Anyone who has spray foam under their roof will be asked, sooner or later, which kind it is. A surveyor asks, a lender may ask, and so does anyone pricing its removal. The question matters because the two families of foam are not interchangeable. One is soft, light and open to the passage of water vapour. The other is hard, heavy and far more resistant to it. Both go on wet and swell as they cure, which is why they are easy to confuse once they are in place. The practical consequences for a roof are real, though. They touch how the timbers beneath are likely to behave, how a leak might show itself, how the foam comes off and what a buyer's lender makes of it. This guide explains the difference without trade jargon and keeps its focus on roof timbers. It is general information, and the only way to know what is in your own roof is to have it looked at. Our spray foam insulation service page explains how we do that.
What the two types are
Spray foam is made on site by mixing two liquid components that react, expand and harden into a cellular plastic. The word cell describes the tiny pockets of gas or air the process traps. The difference between the two types lies in what happens to the walls of those pockets.
In open-cell foam the walls of the pockets are broken or incomplete, so the structure is a bit like a firm sponge. It is light, spongy to the touch and takes up a good deal of space as it expands. Because the pockets connect, air and water vapour can move through it.
In closed-cell foam each pocket is sealed, so the structure is more like a mass of tiny balloons set in a hard shell. It is heavier, stiff and difficult to dent with a thumb. Because the pockets do not connect, it blocks the passage of vapour far more effectively, and it adds some stiffness to whatever it is stuck to.
We keep this description qualitative on purpose. Products vary between makers, and figures quoted for one product do not carry over to another. If you need a precise performance figure for a named product, ask the manufacturer or read the installer's data sheet.
How to tell which you have
It is harder than it sounds. Colour is not a reliable guide, since both types are available in several shades and some fade or darken with age. Even so, a few cues help.
Press gently on a patch that is safe to reach. Open-cell foam gives under a fingertip and springs back slowly, and it may crumble if scratched. Closed-cell foam resists, feels almost like a hard board and may ring a little if tapped. The surface also tells you something. Open-cell often looks fine-grained and slightly furry, where closed-cell tends to look smoother and more like an orange peel.
Thickness gives a further hint, because open-cell is usually applied deeply to reach the same job, and it can bulge out from between the rafters. Closed-cell is often laid as a thinner coat.
None of this should be taken as certain, and the cues can mislead where several coats of different foam have been sprayed over time. The surest evidence is the installer's invoice or data sheet. If that is missing, a trained eye looking at a small, safely exposed sample is the next best route. Our guide on identifying your spray foam type goes through the steps in more detail.
What each type does to roof timbers
Timber in a roof is not meant to stay wet. Through the year it absorbs moisture and releases it again, and in a traditionally built roof the air moving through the space helps it dry. A layer of foam changes that exchange, and the two types change it differently.
Open-cell against timber
Open-cell foam lets vapour pass, which can seem a benefit, since damp air is not shut in. The risk lies in what it does with liquid water. If a slate slips or a flashing lets in rain, the water can soak into the foam like a sponge and sit against the wood, out of sight and slow to dry. A small leak that would have shown as a stain on a ceiling may stay hidden in the foam for a long time.
Closed-cell against timber
Closed-cell foam keeps liquid out and also blocks vapour, so the timber it is stuck to has little chance to dry in that direction. If moisture is already in the wood, or arrives from the outside through a defect, it can be held there. The foam also bonds firmly to the rafters, so a problem behind it is hard to see and hard to reach.
Neither outcome is certain. Plenty of foamed roofs have sound timber, because they were dry when sprayed and the covering outside is in good order. The point is that foam makes the condition of the roof harder to check, and the type decides which kind of problem is more likely to go unseen. The detail is in our guide on spray foam and roof timbers.
Leaks, condensation and what the foam hides
A roof leak is usually found by its trail: a brown ring on a ceiling, a damp patch on the loft floor, a drip on the hatch. Foam can break that trail. If the water is soaked up by open-cell foam, the first sign may be foam that is heavy, discoloured or slumping. If it is held out by closed-cell foam, the water may run along the surface until it finds a gap and appear far from its source.
Condensation is a different problem with a similar effect. Warm, humid air finds its way up out of the living space, and where it meets a cold surface it turns to water. A foamed roof is meant to stop that air reaching cold timber, but a gap in the foam, a poorly sealed hatch or a bathroom fan that vents into the loft can send it where it should not go.
The covering outside plays its part too. Slate nails that corrode over time can let slates slip, and older roofs often have joints and flashings in lead that need attention. Where these let in water, the foam can hold it against the timber. That is why our survey covers the full roof from outside as well as the loft. A foam inspection that ignores the slates, the valleys and the chimney flashings is only half a survey.
How the type changes removal
When foam has to come out, the type sets the method and the pace. Open-cell foam is soft enough to be cut and torn away in sections, and much of it can be pulled free of the timber. The remaining film is then scraped or brushed off. Closed-cell foam is hard and well stuck, so it is usually removed in thin layers with scrapers, chisels and other hand tools, and some of it stays on the wood as a residue that needs patient finishing.
The slower method costs more in labour, and that is one reason the price per square metre in the published ranges is so wide. Typical UK figures for removal are £20 to £50 per m², as reported by MyBuilder's guide (2026), and our page on spray foam removal cost shows how those figures turn into a total. Time on site follows the same logic, and we deal with it in our guide to how long removal takes.
For the methods themselves, the mess and what is left on the timbers afterwards, read the guide on the removal method, step by step. The method for your own roof is settled at the visit, and the written price names it.
How lenders and surveyors treat the type
Foam type often appears on a surveyor's list of questions. A report that names the type, the area and the depth is more useful than one that says only that foam is present. Lenders who accept foam on a case-by-case basis are likely to ask about it, and those who decline it generally do so whichever type it is.
A RICS consumer guide dating from March 2023 suggests that owners ask their lender, ahead of installing foam, whether its policy allows it. Lender positions move, and we avoid describing any as current. Our guide on spray foam and mortgages sets out the reasoning behind the caution and the roof evidence that tends to help.
If you are in the middle of a sale and need a report fast, start with the facts that can be gathered easily: the installer's paperwork, a note of where the foam is and how thick, and a photographic record of the loft. A timber inspection can follow where the foam is opened in a few places. You get pictures taken on your roof and a written account of what was found, and the lender decides how to use them.
Choosing what to do next
Knowing the type does not decide the outcome, but it narrows the choices. Where the foam is dry, well installed and over a sound roof, leaving it alone can be reasonable, and an inspection can confirm that. Where there are signs of damp, a slipped covering or a lender who will not proceed, stripping some or all of it may be the route.
It helps to treat the decision as a series of questions. What type is it, and how thick? What are the timbers like where they can be seen? Is the roof outside in good order? Does a lender or buyer need something specific? The answers point towards one of three outcomes: keep it and gather evidence, strip a section, or strip the lot.
We start with a survey. The photographs you receive are of your own slopes and loft, showing where the foam meets the rafters, whether the timber can be seen at all and any signs of damp or condensation behind it. If a repair or an inspection will do, that is what we recommend. Where the foam has to go, we describe the work step by step before it starts, and we leave the site clear and take the waste away.
It is also worth asking what happens afterwards. A roof stripped of foam still needs insulation, and the choice of material and ventilation should be settled before the first scraper goes in, so the loft is not left bare for longer than it has to be. The same goes for any repairs the timbers turn out to need: agree how they will be handled and priced while the roof is still closed, not after it is open.