Multi-foil insulation – does it work?

The construction industry is seen as being very conservative on the whole. Trying to get new materials to be used is seen as a difficult and costly exercise. We have stuck with brick and block for many years on individual builds and extensions, but there are a few exceptions. Multi-foil insulation being one of them.

Loft conversions have fuelled the demand for multi-foil with its attractive looking headlines of 200mm worth of insulation in 20mm. Headroom issues can rapidly disappear when these figures are being bandied around. Also it is seen as a clean and quick way of insulating. No fibres to worry about and a few staples later the roof is done! Not only that, some come with BBA certificates to prove that they work.

I like to think that I am not too sad when it comes to looking at product data, but I did feel compelled to have a closer look at the multi-foils, because if they did work, it would be a great relief (given all the roofs that they are ‘insulating’) and also something to promote. This is what I found:

The BBA certificate for a leading brand was tested under the following conditions:

Roof make-up was:


Batten (non vented void)


Batten (non vented void)

50mm phenolic board insulation

With these factors and additional insulation in place it worked. Hooray!


1. You need another insulation in place – they do not pass the test on their own

2. You need to tape all the foils together to help form the airtight (non vented) layer

3. You need to create a permanent airtight seal against the wall / structure to ensure that the non-vented layer stays unvented

Only three BUTs, however they are pretty major ones. I don’t know of any jobbing builder who would really know about the need for the additional insulation or the reliance on airtightness. So if the insulation is not fitted correctly, diligently and ultimately checked for airtightness then what? Well I think that we can safely say that virtually all homes insulated with multi-foils are not performing as specified and consequently we have created another potentially great product (except that it doesn’t deal with decrement issues) that has been mis-used, potentially mis-sold and has led us to higher energy use than would have normally been expected.

So if you are thinking about using the multi-foils for a loft conversion you will have to insist on:

All foils being taped together using a very long lasting tape

50mm of additional insulation being added

All joints with the wall, roof etc being sealed with a very long lasting airtight sealer / tape

Good luck!