually involves
Where decay is localised, a borate-based fungicidal treatment is brushed or sprayed onto the affected timber after the foam is stripped. For more advanced damage, a 'sister' rafter is bolted alongside the original to restore load capacity. Full rafter replacement is uncommon and only required where the original timber has lost cross-section.
Will the lender still lend if there is decay?
Localised treated decay, properly documented, is acceptable to most UK lenders. Significant untreated decay is not. The removal contractor's inspection report should describe the timber condition clearly and confirm any treatment applied. That paperwork is what underwriters rely on.
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