The short answer
It can. A traditional cold roof relies on a steady draught of air, entering low and leaving high, to carry moisture away. Spray foam sprayed over the rafters and into those gaps can block that route. If the foam also traps damp against the felt and timber, the roof has nowhere to dry. A survey shows whether the ventilation paths are open or sealed.
Roofs shed moisture by moving air through them. Foam can close those paths, and the damage shows in the timber long before the ceiling.
How a roof is meant to breathe
Moist, warm air drifts up from the rooms below into the loft. With a cold roof, the loft floor carries the insulation and the void above is left cold and ventilated, so the moisture is carried out before it condenses on the felt and timbers.
What does BS 5250 say about ventilation?
The standard behind this is BS 5250:2021, which covers moisture in buildings. Marley's guidance shows how it works for a cold roof under an HR underlay, a non-breathable type: a continuous 25mm opening along the eaves for a pitch of 15 degrees or less, and 10mm where it is steeper. Under an LR breather underlay with a well-sealed ceiling, the same guidance allows the gaps of ordinary eaves construction, about 3mm, to do the job, provided third-party certification supports it.
The point for you is simple. The right openings at the right places are part of how the roof was designed. They are not an afterthought, and a roof built without them has to be rethought, not simply insulated.
How foam changes that picture
Spray foam is usually applied against the underside of the roof covering. That turns a cold roof into a warm one: the insulation moves up to the slope and the roof void is meant to be sealed off, not ventilated. Done properly, that can work. Done onto a roof built to be ventilated, it can cause trouble.
How does foam block the airflow?
Foam fills the eaves gap where air should enter, covers the vents at the ridge, and sits hard against the felt so that nothing can circulate behind it. Any moisture that reaches the felt, from a small leak or from warm air getting through the ceiling, stays there, because the foam leaves it no way out. Timber that cannot dry stays damp, and damp timber is where rot begins. Our page on spray foam and timber rot explains how that develops.
This is one reason surveyors often question foam on older roofs, where ventilation was a design feature from the start. It does not follow that every foamed roof is failing, but a roof where the ventilation paths have been sealed deserves a proper look from underneath.
What we look for in a foamed roof
We start in the loft and look at the places a roof breathes. At the eaves, we check whether the gap above the wall plate is open or packed with foam. At the ridge and verge, we look for vents or dry-fix fittings, the plastic systems that clip tiles down without mortar, to see whether air can pass through them.
Can you tell if the timber is damp?
We can see and feel the signs: dark staining on rafters and battens, a musty smell, soft spots under a probe, rusted nails and white or green marks on the felt. A moisture meter gives a reading for the timber we can reach. We do not diagnose from a meter alone, because the pattern across the whole roof says more than one reading.
The Energy Saving Trust (advice last updated 8 May 2026) comments on foam sprayed over a roof that already leaks: "We don't recommend doing this." Water finding its way in under foam can go unseen for years. Our answer on how spray foam hides roof problems goes into that.
Restoring ventilation after removal
Removing the foam brings the roof back to the structure it was built as, and the next question is what keeps the loft dry and healthy over the years that follow. That depends on the roof, and we decide it with you after the survey.
What replaces the foam?
For a traditional cold roof, mineral wool or similar insulation goes on the loft floor, the eaves are left clear with ventilation trays holding the insulation back from the gap, and ridge or eaves vents are checked or added. Where a loft room needs insulation in the slope, the design has to account for the air path as well. Our guide to what happens after spray foam removal goes through the options.
If the roof was foamed because ventilation was already poor, fixing that is part of the job. Insulation work in a loft has to keep the roof ventilated, and your local building control team can confirm what applies to your project. Photos of the eaves and ridge taken from the loft help when you write to us. After the survey you get one written price, and the spray foam removal page walks through the job.
More on foam and ventilation
Is a roof with foam on the underside always badly ventilated?
No. Some roofs were designed as warm roofs, with foam or another insulation fitted against the slope as part of the design. The problem is foam sprayed onto a roof that was built to breathe, where it blocks the eaves and ridge paths. A survey shows which case you have.
Will the loft feel colder once the foam is gone?
Until it is re-insulated, yes. The foam had been your insulation, so the loft and the rooms below lose heat faster without it. That is why we plan the replacement insulation with you, before the foam comes out, so the gap is as short as possible.
The full picture: Spray foam insulation removal in Taunton ยท More short answers