Is spray foam insulation bad? The real risks, explained
Spray foam has a bad reputation, but the official position is more balanced. Government guidance says spray foam "can be used effectively in many settings" but isn't always the right product for some homes. Whether yours is a problem depends on how and where it was installed.
The main risks
Trapped moisture and timber decay
Most pitched roofs are designed to be ventilated. RICS explains that spray foam on the underside of the roof turns a ventilated 'cold roof' into a 'warm roof', sealing out draughts but also potentially sealing in moisture unless the right precautions are taken. Incorrectly installed insulation can lead to uncontrolled condensation and damp.
Hidden problems
RICS notes that when timbers are covered in foam they can't be fully seen or inspected, so a roof leak behind the foam may go unnoticed until timber has started to rot. This is the main reason surveyors and lenders get nervous – they simply can't confirm the roof is sound.
Fire
RICS says polyurethane spray foam is combustible, typically rated Class E under BS EN 13501-1, and that in a house fire it can increase the likelihood and extent of damage to your home and adjoining properties. RICS advises checking with your insurer about cover.
It may not save as much energy as claimed
RICS points out that topping up mineral wool at ceiling level to current standards can outperform the typical maximum of about 100mm of spray foam between rafters, and is likely to be substantially cheaper. In large roof spaces such as bungalows, heating the whole roof void may not be cost effective.
What the government-published research found
In 2024 the Building Safety Regulator (part of HSE) published computer modelling of moisture risk when spray foam is added to timber pitched roofs. In summary:
| How the foam was applied | Modelled risk of timber decay |
|---|---|
| Open-cell foam installed to the BS 5250:2021 prescriptive designs (vapour control layer plus a gap behind the foam) | Low |
| Applied directly to a low-resistance (breathable) underlay | Low in a London climate; medium in Newcastle in most scenarios |
| Applied to a high-resistance underlay (e.g. traditional bitumen felt) | High in London for open-cell foam with no vapour control layer; lower with closed-cell foam or a vapour control layer |
| Sprayed directly onto tiles or slates | High in every scenario modelled |
Simplified from the report's executive summary. Modelling is theoretical and not a judgement on any individual home.
That matches the industry inspection protocol, which treats foam sprayed directly onto tiles or slates with no underlay differently from foam applied to a membrane – it can mean reactive tile repairs are no longer possible, and re-roofing or future removal may be recommended.
When it may be fine to leave it
The Insulation Manufacturers Association says that when spray foam is installed in line with the manufacturer's instructions there should be no need to remove it, and warns about unscrupulous firms frightening people into unnecessary, expensive removal. Government guidance says only specialist surveyors experienced in spray foam can tell you if it should be removed. If a competent surveyor confirms your installation meets the standards and manufacturer guidance, it's likely to be considered low risk with no further action needed.
Signs it's worth getting checked
- Foam sprayed straight onto the back of tiles or slates, with no felt or membrane
- Damp patches, staining, mould or a musty smell in the loft or rooms below
- Foam that's cracked, flaking, coming away, or covering cables and pipes
- No installation paperwork, or an installer who's no longer trading
- A sale, remortgage or equity release coming up
Next step: read what an independent spray foam survey checks.
Related guides
- Spray foam & mortgages
- Removal costs
- Selling a house with spray foam
- Spray foam & equity release
- Spray foam surveys explained
Sources
- GOV.UK – Spray foam insulation: remediation, removal and lending (22 Oct 2026)
- RICS – Spray foam insulation consumer guide
- HSE / Building Safety Regulator – Spray foam insulation applied to timber sloped roofs in dwellings: modelling of moisture risk (Feb 2024)
- PCA/RPSA – Sprayed Foam Insulation Inspection Protocol (March 2023)
- Insulation Manufacturers Association – A guide to spray and injected foam insulation (Oct 2024)
- PCA, IMA, RPSA et al. – Spray Foam Customer Guidance (2025)
Sources checked 29 October 2026. Guidance changes – always confirm with your lender and a qualified surveyor.