Steel Buildings in Europe

Part 8: Building Envelope 8 - 18 loss from a building, thereby increasing the operational energy requirement. It can also lead to a reduction in the internal surface temperature of the cladding, causing condensation to form under certain conditions. 3.3.4 Airtightness The airtightness of a building is central to the requirements of the building regulations and is likely to become even more important as architects strive to improve the thermal performance of the building envelope without significant increases in insulation thickness. The airtightness of a building is quantified in terms of its air permeability, which is defined as the volume flow rate of air per square metre of building envelope and floor area at a given pressure. The maximum permissible air permeability for a given building will depend on a number of factors including the requirements of the building regulations, the specified CO 2 rating for the building and the means by which this rating is to be achieved (e.g. the architect may specify a very low level of air permeability as an alternative to increasing the thickness of insulation). In many countries, achievement of the specified air permeability must be demonstrated by post-construction testing. 3.4 Interstitial condensation Interstitial condensation occurs within the layers of the cladding construction and is due to warm moist air from within the building penetrating the liner and condensing on the cold outer sheet and other components. The severity of the problem will depend on the relative humidity of the air within the building, the external air temperature and humidity, and on how well the liner is sealed. Buildings in cold climates and those containing swimming pools, laundries or other similar applications are most at risk, as are cladding systems that incorporate a perforated liner and separate vapour control barrier. In extreme cases, the condensation could result in corrosion of steel components within the roof assembly or in wetting of the insulation. Recommendations for avoiding interstitial condensation are usually given in National Standards. 3.5 Acoustics Depending on the application, acoustic performance can be an important consideration when specifying roof and wall cladding. There are three categories of acoustic performance to consider, as illustrated in Figure 3.3. 3.5.1 Airborne sound transmission Where there is a need to limit the passage of sound through the building envelope, the cladding specifier needs to consider the Sound Reduction Index (SRI) of the cladding. The SRI is a measure of the reduction in sound energy (in decibels) as sound passes through a construction at a given frequency. The acoustic performance of a particular cladding system will depend on the insulation material, the weather sheet and liner sheet profiles and the method of assembly. Of these, the insulation is the dominant factor, with soft mineral wool insulation giving better sound insulation than rigid board (dependent upon density).