Foam and breathable felt
Can spray foam go over a breathable membrane?
The short answer
It can be done, but it works against what the membrane is for. A breathable underlay lets moisture vapour pass out through the sheet into the ventilated space above. Foam sprayed against its underside covers that route and leaves the wood unable to dry. Whether a given roof is at risk depends on the membrane, the foam and the ventilation, and only a look inside can say.
A breathable membrane and a layer of foam work in opposite ways. This is how the two interact, and what is worth checking.
What a breathable membrane is meant to do
Most modern pitched roofs carry a sheet of underlay below the battens and tiles. A breathable, or vapour-permeable, membrane lets moisture vapour leave the roof space through the sheet itself while keeping liquid water out. The idea is that damp air from the house can pass through it instead of lingering on the timber.
That only works if the vapour has somewhere to go. It crosses the membrane into the batten space and out through the tile laps or vents. And on the room side the membrane needs air, or a cold roof space, against its underside, not a solid layer pressed to it.
This is a general description. Each product comes with its own fixing instructions, and the maker's data sheet is the reference for what that membrane tolerates. A roofer or surveyor looks the sheet up once its type has been identified, and that identification is the first task on any foamed roof.
Vapour-permeable and vapour-resistant underlays
Not every roof has a breathable sheet. Older roofs often carry bitumen felt, which does not let vapour through, and some modern products are sold as high resistance. Marley classes high-resistance underlay as a product that does not pass vapour, and gives eaves ventilation of 25mm along the eaves for pitches of 15 degrees or less and 10mm for steeper ones. For low-resistance underlay over a well-sealed ceiling it describes the small leakage of ordinary eaves construction, with conditions attached.
So the answer depends on what you have. Vapour-resistant felt needs the roof space ventilated, and foam that fills the eaves removes that. A breathable sheet needs space beside it to take the vapour, and foam pressed to its underside does the reverse. Neither pairing is simple.
Our answer on how to tell if you have spray foam helps if you are unsure what is in your loft.
What foam changes for the sheet
Foam sprayed to the underside of a breathable membrane covers the very surface the vapour was meant to leave by. If the foam is closed-cell, that surface is largely sealed. If it is open-cell, some vapour may pass, but the foam can also hold moisture against the sheet and the wood behind it.
RICS, in its consumer guide of March 2023, links badly installed insulation with dampness, and tells owners to put the question of foam to their lender before relying on it. Both points apply to a roof where the membrane sits behind the foam.
None of that proves a roof with foam under a breathable membrane is damp. It says the way the roof was designed to dry has been altered, so the timber should be checked and not assumed sound. Weather, ventilation and the condition of the felt all play their part, and no two roofs behave quite alike. Each slope is therefore judged on its own timber, not on the evidence of its neighbour.
Finding out what lies behind the foam
From the hatch you see foam, not the sheet. Working out what the membrane is takes a closer look. We check the eaves for any visible edge of the underlay, examine a few tiles at the roof edge where that can be done safely, and compare the findings with the age and type of the roof.
The wood is the real evidence. A probe pushed into rafters at several points tells you whether they are sound. Dark staining, a white or grey bloom or a spongy feel mark places where the membrane has not been letting the timber dry. Nail heads and the zone near the eaves get extra attention, since water tends to gather there.
Pictures of what turns up go with the written quote. If there is damage, the proposed repair is shown to you before work begins, and our guide to a spray foam survey covers the process in more depth.
Taking the foam off, or leaving it
When the foam lies on bare timber and the membrane is loose, it can normally be taken out through the hatch while the tiles stay put. If it has glued itself to the sheet, the membrane tears and needs replacing, which means lifting tiles in sections.
Once the foam has gone, the sheet is kept if it is undamaged, or renewed if not. Air paths are reopened at the eaves and ridge, and the question of insulation is settled. Mineral wool laid across the ceiling is a common replacement, and it leaves the roof space cold and ventilated, and your building control body can tell you what it expects to see.
The decision is yours, taken on pictures, and the survey ends in a price set out as one total. Our spray foam insulation removal page describes the service as a whole.
Related questions
Foam and membranes
How do I know whether my underlay is breathable?
Often it cannot be seen without lifting a tile or reaching the edge of the sheet at the eaves. The age of the roof helps, as does any paperwork from when it was laid. A roofer can usually identify the type from the sheet's edge or its printed markings.
Is open-cell foam safer against a breathable membrane?
The Energy Saving Trust describes the leak risk as lower with open-cell foam, but that is about leaking roofs, not membranes. It does not make the pairing safe in every case, and the membrane maker's instructions still decide what is acceptable. Checking the timber shows what has actually happened.