Condensation is water that was already in the air of your house. Cooking, washing, drying clothes and simply breathing all add vapour, and warm air holds a good deal more vapour than cold air. When that air meets a surface cold enough, the vapour turns back into liquid. In a traditional loft this happens, in small amounts, on the cold face under the roof covering, and the ventilated space lets it dry out again. Spray foam changes the arrangement. It is sprayed onto the slope itself, often over rafters, battens and the back of the slates, and it can close that ventilated space off completely. The mechanism is simple enough once it is laid out: the moisture has a source you can trace, wood is the part that suffers, ventilation can do only so much once foam is in place, and a roof that has already been sprayed needs checking in a few specific places when we are asked to look at it.
Where the moisture in a roof space comes from
A house is a steady source of water vapour. A kitchen in use, a shower, a tumble dryer vented badly, even a houseful of people overnight: each adds moisture to the air, and warm air rises. It leaks upwards through gaps round the loft hatch, light fittings, pipes passing through the ceiling and the top of internal walls. It reaches the roof space carrying that vapour with it.
In an ordinary cold loft, with insulation laid on the floor, the roof space stays close to the outside temperature. The slates or tiles are cold, the underlay is cold, and the timbers are cold. A little vapour that gets through will condense lightly and then be carried away, because outside air moves through the eaves and, on many roofs, through the ridge or the gaps in the covering itself. The roof is built to breathe. That is not a defect. It is how a pitched roof has always coped with the moisture that rises into it.
The size of the problem depends on how much vapour the house makes and how well the ceiling below holds it back. A bathroom without an extractor fan sends far more upward than a well-ventilated one. The same goes for a loft hatch that sits loose in its frame. We often start a survey in the rooms beneath the roof for exactly this reason, because the cause of a damp roof timber is sometimes a damp house.
None of this makes foam unsafe in every case. It does mean that any insulation which changes where the cold surface sits has to be judged against the moisture load of the house beneath it.
What spray foam changes inside the roof
Spray foam is applied as a liquid that expands and sets. Sprayed to the underside of the roof, it bonds to whatever it touches and fills the space between the rafters. Closed-cell foam sets dense and is hard to pass vapour through. Open-cell foam is softer and lets some vapour pass. Our page on open and closed cell foam sets out the difference. The two behave differently in a roof, and the type matters when we judge what is going on behind it.
Two things happen at once. First, the foam holds the warmth of the house against the underside of the roof, so the slope above it becomes colder than it was. Second, it often covers the timbers completely. The rafters, the underside of the battens and sometimes the back of the underlay now sit behind a layer that is hard to see through and difficult to dry through.
Where foam is well adhered and the house has little vapour in it, the wood behind may stay perfectly dry. Where the foam has gaps, where it has pulled away from a rafter as it set, or where moist air finds a route round its edges, the vapour reaches cold wood and condenses there. The foam now stands between that moisture and the air that used to carry it off. Wood that is wet for long periods can soften and decay, and fixings can rust. The RICS consumer guide on spray foam, from March 2023, names "uncontrolled condensation" as a risk. That is the worry in two words, and it is why a check of the timbers matters more than the foam's own appearance.
Why the timbers are the part that suffers
Wood is a living material in the sense that it takes up and gives off moisture with the air around it. Dry wood in a ventilated loft swings gently with the seasons. The trouble comes when it stays damp. A rafter that sits wet against a cold slope for a long time becomes a place where rot can start, and once it does, the strength of the member is at stake, not just its surface.
On an older house the rafters may be sawn softwood from a period when timber was cut and seasoned differently, and the roof was designed to be draughty. On a Bath terrace or a village stone house, that same roof may carry heavy stone slates, so the timbers are also doing real structural work. Foam over them gives you no way of seeing what they look like.
What we look at
When we are asked to look behind foam, we start with the visible edges: the foam where it meets the wall plate, around the chimney breast, at the eaves and at the ridge. Staining on the foam or timber at an edge is a prompt to go further. We probe the wood where we can reach it, take a reading with a moisture meter, and look at the nails and fixings, which tend to show rust before the wood shows decay. A damp patch on a ceiling below may be a leak, or it may be condensation, and the difference usually shows in the pattern of staining and in whether it follows the weather.
