The short answer
It depends on the roof build-up that replaces the foam. A cold roof, with insulation on the loft floor, needs air moving through the void, usually through eaves vents and sometimes a high-level vent as well. A warm roof, where the insulation sits at rafter level, is designed on a different principle. Either way, the ventilation is decided at the survey, written into the quote and checked before the job is signed off.
Why the foam changed the airflow
Spray foam is normally applied to the underside of the tiles or boards and fills the gaps between rafters. That seals the roof void, closes off any vents the roof originally had and traps the timber behind a layer that does not breathe the way an open loft does. Moisture that used to leave through the vents has nowhere to go, so it settles in the wood.
That is one reason removal is considered at all. Once the foam is cut and scraped back, the rafters are bare, which is the right moment to look at the timber and decide how the roof will work in future. Simply putting loose insulation back and hoping is not a plan. The airflow has to match the new layout, and the sizing of vents, trays and gaps should be written down before work starts, not worked out on the day with the roof open to the weather. Our spray foam insulation removal page describes the whole sequence from survey to finish.
Cold roof ventilation: the usual route
When the insulation lies across the loft floor, the void above stays cold and needs air to pass through it. Marley's guidance on ventilating a cold roof, which refers to BS 5250, gives the openings: with a non-breathable HR underlay, a continuous 25mm strip along the eaves on pitches of 15 degrees or less, and 10mm on steeper pitches. A further continuous 5mm at high level applies where the pitch exceeds 35 degrees, for spans over 10 m, and on lean-to roofs.
A roof with a breathable underlay and an airtight ceiling beneath can need less, but the manufacturer's instructions and the product certification settle that. Ask your roofer which system is specified, which underlay it assumes and why, and keep the answer with your house papers. Our page on spray foam on a breathable membrane covers what the underlay itself can and cannot do for a roof that has been foamed.
Warm roofs and living in the loft
If the loft is a room, or you want insulation kept at rafter level, the design changes. A warm roof puts the insulation in the rafter depth or above it, and moisture control comes from the build-up rather than from open eaves. Options include rigid boards fitted between the rafters with a ventilated gap above, or boards laid over the rafters. Each needs the right membranes and a continuous, sealed air barrier on the warm side.
This is where the Building Regulations come in. Under Part L, stripping to the rafters and rebuilding is classed as renovation of a thermal element, which means the new insulation may need to reach a set U-value. Our page on spray foam on a flat roof covers the separate problem of flat decks, where airflow is much harder to provide. Where the Building Regulations require notification, we explain what is needed and look after it.
What gets checked before the loft is closed
Ventilation is easiest to get right while the roof is open. Before any new insulation goes in, we look at the exposed rafters, battens and boards for dampness, staining, softness or rot, and replace what has failed. We clear anything blocking the eaves, such as foam overspill or debris, and make sure the soffit or the tile-level vents genuinely open into the void.
New insulation is then laid so it stops short of the eaves instead of squashing into them. Rafter trays or vent spacers hold that path open. Photographs are taken at each stage on your own roof, which is handy if you sell later and a buyer's surveyor asks what was done.
Our guide to selling a house with spray foam insulation explains why that record carries weight with buyers and their lenders.
Finally, the covering outside gets a check too. A slipped tile or cracked ridge can feed water into the void faster than any vent can clear it, so we deal with those faults during the same job when we can, and tell you plainly if one needs its own visit.
What it costs and how to decide
Ventilation is rarely a big item on its own. Fitting eaves vents or rafter trays is small beside the removal itself, which MyBuilder's guide, updated in August 2026, puts at £20 to £50 per square metre nationally, with repairs and new insulation typically £500 to £3,000. Treat those as national orientation figures. Access, roof size, the state of the timber and your choice of insulation all move the total, and a written quote follows once we have surveyed.
Whichever figure lands on your doormat, ask whether it covers VAT and the scaffold. If the specification is silent on ventilation, ask why, because an insulated roof with no planned airflow is how the original trouble often starts.
The quickest way to settle the question is a visit. We examine the roof, tell you what the build-up should be and put it in writing. Ring 01793 626096, write to us on WhatsApp, or send the form. The work comes with a 10-year guarantee in writing, and the old foam leaves with us at the end.