Spray foam insulation is a liquid that is sprayed onto a surface, where it swells and sets. Two families are used under roofs. Open-cell foam sets soft and spongy, full of tiny cells that are broken open to the air. Closed-cell foam sets hard, with each cell sealed shut. From the loft floor they can look alike, a pale mass stuck to the underside of the roof, yet they respond to damp in opposite ways, and they come off timber in opposite ways too.
This guide explains the two types, what each does with moisture, why removal is easier or harder depending on the type, and how to work out which one is above your head. It sits under our spray foam insulation removal service, where the wider picture of assessment, removal and re-insulating is set out. Nothing here says foam must come out of every roof, and the type alone never settles that question.
Two foams, two structures
The difference starts with the cells. Foam is mostly gas trapped in a plastic matrix. In open-cell foam the cell walls are incomplete, so air and vapour move through the material. That is why it feels like a soft sponge and can be pressed with a thumb. In closed-cell foam the walls are intact, so each pocket of gas is sealed and the whole mass is firm, closer to a hard board than a sponge.
Installers choose between them for different reasons. Soft foam is generally sprayed thick to fill a cavity and is often the lighter, cheaper product to put on. Rigid foam bonds hard and adds some stiffness to whatever it is stuck to, which is one reason it is chosen for places where an installer wants a continuous, airtight skin. We do not publish a price for either, and the national cost guides we use do not split their figures by type.
Where it was sprayed matters as much as which foam
A roof can carry foam on the underside of the rafters, between them, over the roof boards or directly onto the back of the tiles or slates. Soft foam between rafters behaves differently from rigid foam sprayed across every timber and the gaps beside it. Some roofs hold a mix of both, laid in layers or sprayed by different installers at different times, so we never assume that one test tells the whole story. A bathroom extract that ends in the loft, a chimney breast that runs through the foam or a dormer cheek that has been sprayed all create local conditions, and each deserves its own look.
Every statement in this guide describes general behaviour. Products vary by manufacturer and thickness, and your installer's paperwork, where it exists, is better evidence than anything we can say in general terms.
How each type handles moisture
Moisture reaches a roof in two ways. Liquid water finds its way in where a tile has slipped, a flashing has lifted or a valley has split. Water vapour comes up from the rooms below, carried in warm air from kitchens, bathrooms and breathing. Warm, moist air that reaches a cold surface drops its water there as condensation. RICS warns that foam fitted wrongly can bring damp problems, with uncontrolled condensation among them, and harm to other parts of the building.
Open-cell foam
Because its cells are open, soft foam lets vapour pass and can take on liquid water the way a sponge does. In a roof, that cuts both ways. A small leak may soak into the foam rather than showing as a drip, so the first sign is a stain or a sour smell, not a puddle. Meanwhile the foam holding that water sits right against the timber. How long it stays there depends on how thick the foam is and whether the roof above can dry it.
Closed-cell foam
Rigid foam largely stops water and vapour passing through it. That helps keep warm air from reaching cold surfaces from below. The catch is that anything which does get behind it, from a leak above or from damp already in the timber, has little way out. The foam does not soak up the water, but it does seal the wood against drying.
Neither type is automatically the safer choice for the timbers. What decides the outcome is whether the roof was dry when it was sprayed, whether it has stayed dry since, and whether moisture has anywhere to go.
What the difference means for the rafters and boards
A traditional pitched roof timber lives in moving air. Gaps between slates, tiles and underlay, along with vents at the eaves, let the wood dry out after a wet spell. Spray foam across the underside takes the timber out of that airflow. In the open-cell case, the wood is held against a material that may stay damp. In the closed-cell case, the wood is shut behind a barrier that slows any drying at all.
This is why the RICS consumer guide asks for thermal calculations and condensation risk checks, carried out by a qualified professional, before foam goes in. Where those checks were skipped, the question to ask afterwards is simply whether the timbers underneath are sound. We cannot answer that from the loft hatch. Our guide to inspecting roof timbers under spray foam describes how sample openings and moisture readings answer it.
