Spray foam or mineral wool for a loft?

Mineral wool comes as a quilt, a thick blanket of fibres that holds still air. Laid between the ceiling joists with a second layer across them, it keeps the heat in the house while the roof space above stays cold and ventilated. That arrangement is called a cold roof, and it is how most houses built before foam existed were meant to work.

Foam is sprayed on as a liquid, swells against the underside of the felt or tiles and sets hard. It turns the roof into a warm roof, with the rafters pulled inside the heated envelope. The trouble is that a cold roof was never designed for that. The air paths at the eaves are often blocked, and moisture that used to drift out through the roof has nowhere to go. Our guide to open-cell and closed-cell foam explains the two foam types, and the same worry applies to both. A quilt makes no such demand, because it leaves the roof to work as it was built.

This is where the two part company most clearly. With a quilt on the loft floor, the rafters, felt and battens are in plain view from the hatch. You can shine a torch along them on a wet evening and look for staining, drips or daylight. If something is wrong, the quilt is rolled back in one place, the fault is mended and the wool goes down again.

Under foam, the same timbers are buried. Nobody can probe a rafter that is set in a hard layer, and a slow leak can run for years without a sign. RICS, in its March 2023 consumer guide, says that foam could affect a surveyor's valuation opinion and that taking it out risks damaging the structure. A buyer's surveyor reads that as a cue to ask questions. Our page on the spray foam survey shows how an inspection gets round the problem, using a probe, a meter and a small opening.

The difference matters most when you are the one living with the roof for another twenty winters.

RICS suggests you put the question to your mortgage provider before anything else. Today's Conveyancer reported in November 2024 that certain lenders decline loans on houses with foam in the roof, while others weigh each property individually. A quilt that any valuer can lift and look beneath does not raise the same difficulty, which is one reason buyers' surveyors are comfortable with it.

That does not make wool a cure for every sale. A lender looks at the whole roof, and a quilt laid over a damp or neglected loft will not hide the problem. What wool does is keep the roof open to inspection, and inspection is what a valuer wants. Keep the receipt, the product name and a dated photograph of the finished loft. Those few papers answer most of what a conveyancer asks.

Foam has a case where the roof space is lived in, such as a bedroom under the slope, a vaulted ceiling or a knee wall, where no cold void exists for a quilt to sit under. Even there, rigid boards fitted between and over the rafters are an alternative that can be checked and altered later. If foam is still being considered, ask who owns the ventilation plan and the moisture control, and get the answer in writing before the machine arrives.

Foam also tempts people with a leaking roof, because it looks like a quick seal. The Energy Saving Trust (8 May 2026 update) says it does not recommend foaming over a roof that leaks while the fault stays unrepaired. Burying a fault under a layer of foam only makes it harder to find, and a hidden leak does its damage slowly. A dry roof has to come first, and a quilt can be laid on it the same afternoon without changing how the roof behaves. That ordering is the reason we ask to see the roof before anyone talks about materials.

For an ordinary loft with a hatch and a cold roof, our usual advice is mineral wool at ceiling level, laid so that it stops short of the eaves and the air keeps moving. It is quick to fit, easy to top up and simple for the next owner to understand. Where the roof is being renewed anyway, the best moment to plan the insulation is when the covering is off.

If foam is already there, the first step is a survey, not a removal quote. The photographs and the timber checks decide whether to leave the foam alone, open it in a few places or take it out. Our page on insulating again after removal covers what goes back, and the spray foam removal hub explains the service as a whole. Send us photographs from the hatch, and we will tell you which route fits your roof, whether that means leaving things as they are, a repair or a fresh quilt.