Mulit foil insulation, if the manufacturers claims are believed, looks an easy way to meet the tougher 2006 Building Regulations for building insulation. Like spray foam, it can be used for both new and retro fit. But is it so simple? We investigate below.

The 2006 Building regulations raised considerably the insulation values required in UK homes and buildings. Part L of the Building Regulations deals with the U values required to meet the standards. For example, the U value to meet a ceiling against rafter section is 0.2. This is difficult to achieve with conventional insulation products due to the need to top vent the top or cold side of the insulation material by a continuous 50 mm air gap vented to the outside to prevent condensation. The amount of space required to fit the insulation product has become critical which is why spray foam has become a big winner in recent years; nothing can pack as much insulation value in the smallest of depths as Rooftherm polyurethane spray foam with a lambda value of just circa 0.020 when new. Space and ventilation make a ceiling against rafter section extremely difficult to achieve the U value of 0.20 with conventional products so how does 1) Rooftherm Spray Foam and 2) Multi Foil insulation compare?

Rooftherm spray foam on its own can achieve U value of 0.20 with just 100 mm given a k value of 0.020. Not only that, but because Rooftherm is an air sealer it requires no top side or cold side ventilation as relatively warm (moist) air cannot permeate to the cold side and produce condensation. Obviously, due allowance in the real world must be made for cold bridging, typically the timber rafter section itself will drag the overall U value up so an increase in Rooftherm will be needed.

The manufacturers of multi foil have claimed in the past R values up to 5 which would provide a U value of 0.2. Again due allowance would have to be given for cold bridging and also care would be needed when installing to provide a an air tight seal by careful jointing. However, a major spanner has been thrown in the works as the Local Authority Building Control association has now declared that they will not accept multi foil as achieving anywhere near the U value of 0.20 as a stand alone insulation product. The April 2008 report that has been circulated to all Local Authority Building Control departments makes the position very clear, Building Control officers cannot now accept multi foil in its own right as meeting Building Regulations on U values.

Please feel free to download the LABC Technical Guidance on Multi-foil insulation report here.

This would appear to indicate that whatever the merits of multi foil it is no longer an acceptable method of insulating a property on its own to meet Building Regulations U values. It would further appear from one British Board of Agrement certificate that the U value of multi foil is of the order 0.54 which is well outside Building Regulations where U values now start at 0.25 and below. So even a British Board of Agrement certificate will not help as this merely confirms the inadequacy of the product in meeting U values down to 0.20.

Some mulit foil manufacturers have responded that the method of testing for U values (the hot box) does not adequately test how multi foil performs in the real world but it is interesting that the National Physics Laboratory who are UKAS audited and who have tested multi foil are sticking with the scientific evaluation made (whilst leaving the door open for multi foil manufacturers to come up with an accepted scientific test methodology), multi foil as tested in the laboratory does not meet manufacturers claims. The bottom line is simply that if you wish to meet Building Regulations for U values you cannot achieve them in the eyes of LABC and the National Physics Laboratory by using multi foil insulation. Another reason to use tried and tested polyurethane spray foam where the results have already been evaluated and accepted.