The short answer
Sometimes, but not as a general fix. Ventilation can dry a roof void above insulation, yet foam sprayed tight to the roof covering often leaves no void to ventilate. Whether it is accepted depends on the foam type, the build-up and what the surveyor or lender will take. A specialist assessment should come before any decision.
Why ventilation is the first idea people reach for
A conventionally built cold roof relies on moving air. Air enters at the eaves, moves under the covering and leaves at the ridge or high level, carrying moisture with it. When foam goes onto the back of the covering, that air path is usually blocked, and the roof becomes a warm roof without having been designed as one.
The thought follows naturally: cut a few vents, fit tile vents or ridge vents, and let the roof dry. It is cheaper than stripping foam and keeps the insulation. In the right roof, that thinking is sound, and BS 5250 (moisture management in buildings), treats ventilation as a normal tool in cold-roof design.
The question is whether the roof in front of you still has a space for air to move through. If the foam fills the rafter depth and touches the underside of the covering, vents in the covering open onto foam, not a void. Our guide to spray foam, condensation and timber decay explains where that moisture goes instead.
Where added ventilation can genuinely help
There are cases where it is a reasonable response. Some installers left a gap between the foam and the underside of the covering, or sprayed only the rafter faces, leaving a channel along the slope. Others foamed the loft floor rather than the rafters, so the roof void above is still a normal cold space that needs the usual eaves and ridge openings.
Marley's published guidance on cold-roof ventilation, which cites BS 5250, gives the pattern. For a vapour-closed underlay it asks for 25 mm of eaves ventilation on pitches of 15 degrees or less and 10 mm above that, with extra high-level ventilation on steeper or wider roofs. Where the void is clear, those figures show what a conventional roof needs.
In those roofs, we check the openings, clear any blocked eaves and fit vents where the survey shows a shortfall. Insulation pushed into the eaves is a common blockage, and an early check on any survey, because a vent that opens onto a packed eaves space does nothing. The result is recorded with moisture readings so that the surveyor sees the before and after.
Even then, ventilation is a managed risk, not a cure. Timber that was wet before the vents went in needs time to dry, so a second set of readings some weeks later, once the roof has had a spell of ordinary weather, is the only fair test of whether the change worked, and the readings are shared with whoever asked for the work. We will say when that recheck makes sense.
Where ventilation cannot do the job
Three situations stand out. First, foam stuck to the whole underside of the covering, where no air gap exists to ventilate. Second, closed-cell foam against timber that is already damp, because vents cannot dry a rafter that is sealed under a layer of foam. Third, roofs with a breathable membrane under the battens, where adding vents above the foam changes little for the timber below it.
Roofs of those kinds may only be resolved by removing the foam from the affected timbers, either fully or in part, and re-insulating with a build-up that can move moisture. Our answer on spray foam over breathable membrane covers that arrangement in more detail.
A surveyor who sees foam hard against the covering will often say ventilation is not a fix for that reason. The RICS consumer guide itself looks for roof-specific condensation checks from a competent person, so a calculation of the actual roof beats a rule of thumb.
How a surveyor or lender is likely to treat the idea
Ventilation as mitigation is a proposal, and the party who raised the concern decides. Some will accept it with evidence that the void is clear and the timbers are dry. Others will want the foam out regardless, particularly where a lender refuses foam in principle.
The evidence that tends to matter is practical: a drawing or photographs showing the air path, moisture readings taken before and after, and a note that the roof will be rechecked later. A ventilation scheme without those is hard to defend, because nobody can see whether it worked.
Where foam is on a flat roof the situation differs again, because a warm flat roof has no air path to restore. See spray foam on flat roofs and dormers for how those roofs are assessed.
Cost, regulations and what we would do
Vents themselves are modest compared with removal. MyBuilder's 2026 guide puts foam removal at about £20 to £50 per m², and an assessment at £100 to £200, so a correct diagnosis before spending is worth having. Ventilation work is priced on survey, since it depends on how many openings the roof needs and how the eaves are built.
Altering the roof edge or covering can bring Building Regulations in, and insulation changes can raise Part L; we explain which applies and look after any notification. Your quote shows VAT; fitting roof insulation to a home is zero-rated until 31 March 2027, whereas general roofing is standard-rated.
We would begin with an inspection from the loft and outside, open a small section if the surveyor agrees, and photograph your roof as it stands. Our written quote follows. For the whole service, see spray foam insulation removal in Stroud.