Covering above the foam
The roof covering changes the picture. Stone, clay and slate coverings were designed for old roofs that breathe, and they let some moisture escape through their joints. Foam sprayed to the back of them can block that route. A modern roof with a breathable membrane and a ventilated batten space behaves differently again. This is the reason a surveyor at one house flags the foam while a surveyor at the next barely mentions it.
Two houses with identical foam can therefore carry quite different levels of concern, which is why a written assessment, not a general rule, decides what happens next. A roof that was re-covered shortly before spraying, with new underlay and sound battens, starts from a better place than one sprayed over old, damp boards. Age counts too: foam that has had years to shrink away from the timber can leave gaps where warm air finds its way in.
Removal difficulty: soft against rigid
Foam that was easy to spray is not always easy to remove. Taking it off a roof means separating it from wood without gouging the wood, because the aim is a rafter that can be looked at properly and then insulated again.
Taking off open-cell foam
Soft foam tends to lift off the wood in clumps. A blade frees it, hands pull it away, and a scraper takes the rest, while a vacuum collects the crumbs as the work moves along. It raises less dust than the rigid kind, but it is bulky, and a thick layer fills a surprising number of waste bags. A stiff brush deals with the thin film that stays behind.
Taking off closed-cell foam
Rigid foam grips the timber and the roof boards firmly. The usual method is to shave and chip it away in fragments, one rafter after another, then clean what is left of it from the surface. It takes longer, it makes more dust and debris, and it tests the patience of whoever is holding the scraper. That extra effort lands in the labour cost, not on any rate we publish.
For a sense of scale, MyBuilder's 2026 spray foam removal guide gives £20 to £50 per m², averaging about £1,500, with an assessment at £100 to £200 and disposal at £150 to £400. Those are national figures that do not separate the two types. Our spray foam removal cost page sets them out in full.
Telling which foam you have
You can learn a lot without touching anything, and you should not cut into foam on a hunch. Dust from some products is unpleasant to breathe, and a damaged foam surface tells you nothing the cut face would not.
What you can see from the hatch
Soft foam looks fluffy or ragged, with a rough, almost bread-like surface, and it is often thicker. Rigid foam looks smoother and denser, sometimes with a skin, and it can look more like a hard plaster coat. A loft that has been sprayed in layers may show both. Colour is a weak guide, since pigments vary and foam darkens with age.
What the paperwork says
The installer's invoice, data sheet or guarantee should name the product. Look also for any certificate or condensation calculation. If your surveyor flagged the foam, the report may already say which type they saw. The guide on what to do when a surveyor flags spray foam covers that first conversation.
Our part of the survey
We look at the foam from several places in the loft, photograph it, and note how it sits against the rafters, the boards and the wall plate. A short test cut at the edge, done carefully, shows the structure of the cells. Our pictures of the loft are attached to the assessment, which means you see the same view we did.
Does the type decide whether foam comes out?
No. RICS does not say every installation must be removed, and we would not say it either. Plenty of foam sits happily on dry, sound timber for years. What matters is the evidence from your own roof: the readings, the condition of the wood, the age of the foam and the condition of the covering above.
The foam type shapes the plan in three ways. First, it changes how hard the inspection is, since soft foam can be opened with a blade while rigid foam needs more effort. Second, it changes the removal method and therefore the programme and the waste. Third, it affects which options are worth considering, which brings in the idea of taking off only part of it. Our answer on whether spray foam can be partly removed deals with that one choice.
Lenders and the type
Lending attitudes are moving. Today's Conveyancer, writing in November 2024 about a BBC investigation, named several lenders that decline such properties and several that decide each application on its merits. That was a snapshot of policy at one date, not a verdict on either type of foam. Ask your own lender what applies today.
Once any foam is off, the next question is what goes back, and our guide to re-insulating after foam removal covers the options. Afterwards the quote covers scaffold access, foam removal, cleaning, waste and making good.