Is Spray Foam Insulation Bad for Your Roof?

Why the answer is not yes or no

Spray foam has been fitted to many thousands of roofs, and many of them show no sign of trouble. The product works as an insulator, and when it is applied to a dry, well-built roof in the right way, it can last. The concern is that it narrows the margin for error. A cold loft with conventional insulation forgives small faults: a slipped tile leaks, the drip is spotted, the wet area dries. Under foam, the same leak soaks the rafter, the foam holds the water in, and nobody sees it for years.

This answer gives the verdict only; the causes, symptoms and checks for each fault are in our guide to spray foam insulation problems.

So the question to ask is not whether foam is bad but whether your roof is in the condition to take it. Age of the covering, state of the felt, ventilation history and humidity inside the house all feed into the answer. A properly designed warm roof, planned from the outset, is a different thing from foam added later to a roof designed to breathe.

What foam changes about a roof

The RICS consumer guide explains that spray foam converts a cold roof into a warm roof. The insulation moves to the slope, the roof space is no longer ventilated, and the structure sits on the warm side of the insulation, or partly on the cold side if the foam is thin. That is fine if the build-up was designed for it. If it was not, condensation can form where warm air meets a cold surface, as our page on whether spray foam causes damp explains.

It also affects inspection. Rafters, battens and the underside of the covering are covered, so a surveyor cannot see them. The RICS guide notes that timbers can look dry on the visible face but be wet behind the foam. This is why some surveyors flag foam and some lenders have declined mortgages on homes with it, though positions vary and change. The consequences are covered on our page about spray foam and mortgages.

When foam does least harm

Foam is least risky where the roof is dry and was dry when the foam went on, the covering is in sound condition, the ventilation of the house is good, humid air is not leaking into the roof space, and the installer used a suitable product at a sensible thickness with proper detailing at the eaves, flues and hatches. Open-cell foam is more vapour permeable than closed cell, which matters in some roofs, as our comparison of open-cell and closed-cell foam explains.

Foam does most harm where it was sprayed over wet or already decayed timber, applied to a roof with an unrepaired leak, used on a roof covering that needs replacing, or fitted without regard to chimneys, flues and ventilation. Where a tile is loose or a gap lets rain in, foam makes the damage worse by hiding it.

How to judge your own roof

Look for the signs: a musty smell in the loft or on the landing, mould on the foam face, ceiling stains, condensation on the hatch, slipped tiles or flashings outside, and rust on the nail tips. Check your paperwork for the product, thickness and guarantee. If the foam is in good order and you have none of these signs, the roof may be doing its job. It is still sensible to have it looked at before selling.

If you want a proper opinion, we survey for free, photograph the tiles, flashings, foam and any exposed timber, and read the timber with a meter where we can. We tell you plainly whether the foam can stay. We see the roof as roofers, and the result is a recommendation, not a sales pitch. If the roof is fine, we leave it alone.

Age of the roof is a factor many owners overlook. A covering with only a few seasons left will need work soon, and any repair under foam is more awkward than in an open loft. If you know the tiles, slates or felt will need attention within a few years, it is often wiser to deal with the foam first, since removing it later around a half-finished roof costs more. We take that into account when advising on whether the foam can stay.

Mixed advice is common, and it helps to ask whoever recommends keeping the foam what evidence they have. A moisture reading and a photograph of the timber behind the foam is evidence. A reassurance on the phone is not.

If the answer is that it has to go

Removal is a straightforward process, but one that needs timber repair and re-insulation planned alongside, as described on our spray foam removal page. Typical UK ranges reported for removal are £40 to £80 per square metre, and a fixed price follows the survey. You can find the wider service on our spray foam page.