Achieving the correct U Value using Multifoil Insulation

2 Minute read

• Materials such as Multifoil insulation do not have their ‘own’ U Value

• A U Value is the measurement of heat loss through a structure

A U value is a measure of how much heat is lost across an insulation build-up. This includes 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 the core R value of the 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 calculation

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 a U value of 0.18 W/m²K.

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