There has been considerable debate over the development of simplified calculation methods for thermal resistance (R-value) and thermal transmittance (U-value) for light steel frame constructions that designers can easily use. Such simplified methods have presented a particular difficulty for light steel framing constructions as compared with many other forms of construction because they must accommodate the effects of non-homogenous layers and thermal bridges where the difference between the thermal conductivity of materials is large.

This paper describes the development of a simplified method for calculating U-values of prefab house suitable for incorporating into U-value calculation software. The new method is based on research carried out by returning to first principles to use finite element modelling to analyse the heat flows through steel framed constructions, and comparing these with predictions made by simplified methods. It was found that for warm frame constructions, where all the insulation is placed outside the steel frame, the normal U-value calculation method as set out in BS EN ISO 6946 can be applied. However, for constructions where some or all the insulation is placed within the thickness of the steel frame, the methodology in BS EN ISO 6946 has to be adapted to allow for the increased effect of the steel.

A new method is proposed that is similar in principle to that used in BS EN ISO 6946 but adapted to increase accuracy for these types of construction. It is designed to be easily implemented in U-value calculation software. It was found that with the proposed method the mean error of prediction compared with finite element modelling is less than 3% with a maximum error of 8% for a range of 52 assessed constructions. This method has been adopted for use in the UK to demonstrate compliance with the amendments to the thermal Building Regulations.