The Development of a Simplified U-value Calculation Method for Twin Skin Metal Cladding Systems

Abstract

This paper describes an investigation to develop an appropriate, simplified U-value (thermal transmittance) calculation method for metal cladding systems. Since the introduction of the 2002 changes to the thermal requirement of the Building Regulations in the UK the lack of an appropriate simplified method meant that these constructions had to be assessed using complex numerical analysis. This is expensive and complicated. As a result, appropriate simplified methods of calculating U-values for such construction were investigated. Numerical analysis was carried out for twenty-three typical constructions, and the results compared to a simplified method originally developed for light steel frame construction. The results demonstrated that good agreement is achieved and consequently the simplified method has been proposed as an acceptable way of meeting the requirements the 2002 editions of Approved Documents L1 and L2. The method enables U-values for the relevant constructions to be calculated easily using simple algorithms. Software is available, incorporating these simple algorithms that can be readily used by designers to carry out the U-value calculations. Alternatively, graphs are available that can be used to determine U-values for typical specifications. The proposed method is based on the principles in BS EN ISO 6946 for other constructions but with some important differences to account for the effect of the linear and point thermal bridges.