Achieving the correct U Value using Multifoil Insulation


A U value is a measure of how much heat is lost across an insulation build-up – insulation materials, rafters, air gaps and all.

Materials by themselves e.g multifoil insulation don’t have U values; they have R values which are a measure of the resistance to the flow of heat – the very opposite idea to the U value. And this is hardly surprising since the U value equals 1 divided by the R’s of all the materials added up in a certain way [U = 1/(all the R’s)].

So a particular product of, say, Rigid insulation board (PIR) that is 100mm thick will have twice the R value of one that is 50mm thick.

So can you just add up the R values of all the layers? 

Only if you have continuous layers next to each other (as in a warm deck flat roof). Otherwise you need to have specialised software or else rely on the manufacturers’ specifications.

[For those of you that remember electricity from school science, it’s just like adding the resistances up both in series and parallel but with heat rather than electrical current].

Why do some multifoil manufacturers give different R values depending where they are used and others just give core R values? 

Multifoils rely on reflecting heat back across unventilated air gaps next to them, and these air gaps also have R values – but they depend on whether the air gaps are horizontal or vertical, and also on their depth – though unlike the example above, it’s not a case of twice as thick, twice the resistance – software is needed to calculate it. 

The core R Value is key

So the R value quoted is in fact the core R value of the actual product plus two optimum thickness air gaps. However, when it comes to the testing required for BBA certification, it’s the core resistance that is tested and quoted – and this is related to the thickness of the product. Read more on R values here.

We can help you with a bespoke calculationbespoke u value calculations by tlx insulation

So unless you have the U value calculator to cope with air gaps and R values, it’s perhaps wisest to simply look at the Approved build-ups that give the U values required for particular situations.

For a pitched roof with rafter-level insulation current Building Regulations require refurbishments and extensions to achieve a U value of 0.18 W/m2K.

If you would like a bespoke U Value calculation, call us on the number below, or email:

tlx insulation technical helpline