Steel Buildings in Europe

Part 8: Building Envelope 8 - 17 Table 3.1 Limiting U-values Element Area weighted average ( Wm -2 K -1 ) Wall 0,35 Roof 0,25 Window 2,2 Pedestrian door 2,2 Roof ventilator 6 Over recent years, the drive to improve the energy performance of buildings has resulted in a significant reduction in the U-values for building envelope elements, resulting in a considerable increase in insulation thickness. This has had important implications for the structural performance of the cladding system and its relationship with other structural elements. Of particular concern to the structural engineer are the increased depth and weight of the cladding and its ability to adequately restrain the purlins or side rails. Inevitably the trend will continue towards improved thermal efficiency. However, the diminishing returns obtained from further reductions in U-values means that in future more emphasis is likely to be placed on airtightness and the performance of mechanical services, rather than ever increasing insulation thicknesses. While some countries have adopted the U-value as the preferred means of quantifying the performance of the envelope, elsewhere the chosen parameter is the R-value or thermal resistance. The R-value is simply the reciprocal of the U-value and the points noted in the preceding paragraphs are equally applicable in these countries. Typical U-values for different cladding systems are shown in Table 3.2. Table 3.2 Typical U-values for cladding Element U-value ( Wm -2 K -1 ) Built-up system, 180 mm insulation 0,25 Built-up system, 210 mm insulation 0,2 Composite panel, mineral fibre, 120 mm 0,34 Composite panel, mineral fibre, 150 mm 0,27 Composite panel, PIR, 60 mm 0,33 Composite panel, PIR, 100 mm 0,20 3.3.3 Thermal bridges Thermal bridges are areas or components within the roof or wall cladding assembly whose thermal insulation properties are lower (often much lower) than those of the surrounding material, thereby permitting local high heat flows through the building envelope. A common example of a thermal bridge would be an all-metal spacer in a built-up cladding system. In general, all metal components will act as thermal bridges, because of their high thermal conductivity, unless specific measures are taken to interrupt the heat flow by introducing a layer of thermal insulation. Thermal bridging increases the heat