Spray Foam Insulation: Pros & Cons

Spray foam is aggressively marketed and, applied to the underside of a pitched roof, it is the single most expensive mistake a homeowner can make to a property they intend to sell. This is the honest version: what is actually documented, what is myth, and what it costs to undo.

What is spray foam insulation?

Spray foam is a liquid polyurethane insulation sprayed in place, where it expands and cures. It comes in two forms and the difference matters. Open cell is lighter, softer and vapour permeable, so moisture can pass through it. Closed cell is denser, rigid, better performing per millimetre and far less permeable. Closed cell is frequently sold as a "vapour barrier", but on UK certification it is a vapour retarder, not a vapour control layer, which is a meaningful distinction when a designer is relying on it.

Sprayed onto the underside of roof tiles, felt or sarking, either type bonds hard to the rafters, battens and felt. That single property causes everything that follows.

Does it even meet current standards?

Often not, which surprises people who paid a premium for it. The BBA certificate for a widely installed open cell system prints its achievable wall U-values in a table, and against 0.18 and 0.19 W/m²K it prints a dash: the product cannot reach those figures at any thickness in that build-up. It reaches 0.26 W/m²K at 140mm plus 20mm between battens, and 0.28 at 140mm plus 10mm. Those are respectable numbers, but they are not modern Part L numbers.

There is a fire point too. Both open and closed cell products are Euroclass E for reaction to fire, and the certificate requires a fire-resistant lining board over the foam in all applications. In a loft sprayed between the rafters and left exposed, that lining board is almost never there.

The mortgage and survey problem

This is the part that ruins sales. Because a surveyor cannot see the roof timbers through the foam, they cannot certify the structure, and lenders price that uncertainty as risk. Reporting by the BBC in November 2024, summarised in a House of Commons Library briefing, found that around a quarter of the largest lenders would not lend on a property with spray foam in the roof, and every equity release provider surveyed declined outright. Others will lend only after a specialist inspection report, which you pay for and which may not come back clean.

The scale is genuinely disputed. The figure of 250,000 affected homes that circulates widely originates with the Residential Property Surveyors Association and is repeated by the Property Care Association and the HomeOwners Alliance. No government department has published an estimate, so treat it as an industry figure rather than an official one.

What is properly evidenced is the defect rate. A joint letter from the Property Care Association and HomeOwners Alliance in May 2025, cited in the Commons Library briefing, reported that across more than 500 inspections, 35% of properties had one or more defects caused by the foam. Note what that does not say: no study quantifies how many foamed roofs are rotting, and anyone quoting such a percentage has invented it. The accurate and more useful statement is that the foam removes your ability to know.

Why it damages roofs, and the compliance failure behind it

A cold pitched roof works by being ventilated: air moves through the void, carrying away the moisture that rises out of the house. Spray the underside of the roof and you convert it to an unventilated construction without designing it as one. Moisture that used to escape now condenses where the warm side meets the cold, which is against the timbers you can no longer see.

The certificate anticipates this. It requires a condensation risk assessment to BS EN ISO 13788 and a vapour control layer. That is a condition of the certification, not a best-practice suggestion, and its near-universal absence on domestic installations is the compliance failure at the heart of the whole affair. If your installer did not carry out a condensation risk assessment, the product was not installed to its own certificate.

What removal costs

Around £3,200, or roughly £40 per m², for a three-bed detached house, according to Checkatrade figures cited in the Commons Library briefing. That is the reasonable case. Because the foam bonds to the timbers, felt and battens, removal is slow manual scraping, and it can damage the felt so badly that the roof needs recovering afterwards.

The BBA certificate itself flags this, warning that the foam "forms a strong bond" which "should be taken into account when anticipating future alterations". The bad cases are severe: the HomeOwners Alliance reported a homeowner quoted £17,000 to remove foam that had cost £12,000 to install. Removal can cost more per square metre than installation did, and there is no help with it. A written parliamentary answer in March 2025 confirmed no financial assistance is available, and a further answer in June 2025 said only that the government was engaging with lenders.

Enforcement has been thin. The only formal action to date is two Advertising Standards Authority rulings, against installers in April and November 2023. Both are worth reading for what was not upheld: in each case, the complaint that adverts should warn buyers about mortgage difficulty was rejected. Nobody is obliged to warn you. See our roof replacement cost guide if the felt goes with the foam.

Where spray foam belongs, and what to use instead

Spray foam is not a fraud, it is a misapplied product. Closed cell foam in suspended timber ground floors, and in properly designed warm roof build-ups, is legitimate: it is certified for those uses, it performs, and it does not conceal the structure a surveyor needs to see. What it needs is a designed ventilation and vapour strategy, a condensation risk assessment, the specified lining where required, and building control sign-off.

For a standard cold loft, use the boring options, because they work and nobody will refuse to lend on them:

• Mineral wool loft roll laid between and across the joists, which leaves every timber visible and costs a fraction as much.
• Rigid insulation boards between and under rafters where you are converting the loft into a room and need the height.
• Cavity or external wall insulation for walls, and see damp causes and fixes if moisture is the underlying reason you are insulating.

Our recommendation is unambiguous: do not put spray foam in a pitched roof you may ever want to sell, remortgage or release equity against. The insulation performance is real, the risk is asymmetric, and mineral wool gets you most of the benefit with none of the exposure. If a property you are buying already has it, price an independent inspection and a possible removal into your offer before you exchange.

We plan insulation into our renovations so it protects both comfort and resale value, across London, including home renovation in London. For independent advice, contact us or call 07472 424 226.

For how a job like this is contracted and staged, see what a building contractor actually covers.