Spray foam insulation FAQ

What is spray foam insulation?

Spray foam insulation is a type of insulation that can be used in the roof space at either the joist or rafter level, to help keep warm air in the home during the winter and also prevent the home getting too warm in the summer.

Typically, the spray foam is made up of polyurethane. It is sprayed as a liquid, which then gradually expands to up to 100 times its original volume, and once set it not only creates an effective insulating layer, it also acts to reduce noise pollution.

Can I install spray foam myself?

Spray foam insulation ideally needs to be installed by a professional, since it gives off dangerous fumes and also can actually damage the structural integrity of the building if applied incorrectly.

The reason for this is that the expanding foam can exert some serious pressure on the home’s structure if applied incorrectly and ceilings and walls have been known to collapse or be damaged from DIY installs.

What is open cell and closed cell spray foam?

Foam structures consist of a flexible material that is filled with bubbles of gas (or just air). There are two main types of spray foam, closed cell and open cell.

In closed cell foams, the bubbles are encompassed entirely in the liquid. When the foam sets, each bubble forms a trapped discrete pocket of gas. In an open cell foam, the bubbles all merge with one another, so a gas can travel through the foam (imagine a bathroom sponge).

This gives the two different types of foams different properties.

Features of closed cell spray foam

This spray foam sets rock hard – it is not compressible when it has set, this means that it can help support the structure of the property. However if installed incorrectly it can exert unnecessary strain on the structure of the home. This type of foam will also tend to be much denser as a result and this also means it will be heavier.

The insulating properties of closed cell spray foam tend to be greater than the open cell spray foam, partly because they can put bubbles of insulating gas in the foam (not just air). This helps slow the movement of heat across the foam.

The typical U-value for closed cell spray foam is about 0.16 W/m2K

The final thing about closed cell spray foam is that it will not allow moisture across it, which means that you will need to consider ventilation in the loft space to prevent build up of condensation.

Features of open cell spray foam

Open cell spray foam is less dense than close cell spray foam, so it will not help strengthen the structure of the building. In addition, its chemical makeup means that it doesn’t offer the same level of insulating effectiveness as the closed cell. However on the plus side it has high vapour permeability allowing it to breath far better than the closed cell spray, so condensation build-up should be less of an issue. The other major benefit is that it offers better acoustic insulation, absorbing more sound in normal noise frequency ranges.

The typical U-value of open cell spray foam is about 0.35 W/m2K

Since spray foam has better insulating properties than wool (when the same thickness is used), you can achieve the same U-value by insulating to a greater depth. This means that if space is an issue (remember you need to install 270mm of wool insulation to meet building regulations now), then spray foam could be the answer. Typically you will require a depth of 150mm to achieve a u-value of 0.16 W/m2K, but a depth of just 40mm will help you to achieve a u-value of 0.40 W/m2K.

What should I expect to pay for spray foam insulation?

Spray foam insulation is not cheap – while using mineral insulation might cost £6-7 per m2 to achieve a u-value of 0.16 W/m2K, spray foam might cost £45 per m2 to achieve the same value.

Are there any issues with insulating my loft with spray foam?

So having mentioned all the positives relating to spray foam insulation, we feel there is one fundamental issue with using this product to insulate the loft space that we should draw your attention to – and that is ventilation.

If the spray foam is applied to the undersides of roof slates or tiles, it is not possible to keep this ventilation gap, since the foam encapsulates the roofing battens and the top of the rafters, trapping moisture. As you can read here, we think it is really important that you leave a ventilated gap between any insulation and the tiles in order to vent away any moisture otherwise the wood can’t breathe and it will consequently begin to rot. A new roof is not a cheap thing to install!

That being said, if the wooden rafters are treated properly first and it is installed correctly, it is certainly worth considering, although if you want a more sustainable product to insulate with (remember polyurethane is derived from crude oil) then take a look here!