New Part L convention makes U-value calculations clearer for users

Lack of clarity on how to carry out U-value calculations for those specifying multifoil insulation has been resolved – thanks to the introduction of a new convention brought in under revised Part L of the Building Regulations this summer.


The updated BR443’s clear guidelines will also help building control officers and merchants, whose point-of-sale advice is much sought after by builders.

BRE’s Conventions for U-value calculations – aka BR443 2019 edition, despite its adoption in 2022, came into force in June in England and will be adopted later this year when respective Building Regulations are also changed in Scotland and Wales.

For anyone not wanting to read all 80 pages of the document, a free, one-hour CPD created by insulation specialist Actis, Tomorrow’s Insulation Solutions for Future Homes Standards, will cover all the key facts, alongside tips on how to meet targets laid down in the wider Part L revision.

Thomas Wiedmer, UK and Ireland technical director for Actis, said: “The new BR443, updated for the first time since the 2006 edition, clarifies how U-value calculations are carried out for multifoils and presents a clear, level playing field for specifiers to compare the thermal efficiency of various insulation products, including reflective insulation.”

“The main focus of the revised BR443 is that it gives clear guidance on how to insert product characteristics of multifoils, such as their thickness and thermal performance, within U-value calculations. Clarifying the thickness ratio impacts how much space, for air cavities or additional insulation, is left on either side of the products when installed. Build-ups using multifoils benefit from additional energy efficiency thanks to the associated air cavities.

“These revised calculation conventions will enable specifiers to compare apples with apples when looking at multifoils versus other forms of insulation. Under the previous version of BR443 it was not clearly regulated how multifoil thicknesses impacted associated air cavities and how bridging fractions were applied.”

Multifoils are typically installed in a continuous layer over or under rafters and then fixed by battens. Under the earlier BR443 convention, they were treated as just a single layer, while in the revised edition they are divided into separate layers. This means the multifoil thickness ratio expanding into both the rafter cavity and the batten cavity is reflected within the calculation.

Under the revised convention, in a roof scenario, one of these three layers shows that 30% of the multifoil will ‘balloon’ upwards into the rafter cavity and 70% will ‘puff’ downwards. In a wall construction 50% of the product will act in this way. This also ensures that the residual air cavity element is calculated precisely.

The model for each element of the build-up also includes the different bridging effect of the rafters and battens – elements which are now calculated more accurately.

If the compressed product thickness of a multifoil has been robustly determined under BS EN 832, it can be included within the U-value calculation as a separate layer.

Wiedmer added: “With residual air cavity guidance and puffing and pillowing of multifoils clearly defined, testing the robustness of the calculations is very clear. Now that this convention is part of the revised Part L everyone will need to follow it.”

About Anjali Sooknanan

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