AD 258: U-values for Light Steel Frame Construction

There are now several companies supplying light steel framing to the housing sector, and many developers and RSLs (Registered Social Landlords) are actively using this technology. With the implementation of the 2002 editions of the Approved Documents L1 and L2 for the Conservation of Fuel and Power, U-values for most construction will now have to be calculated by the method given in BS EN ISO 6946 (Combined Method). However, BS EN ISO 6946 specifically excludes from its scope constructions in which insulating layers are bridged by linear metal elements. Therefore it is directly applicable to light steel frame designs only if these are “warm frame” construction, which places all the insulation outside the frame. It is not directly applicable for the many light steel frame designs in which some or all the insulation is placed within the thickness of the steel frame.

U-values for frames that are not “warm frame” construction may be calculated using a new method, described in detail in BRE Digest 4651. This method is based on research carried out by BRE and SCI, together with several key suppliers, returning to first principles to analyse the heat flows through steel framed construction. The method is generally similar to that used in BS EN ISO 6946 and is designed to be easily implemented in U-value calculation software. In addition, BRE is producing downloadable U-value calculation software for use to demonstrate compliance with the new Approved Documents L1 and L2 (http://projects.bre.co.uk/uvalues). This will include separate routines for light steel frame construction enabling U-values to be easily calculated.

S.M. Doran, & M.T. Gorgolewski, 2002, U-values for Light Steel Frame Construction, BRE Digest 465, BRE