Should Spray Foam Go Under a Breathable Membrane?

What a breathable membrane does

Modern pitched roofs are usually built with an underlay, a sheet laid over the rafters under the battens and tiles. It catches wind-blown rain and any water that gets past the covering. The older kind, bitumen felt, is not vapour permeable. A breathable membrane, sometimes called a vapour-permeable or low-resistance underlay, is different because it lets water vapour pass through while stopping liquid water. In a cold loft, that lets humid air that has risen from the house escape through the membrane and out through the tile gaps, and it reduces the need for large ventilation openings at the eaves and ridge.

The design relies on air on the underside of the membrane. The vapour has to have somewhere to go, either into a ventilated loft or a ventilated gap. Take away the air and the membrane cannot do its job, however good the product.

How foam interferes

When foam is sprayed against the underside of a breathable membrane, the vapour cannot pass out of the loft because there is no air gap and the foam, particularly closed-cell, is largely impermeable. The membrane is then a layer in a sealed sandwich. If any moisture is trapped in the rafters, or gets in later through a leak or from warm air, it has no easy route out. RICS makes the same warning in its consumer guide: water can end up held against the wood, and the roof becomes difficult to inspect. That applies equally here. Trading Standards guidance also notes that foam can reduce roof-space ventilation, which raises the risk of condensation, mould and timber rot.

Open-cell foam is more permeable than closed cell, and some designers argue it allows limited vapour movement. That is a judgment about a specific product in a specific build-up, not a general rule, and our page on open-cell and closed-cell spray foam covers how they differ. Where foam is applied under a membrane that was fitted with a ventilated gap above it, the situation can work better, since the vapour path above the foam is open. It all depends on where the air is.

The regulatory side

Insulating a roof is controlled work, and the build-up has to satisfy the requirements for thermal performance, moisture and fire. In practice, that means the product, the thickness and the ventilation all need to suit the roof, and the installer should be able to show evidence of that. A product that carries independent certification for a given use gives you something to point to. Roof alterations and renovation can fall under Approved Document L, where the roof is treated as a thermal element, and your building control body can say what applies. We cover this on our page about spray foam and building regulations.

Do not rely on a salesperson's assurance that foam and membranes are compatible. Ask for the manufacturer's written guidance for your type of roof, and for the evidence that the system has been assessed for it. If that cannot be produced, treat the claim with caution.

Thickness and position change the picture. A thin coat sprayed lightly onto the underside of the membrane behaves differently from a deep fill that touches the membrane everywhere. The more of the membrane that is covered, the less of it can do its job, and foam that has crept up over the battens and into the tile laps leaves nothing open at all. Ask the installer exactly where the foam stops, and whether any air gap was left at the eaves or ridge.

Hips, valleys and the ridge are places where air normally escapes. If foam has filled those junctions, even a roof that appears to have a gap in the main slope can be sealed overall, and the moisture has nowhere to go but into the timber.

Checking an existing roof

If foam has already been applied and you do not know what is under the tiles, there are signs to look for. Is the foam in contact with the underlay, or is there a gap? Is the underlay old felt or a breathable type? You can sometimes tell from the loft: breathable membranes tend to be a fabric-like sheet, felt is black and papery. Look at the condition of the visible timber, the smell, any mould and the ceilings below. A survey can take readings and, if needed, open a small area to see the layers.

We check these things on the survey, photograph the foam, the exposed timber ends and the membrane where visible, and tell you whether the build-up is working. If the foam is fine, it stays. If it has sealed in a breathable membrane that needed ventilation, the usual remedy is to remove the foam, check the timber and re-insulate with a build-up that lets the roof dry, as described on our page about insulating after removal.

When tiles are renewed

A re-roof is the best moment to sort this out. New membrane, battens and tiles over a clean rafter structure, with insulation designed alongside, is far more predictable than fitting foam into an old roof. The wider service is on our spray foam page.