Home Insulation Knauf Tech Talk: U-value calculations explained

Knauf Tech Talk: U-value calculations explained

Bradley Hirst, Technical Services Manager at Knauf Insulation.

Approved Document L (Part L) of the Building Regulations sets out how energy efficient buildings and individual building elements must be. But how do you know whether the build-up meets the required minimum standards? In his latest column, Bradley Hirst, Technical Services Manager at Knauf Insulation, looks at U-value calculations in more detail.

What are U-value calculations?
U-value calculations measure whether the proposed build-up will meet the minimum standards by determining how well it will prevent heat transfer. They are measured in W/m²K (watts per metre squared Kelvin) and the lower the U-value, the more effective the build-up is. This means less energy will be needed to heat or cool a space, so a property with high levels of insulation is more energy efficient.

Do I need a U-value calculation for every project?
Not necessarily. Every project must meet the limiting standards for fabric energy efficiency, so architects usually include a specification in their plans. For insulation, they may list a product name or the thermal resistance (R-value) which has been verified to achieve compliance. If you follow the specification exactly, you won’t need to undertake additional U-value calculations.

“If you change any element, you will need new U-value calculations to demonstrate compliance”

But you may need to change the specification. For example, on a rainscreen façade system build-up, you may decide to improve the building’s fire safety strategy by swapping combustible insulation for a non-combustible solution such as Knauf Insulation’s Rocksilk RainScreen Slab. If you change any element, you will need new U-value calculations to demonstrate compliance.

Numerical calculations (3D U-values) are highly accurate so should be used for rainscreen façade systems rather than the simplified method.
Knauf Insulation Rocksilk(R) RainScreen Slab is non-combustible making it suitable for use on buildings of any height.

For applications such as cavity walls or flat roofs on extensions, many installers draw on experience and use the same build-up for every project. This may save time but remember Part L changes in June 2022, so check with your insulation manufacturer’s Technical Team that your chosen build-up meets the new minimum standards.

Are all U-value calculations the same?
How U-values are calculated is set out by BRE’s BR443 (Conventions for U-value calculations). There are two common methods: simplified calculations and numerical calculations (3D U-values). Which method is used depends on the application and the accuracy required.

Simplified calculations are best for straightforward build-ups with few variables such as brick and block walls. But they aren’t recommended for rainscreen façade systems because the blanket correction factor doesn’t accurately represent the system’s thermal performance.

Instead, contractors and installers should insist on 3D U-value calculations which consider the complex interaction between the insulation, façade, and rail and bracket system. This high degree of accuracy often identifies solutions that would have been ruled out by the simplified method and ensures the thermal performance of the final building matches its design.


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