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U-values adjustment to rainscreen subframes

How are U-values adjusted to account for rainscreen subframes?

Written by David Fraser, Rainscreen System - Business Unit Manager in SFS Group Fastening Technology Ltd.  


Thermal transmittances, or U-values, calculated for external walls with a rainscreen facade must be corrected in accordance with BRE Report (BR) 443, to take account of the thermal bridging effect of the rainscreen subframe system.


A separate blog post looks at how most U-values are calculated using the combined method, which is a simplified form of calculation. This post considers how the combined method is not sophisticated enough to account for the complex heat transfer through a subframe’s helping hand brackets, and what options are available instead to ensure accurate heat loss calculations.

However, the complexity of helping hand brackets is such that their effect on the U-value is much larger than a simple fixing. The U-value must therefore be adjusted in other ways.
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How does BR 443 adjust U-value calculations for rainscreen subframes?

BR 443 is a document describing industry conventions that should be used when calculating U-values. At the time of writing, the most recent version is the 2019 edition. Any construction professional who frequently works with U-values should have an understanding of BR 443.


It offers three routes by which a rainscreen subframe can be accounted for. The first is to carry out a detailed calculation for the complete wall build-up, thereby not using the combined method at all.


An alternative is to calculate a point thermal transmittance for any fixing bracket that penetrates an insulation layer (or a linear thermal transmittance for a fixing rail). This means the thermal performance of the bracket is modelled using a detailed calculation procedure, and its effect applied to a combined method calculation for the rest of the wall.