Steel Buildings in Europe

Part 8: Building Envelope 8 - 16 emissions and meeting this obligation will require significant changes in many sectors of industry, especially construction. A significant proportion of carbon dioxide emissions in Europe is related to the operational energy requirements of buildings (heating, lighting, ventilation etc.). This issue is addressed by European Directive 2002/91/EC: Energy performance of buildings [7] . Although many factors influence a building’s energy efficiency, the thermal performance of the building envelope is significant. Consequently, it has been sought to reduce energy consumption by improving the thermal performance of the cladding and associated components. The main sources of heat loss through the building envelope are shown in Figure 3.2. 1 2 3 1 Thermal bridge (metal spacer) 2 Thermal transmittance through insulation 3 Air leakage through joints Figure 3.2 Main sources of heat loss through the building envelope 3.3.2 Thermal transmittance Thermal transmittance through the building envelope can be a significant source of energy loss within a building, especially if there is insufficient insulation. One measure of thermal transmittance is the “U-value”, which is defined as the rate of heat transfer through an element of the building envelope (e.g. a wall, window, section of roof or rooflight) per square metre. The SI unit for the U-value is W/m 2 K. For an individual component such as a cladding panel, the elemental U-value depends on the conductivity and thickness of the insulation, the profile shape and the presence of thermal bridges. Cladding and insulation manufacturers usually quote U-value for their products for a range of insulation thicknesses. Alternatively, the U-value of a given built-up of envelope may be calculated using software. National regulations generally specify maximum U-values. These are often the weighted average (or similar “overall” figure) for the whole of the roof or wall, with maximum values for individual elements such as doors. The individual elements tend to have much higher U-values than the cladding. Typical limiting U-values are shown in Table 3.1.