If there is spray foam under your roof, the first thing to establish is which kind it is. Open-cell and closed-cell foam are chemically related, and both are sprayed on as a liquid that expands and sets, but they behave very differently once they are on the rafters. They differ in density, in how they handle moisture, in how much they cost, in how they hide or reveal the timber behind them, and in how hard they are to remove. A surveyor, a mortgage lender and a roofer will all ask the question. This guide explains how to tell the two apart, what each does to a roof, and why the answer changes what you do next.
What the two foams are
Both are polyurethane foams made by mixing two liquids at the spray gun. The mixture reacts, expands and sets into a rigid or semi-rigid plastic full of tiny bubbles. The difference is in the bubbles. In open-cell foam the cell walls are broken, so the structure is more like a sponge, with air between connected cells. It is light and soft. In closed-cell foam the cells stay sealed, each one a tiny gas-filled capsule. It is dense, hard and stiffer.
That structural difference explains most of the rest. Open-cell foam is made to expand a great deal, filling a large depth from a small amount of liquid. Closed-cell foam expands less, sets harder and gives more insulation for the same thickness. The two materials are sold for different uses, and in a loft they are applied to the underside of the roof covering, between and over the rafters, to turn the roof space into a warm one. Our page on what spray foam insulation is gives the background.
How to tell which you have
You can often tell by looking and pressing, and a surveyor can confirm it. Open-cell foam is usually pale cream or off-white, the surface is fine-textured, and it feels soft, springy and slightly crumbly. A thumb pressed into it leaves a dent that may recover slowly. Pull a piece off and it tears or breaks into a spongy lump. Closed-cell foam is denser, often yellow or tan, hard to the touch, and it resists a thumb. It is stiff enough that you can feel it adding rigidity to the rafters and sheeting. If it is sprayed thick, it may have a slightly glossy skin.
The invoice or the guarantee should say which product was used, along with the thickness, the date and the installer. If the paperwork is missing, a roofer or surveyor can take a small sample and judge the type. The information matters for removal, for a mortgage valuation and for any claim against the installer. Our guide to what paperwork you should have for spray foam lists what to look for.
Moisture: where the types differ most
The main risk of spray foam under a roof is moisture that cannot escape. The RICS consumer guide of March 2023 notes that foam changes how a roof behaves, turning a ventilated cold structure into a warm one, and that a poor installation can produce condensation nobody controls. It also warns that wood may seem dry on the exposed face while the side hidden by foam is wet. Trading Standards guidance published in September 2025 adds that foam can reduce ventilation in the roof space, raising the risk of condensation, mould and potential timber rot.
The two foams contribute to this in different ways. Open-cell foam is vapour-open to a degree, meaning moisture vapour can pass through it, which in theory lets timbers dry. In practice, if the foam takes up water from a leak or condensation, it can hold it against the timber like a sponge, and the damp may go unnoticed. Closed-cell foam is far more resistant to water and vapour, so it keeps liquid water out of the foam itself. The risk is that any moisture already in the timber, or trapped at the edges and gaps where the foam does not bond, is sealed in with no way out.
Neither type is safe by nature, and neither is a failure by nature. Both can be installed well and both can go wrong. A well-installed foam on a sound roof covering, in a roof that has been designed as a warm roof, is a different case from foam sprayed over a leaking roof, over damp rafters, or over a covering that needs replacing. The underlying problems are discussed in our page on spray foam insulation problems.
Insulation performance and thickness
Closed-cell foam insulates better per centimetre, which is why it is used where depth is limited. A thinner layer can give the same performance as a thicker layer of open-cell foam. Open-cell foam needs more depth, and in a loft with shallow rafters this can mean filling the whole rafter depth and sometimes spraying over the rafters, which buries the timber completely. Burying the rafters is what makes inspection hard, because a surveyor can no longer see or probe the wood. Closed-cell foam, applied thinly, sometimes leaves the rafter faces partly visible.
Neither foam is a vapour control layer for the house in the way a good membrane is, and both rely on the installer getting the detailing right at the eaves, the gable ends, around pipes and at the ridge. Gaps in those places are where condensation tends to start.
