Why lenders balk
The House of Commons Library, which has tracked the issue for Parliament, puts spray foam in around 250,000 UK homes and records the core difficulty: when a mortgage valuer finds foam in the loft, they often cannot assess the condition of the roof timbers beneath it. Some lenders respond by declining to lend; others require specialist inspection or removal. RICS has published consumer guidance and an inspection methodology for exactly this situation, and the surveying profession’s default has shifted from ignoring loft foam to flagging it.
Note what that means: the mortgage problem is an information problem as much as a building one. A roof with well-applied, documented, appropriate foam can still fail a transaction because nothing about it can be verified from the loft hatch.
What the moisture research actually says
The government commissioned modelling on this, completed by the Health and Safety Executive as Building Safety Regulator and published in 2024: Moisture risk of spray foam insulation applied to timber sloped roofs. Its findings are more nuanced than the scare stories:
- The highest-risk scenario is foam sprayed directly onto the underside of the roof covering — tiles or slates with no underlay — which produced high risk in all modelled cases. Many older Oxford roofs, built before modern underlays, are exactly this construction.
- Risk depends on the combination: foam type (open-cell breathes; closed-cell does not), the underlay behind it, and climate. Open-cell foam applied in line with BS 5250 (the British Standard for moisture control) modelled as low risk in several scenarios.
- The mechanism of concern with the wrong combination is moisture reaching the rafters and being unable to dry — slow, hidden decay in the timbers that hold the roof up.
So: not all foam, not all roofs — but old roofs with no underlay, sprayed with closed-cell foam, are the worst pairing, and they are common, because that roof type is precisely what cold-call installers found in older housing stock.
The honest assessment route
If your roof has spray foam — or you are buying or selling a house that does — the sequence that protects you is:
- Identify what is actually there. Open-cell or closed-cell, thickness, coverage, and critically what sits between foam and tiles. Installation paperwork helps; most households have none.
- Inspect the timbers where possible. Moisture readings and borescope checks at accessible points establish whether the rafters are dry or the decay concern is live.
- Decide on evidence, not fear. Sound, documented, appropriate foam may only need a written assessment for a lender. Foam on bare tiles, damp readings, or an imminent sale usually points to removal.
- Remove properly where removal is the answer. Delpol removes spray foam: cutting and stripping the foam clear of the rafters, checking every timber as it is exposed, and leaving a roof a surveyor can actually survey. Removal is exacting, dusty work — the value is in what is verifiable afterwards.
- Replace the performance you take out. Removal without reinsulation just restores a cold roof.
What we install instead
Our default for older pitched roofs is vapour-open, inspectable insulation. On a 1920s detached house in the Iffley conservation area we re-roofed around 160 m² with wood-fibre insulation between and over the rafters — 100 mm plus 40 mm — over an airtightness membrane, with counter-battens, a vapour-permeable underlay and a ventilated dry-ridge system moving air from eaves to ridge. Design performance improved from an estimated U-value of 1.2 W/m²K to a target of 0.2–0.3 — a five-fold improvement, achieved with materials that buffer moisture rather than trap it, on a roof whose timbers remain visible from the loft.
Where the loft is unused, the better answer is simpler still: insulate at joist level — 300 mm of mineral wool with the eaves ventilation kept clear — and leave the rafters cold, dry and inspectable. See our guide to diagnosing cold rooms for how roof-slope insulation fits the wider picture.
The short version
Spray foam’s problems are real but specific: unverifiable installs, and moisture risk from the wrong foam on the wrong roof. If you have it, get it assessed before a lender does. If it has to go, remove it properly and put breathable performance back. We do all three — book a free survey and we will tell you which of the three your roof actually needs.