Foil insulation: does it work?

When we talk about insulation in lofts, we are usually referring to rock wool or glass fibre insulation. There is another- sometimes overlooked – category of insulation: foil. We know that foil makes a good insulator because of its very highly reflective properties, but does a thin layer of foil actually perform as well as a thick slab of insulation? Is there any way to get those ultra-low U-values required of modern buildings without having a thick layer of rock wool? Let’s take a closer look.

U-value of foil insulation

The U-value is a simple measure of how easily heat is able to pass through a material. Lower U-values mean better thermal performance. Most brands of insulation have a U-value displayed on their products as a matter of pride. Typical fibreglass insulation, for example, requires 270mm of insulation to get to a U-value of 0.16. Phenolic board like Celotex or Kingspan requires 175mm or so to achieve 0.16.
If you pick up a pack of foil insulation, the U-value is often not easy to find, if it’s there at all. So what is the U-value of foil?

Radiation vs conduction

Regular insulation works by preventing the conduction of heat through the material. The insulation is tested in a lab to work out the conduction through the material, λ (lambda). This value can be used to work out the U-value for any given width of material.

This works well for most insulation materials, because conduction is the main way they prevent the movement of heat. Foil insulation works slightly differently, however. Whilst there will be some resistance to conduction, thanks to the air gaps between the layers of foil, most of the performance of the material lies in its ability to reflect radiative heat back into the room. This characteristic is not an important player when it comes to fibreglass or wool insulation, but is crucial for foil.

It means that foil insulation is going to have a higher U-value, but may well perform better than expected in the real world. There is not much unbiased data out there to give you on this, but some manufacturers have done real world tests comparing foil with standard insulation, with encouraging results. Obviously, it depends on the type of foil and the amount of layers present, but some evidence suggests a performance better than standard insulation at much reduced thicknesses.

Foil insulation is used by NASA, right?

Foil is used in space-faring craft because of these great properties preventing radiant heat loss. Because space is a vacuum, there is very little heat lost by conduction, so foil is absolutely perfect.

How do I use foil insulation?

Foil insulation is worth considering if you have restricted space in your loft or roof space that means regular insulation is not viable. You will have to ensure there are good air gaps either side of the foil to get the best performance. We would recommend going for one with plenty of layers and some heft to it, as the thinner ones will not give a great performance.

Building regulations and foil

You might wonder – if the U-value is not apparent – whether foil insulation can be used to meet building regulations requirements for insulation. The answer is that it is at the discretion of the planning authority, and each authority will probably treat things differently. Because foil insulation companies will provide their own energy saving data on their product, it is down to the individual planning authority as to whether it can be used and what thickness is required to meet the standard.

Should you use foil insulation?

We would probably recommend shying away from it unless it really is the most practical option, due to space constraints. In this case, speak to your architect and planning authority to find exactly what would be the most suitable thickness.

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