Why one was chosen over the other
Understanding why a particular foam ended up in your loft helps you judge the installation. Open-cell foam was often offered because it is cheaper and fills a deep rafter space quickly, which suits an installer selling a fast, low-priced job. Closed-cell foam was offered where a thinner, stiffer, more water-resistant layer was wanted, or where the salesperson claimed it would strengthen the roof. Neither choice tells you whether the installer checked the roof first. The questions that matter are the ones that were often skipped: was the roof covering sound, were the rafters dry, was the cold-roof ventilation dealt with, and was a moisture reading taken before the spraying began.
Foam sprayed in a doorstep sale, with a quick visit and a promise of lower bills, is the case that causes the most trouble later. Our page on spray foam cold calls describes how to recognise it. It does not follow that every installation was bad, and plenty of owners have foam that has caused no problem. The aim of an assessment is to find out which group you are in.
Between the rafters or over them
Where the foam sits matters almost as much as what type it is. Foam sprayed between the rafters leaves their faces and sometimes their undersides exposed, so a surveyor can see and test some of the timber. Foam sprayed over the rafters, as a continuous blanket, hides the wood completely. This is more common with open-cell foam because of the depth needed to reach the target thickness. It gives a smooth finish and a good air seal, but it makes inspection by sight and probe impossible.
Foam can also be found stuck to the underside of the roof tiles or slates, to the felt, and over the wall plate and the top of the walls. In those places the foam sticks to the covering itself, which is why re-roofing a house with spray foam is a job with its own problems; see our page on re-roofing a house with spray foam on the underside. Where the foam has been applied over felt, the felt cannot be inspected or replaced without disturbing the foam.
Cost: install and removal
For orientation, a published guide gives typical UK ranges for installation of £20 to £35 per m² for open-cell foam and £40 to £60 per m² for closed-cell foam. We do not install new spray foam, so these are here to help you understand what was paid and what was bought. The interesting number for most readers is the cost of taking it out. Published guides give typical UK ranges for professional removal of £40 to £80 per m², with a standard three-bedroom loft at £2,000 to £4,500, and a lower estimate of £20 to £50 per m² from another guide. Removal costs more for thick, well-bonded closed-cell foam than for open-cell, because it is harder to cut and scrape away, and the surface has to be cleaned back to bare timber. The price you are given is fixed only after a survey, and our page on spray foam removal sets out the process.
| Point | Open-cell | Closed-cell |
|---|---|---|
| Structure | Broken cells, sponge-like | Sealed cells, dense and rigid |
| Feel | Soft, springy, crumbly | Hard, resists a thumb |
| Install cost (typical UK range) | £20 to £35 per m² | £40 to £60 per m² |
| Moisture behaviour | Vapour-open, can hold water like a sponge | Resists water and vapour, can seal in damp |
| Hides the rafters | Often, as it is sprayed deep | Sometimes less so |
| Removal | Easier to cut away | Harder, more labour |
What it means for surveys, mortgages and sale
Surveyors and lenders are concerned about what they cannot see. A number of lenders have refused to lend against houses carrying spray foam, though stances vary between lenders and change over time, so it is worth checking the current position at the time you need it. Lenders mainly want evidence that the foam was installed properly and that someone is accountable for it. An open-cell layer buried over the rafters raises more questions than a thin closed-cell layer with the timber partly visible, because the inspection protocol relies on being able to check the wood. Our guide to spray foam roof inspections explains what a surveyor looks for.
If you are selling, the type, the thickness, the date and the guarantee are the first things a buyer's side will ask for. Having them to hand makes the sale simpler.
What to do if you are unsure
Do not strip anything yourself. Foam dust and fragments can irritate, and cutting into foam can damage the roof underlay and timber. Photograph the underside of the roof, note whether there are signs of damp on the ceiling below, and find the paperwork. Then get an assessment from someone who understands roofs as well as insulation.
Our survey covers the roof covering from the outside, the underside from the loft, and where it is safe, a moisture check of accessible timber. We photograph what we find, say whether the foam looks open-cell or closed-cell, explain the options, and give a fixed price in writing for any work. If the foam can stay, we say so. If it needs to come out, we explain how, and the work carries our ten-year workmanship guarantee. See spray foam insulation assessment and removal in Basingstoke, or ring 01256 213956, or use WhatsApp.
