Condensation is water that forms when warm, moist air touches a surface cold enough to make the moisture drop out of the air. In a house it is familiar on windows and in bathrooms. In a roof it can happen out of sight, on the sloping timbers, the boarding and the underside of the covering. Timber that stays wet for long enough can decay, which is why surveyors take an interest when they find foam sprayed across the underside of a roof.
The risk is real but it is a risk, not a certainty. Many foamed roofs stay dry, and plenty of owners never have a problem. This guide explains how moisture moves in a pitched roof, how foam can change that, what RICS says, and how we assess a roof, one factor at a time. It belongs to our spray foam insulation removal service, and it explains the problem so that the other pages on mitigation, selling and listed homes make sense.
How moisture behaves in an ordinary pitched roof
Every household produces water vapour: cooking, showering, drying clothes and simply breathing. Warm air holds more vapour than cold air, so the air in your rooms is nearly always wetter than the air outside. That vapour is not stuck in the rooms. It seeps through ceilings, around loft hatches, past pipes and up through light fittings, and some of it ends up in the roof space.
A traditional roof was built to cope. The covering of tiles, slates or stone slates is not watertight in the way a sheet of plastic is, because it has gaps between units. Underlay may be a loose felt or a modern breather membrane. Air moves through the eaves and perhaps at the ridge, and the timbers sit in that moving air. Any moisture that arrives is carried off before it does harm. Dry timber in a ventilated roof can stay sound for centuries.
What this system relies on
It relies on three things: a ceiling that limits how much vapour rises, airflow that removes what gets through, and a covering that sheds rain. If one fails the others carry the load. If two fail together, moisture starts to accumulate. This is the baseline against which foam should be judged, and it is why the details of each roof matter more than any general rule.
Some roofs also have a lining above the rafters, either a boarded deck with felt over it or a membrane draped between the battens. Those layers change where condensation is likely to form, since a cold, airtight surface collects it. A roof with a traditional torched or lime-pointed underside behaves differently again, because lime and old timber absorb and release moisture slowly. None of these is better or worse in itself. They simply mean that a house in Nailsworth and one in Dursley can need different answers.
What foam changes
Spray foam is applied to the underside of the roof, so it sits in exactly the zone where the air used to move. It can change the picture in four ways.
It takes the timber out of the airflow
Rafters and boards that were once washed by air are wrapped in insulation. Wood that gets damp there dries slowly or not at all, because the air that would carry the water off cannot reach it.
It may block the ventilation routes
Foam sprayed hard into the eaves, around the wall plate and into the ridge can close the openings that were part of the original design.
It can leave the timber colder
Insulation makes the structure behind it colder in winter, since less heat from the house reaches it. A colder surface is a better place for condensation to form. If warm, moist air can find its way through or around the foam, it meets that cold surface and gives up its water there.
It can hide what is going on
Once the foam is in, a small leak or a damp patch cannot be seen or probed from inside. Staining may appear late, and by then the timber has been wet for some time. A homeowner who sees nothing from the landing may therefore feel reassured when there is no real evidence either way. RICS states the consequence plainly: a poor installation may end in damp, with uncontrolled condensation as one example, and in harm to the fabric of the building. Our guide on how roof timbers are inspected shows how we look behind the foam.
What RICS says, and what it does not say
RICS, the professional body for surveyors, brought out a consumer guide on spray foam in March 2023, and Mortgage Solutions reported it on 3 March of that year. You can read the RICS consumer guide yourself; it is short, and it is the document surveyors work from.
The points that matter here
Before any foam goes in, the guide wants heat calculations and a condensation check from a suitably qualified person. It tells owners to ask their lender and their insurer whether their terms permit the product, and it says foam is not regarded as reversible. It also warns of damp following a careless job.
What it leaves unsaid
The guide is written for homeowners, so it stays general. It cannot know how your roof is built, where the leaks are or how the house is used, and it does not pretend to.
Nowhere does it ban foam or demand that every installation be stripped out, and it offers no an exact thickness or formula that guarantees a safe roof. We take the same line. Foam is a risk factor to be assessed, and the assessment, not the label, produces the verdict. Our answer on whether RICS says to remove foam deals with the wording in more detail.
How decay risk comes together on one roof
Decay needs wood, moisture and time. Remove any of those and it does not happen. In a roof, the timber is always present, so the risk depends on how long it stays wet and how wet it gets. A patch that dried within a season sits in a different category from a stain that has been spreading for years.
Factors that push the risk up
- An existing leak above the foam, such as a slipped slate, a lifted flashing or a split valley.
- Moist air entering the loft from bathrooms and kitchens, especially where an extractor discharges into the roof space.
- Foam that covers the eaves or the wall plate and closes off ventilation.
- A covering that was designed to breathe, such as stone, clay or slate, with foam stuck against its back.
- Timber that was already damp when the foam went on.
Factors that pull it down
- A sound, watertight covering and well-fitted flashings.
- A sealed ceiling that limits the vapour entering the loft.
- Extractors ducted outside the building.
- A modern roof with a ventilated batten space above the foam.
- Readings and sample openings showing dry, sound wood.
No single item decides the matter, which is why assessment is a weighing exercise, not a tick list. We write down each factor we find, with a photograph where we can, and the report ranks them so you can see which ones drive the conclusion and which are minor.
How we assess the risk on a Stroud roof
The method moves from the least intrusive checks to the most, stopping when we have enough evidence to advise you.
Step one: the history and the paperwork
We ask when the foam went in, who fitted it, whether any condensation calculation was done and whether the roof leaked before or since. A guarantee or data sheet helps, though many owners inherit a house with none.
Step two: the roof from outside and from the loft
We inspect the covering, flashings, valleys and chimneys for the leaks that bring water in from above, and the loft for staining, smell and foam that has moved. Around Stroud, where steep valley roofs and older stone and clay coverings are common, we take extra care over the detailing at chimneys and valleys.
Step three: readings and sample openings
If the first two steps leave doubt, we take moisture readings and open small windows in the foam at the places most likely to be affected. The result is a report with photographs and readings, set out in the guide on timber inspection. A borderline reading is reported as borderline, with a second visit suggested, since timber moisture rises and falls with the weather and with how the house is heated.
Step four: the verdict in plain terms
The finding comes in one of three forms: low concern, concern at identified spots, or concern across the roof. Each form leads to a different recommendation, from keeping the report on file to repairing a leak, or to removing foam over the affected stretch of roof. We do not give a fixed fee for the assessment on this page. MyBuilder's 2026 guide shows national assessment costs of £100 to £200, and your written quote follows the survey.
Ventilation, removal or leave alone: what follows the assessment
If the assessment finds a low level of concern, many owners decide to leave the foam in and keep the report on file. If a specific leak is the cause, repairing the leak and monitoring the timber may be enough. If the roof is wet and decay has begun, taking the foam off and repairing the wood is the likelier path.
Some owners ask whether better ventilation could make removal unnecessary. It is a fair question, and the answer depends on the roof; our answer on adding ventilation instead of removing foam explains where it can work and where it cannot.
Listed homes and sales
On a listed building, RICS says foam is unlikely to obtain consent, and removal needs its own consent checks, so see spray foam in a listed home. If you are selling, the risk story is an issue for buyers, surveyors and lenders: Today's Conveyancer reported in November 2024 on a BBC investigation in which several lenders declined foam homes and others judged each one individually. Our page on selling with spray foam takes that further.
Whichever way you go, we keep the report, the readings and the photographs of your own roof together, so you have a clear record to show anyone who asks.