What Is Spray Foam Insulation?

How it is made and applied

Spray foam is manufactured on site. A van carries two tanks, one holding a mixture containing isocyanates and the other a resin blend with catalysts and a blowing agent. The two are heated, pumped along hoses and mixed in a spray gun, and they react within seconds, expanding to many times their liquid volume. The installer builds up the layer in passes, so the foam bonds to the rafters, the underlay and the underside of the covering and cures into a rigid or semi-rigid plastic.

Because the product is a liquid until it hits the surface, it reaches awkward corners, fits irregular rafters and seals around pipes and wires. That is its selling point. The same quality makes it difficult to take off, since cured foam is bonded to everything it touched. The installers wear respirators and protective suits during spraying, and the house is normally vacated for a time, as set out by the product maker.

Open cell and closed cell

There are two main types. Open-cell foam is light, soft and spongy, with cells that are not sealed, so it passes some water vapour. It needs more thickness to give the same insulation and usually costs less. Typical UK costs are reported at £20 to £35 per square metre for open cell and £40 to £60 for closed cell, and a firm price follows a survey. Closed-cell foam is denser, stiffer and holds its gas in sealed cells, which gives better insulation per millimetre and a more vapour-tight layer. Setting the two types side by side is the job of our open-cell versus closed-cell comparison.

How it differs from ordinary loft insulation

In a standard loft, mineral wool or similar is laid on the floor between the ceiling joists. The loft above is cold, the roof is ventilated and the timber stays dry because air moves through it. Spray foam moves the insulation up to the roof slope itself. The loft becomes warm and, in principle, usable, and the foam closes the gaps at the eaves that let heat escape. This is called a warm roof, and we compare the two arrangements on our page about warm loft versus cold loft.

That change is the root of the concerns. The RICS consumer guide notes that foam converts a cold roof into a warm roof, and that poor installation can cause uncontrolled condensation and damage. Timber that was once able to dry sits behind a sealed layer, and a leak or condensation that would show in a cold loft can be hidden. Where installers took care and the roof was sound, many foamed roofs are fine. Where they did not, the problems have been serious enough to produce the removal work we do.

Where it has been used, and why people chose it

Spray foam was sold for its promise of lower heating bills, draught reduction and a quick, tidy job in one visit. It has been used in lofts, under flat roofs, in outbuildings and in cavity walls and floors, and some of it went on under government-backed schemes. It is a legitimate product for some uses, particularly in new builds designed around it. The trouble in this country has mostly come from older houses with cold-roof construction, where foam was added without the changes in detail that make it safe.

If you are considering it, read the risks first. Our page on spray foam insulation problems sets out what can go wrong, and a free survey from a roofer with nothing to sell you is a useful second opinion.

Cost is the other thing people ask. Typical installation costs are as above, and removal costs are reported separately in the removal guides, so it is worth knowing both figures before deciding. Put simply, the cheap option at the start can become the expensive one later if the roof has to be opened up.

Planning is the last point. Foam goes on quickly, often in a day, which is part of its appeal, but a day of spraying is not the end of the decision. Cured foam is hard to alter, so it is worth being sure the roof is dry, the covering is sound and the build-up suits the house before anyone mixes a drum.

What we do

We do not install spray foam. We look at roofs that have it, tell you whether it is doing any harm, and take it out where it has to go, then repair the timber and re-insulate. You can read how the work runs on our spray foam insulation in Basingstoke, where every price is written down once the roof has been seen.