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. Â
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.
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.