The honest position is that we cannot always tell from outside. Sometimes the one way to be sure is to cut a small inspection hole or lift some foam, and we tell you before we do it.
Ventilation: what it does and where foam blocks it
A cold roof relies on air moving through it. The standard guidance, BS 5250, is summarised by Marley in its advice on how to ventilate a cold roof. Where the underlay is the non-breathable type and the insulation lies at ceiling level, Marley gives continuous eaves ventilation of 25 mm for a roof at 15 degrees pitch or less, and 10 mm above that, with a further 5 mm at high level on steeper roofs, wide spans and lean-to or mono-pitch roofs. With a breathable membrane the figures differ and turn on the airtightness of the ceiling and the permeability of the covering. These are typical manufacturer figures and the actual requirement depends on the roof, so read them as an indication of what a roof is designed with, never as a specification for your house.
Foam breaks that arrangement in a simple way. If it is sprayed hard into the eaves, it closes the gap through which air entered. If it is sprayed to the back of the underlay or the slates, the ventilation space between the insulation and the covering, which the original design relied on, no longer exists. Some foam systems are designed as a sealed "warm roof" with no ventilated void at all, and in those the moisture strategy is different: the foam has to be airtight and the house has to manage its own vapour. The problem comes when foam has been added to a roof built to breathe without anyone changing the way the roof manages moisture.
That is why adding vents after the event is not a fix by itself. A vent cut through foam gives air an exit but not a route in, and may bring moist air towards the very timber you wanted to protect. Whether it helps depends on the roof, and we would want to see it before saying.
Signs that moisture may be building up behind foam
No single sign is conclusive, and many of them also have ordinary explanations. Together they tell you when to get the roof looked at. Signs worth acting on include:
- Dark staining or damp patches on a bedroom or landing ceiling, particularly after cold nights rather than after rain.
- A stale, musty smell in the roof space, or a loft hatch that feels damp.
- Rust on nail heads visible at the edge of the foam, or rust runs on the surface of the foam itself.
- Foam that has cracked, shrunk or pulled away from a rafter or the wall, leaving a gap along the timber.
- Slates or tiles that look wet on the underside, or daylight where there should be none.
- Mould spots on rafters or walls at the edge of the sprayed area.
The pattern matters. A leak follows rain and shows where water entered; condensation shows where cold surfaces are, and it appears in cold weather even with a dry roof outside. It is also possible for both to be present at once. A leaking flashing or a slipped slate can wet foam from above, and the foam can then hold that water against the timber. A slipped slate also looks different from the inside once the roof is foamed, because there is no gap to see light through.
It is also worth knowing that a roof can have a problem with no outward sign at all. If the foam has been in for some time and there is any doubt about its type, thickness or the ventilation, a check is a small cost against the risk of finding rot later. A separate page, spray foam and roof timbers, looks at the wood itself.
What a check involves and what comes after
A check starts with the roof and the loft, with the house below it in mind. We look at the type of foam, where it stops and starts, the state of the eaves and ridge, and the timbers we can reach. We take moisture readings, photograph what we find, and describe it in terms you can use. The typical UK range for an assessment is £100 to £200, according to MyBuilder's spray foam removal cost guide (2026). Your fixed quote for any work follows the survey. Our guide to what a spray foam survey looks at sets out the checks in order.
What happens next depends entirely on what we find. If the timbers are dry and the foam is sound, the answer may be to leave it, monitor the loft and manage the house's moisture. If there is damp in the wood, the options range from removing foam over an affected section to taking it off the whole roof and re-insulating in a way that lets the roof breathe. MyBuilder gives removal as a typical £20 to £50 per square metre, averaging about £1,500, with disposal at £150 to £400. The cost of repairing or replacing timbers is separate and depends on what the survey finds.
Many owners ask about the effect on a sale. Lenders and surveyors have taken different positions on foam, which our page on selling a house with spray foam covers. If a new roof is planned, foam interacts with that work too, which our guide to spray foam and a new roof. And for the older houses where condensation is most dangerous, see foam in older and listed Bath houses. The wider picture of our work on foam is on the spray foam insulation page.