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Optimizing building envelope properties results in efficient regulation of the heat transfer between the building’s exterior and interiors. Critical parameters in the building envelope include thermal performance of the exterior walls, roof, ground floor slab and fenestrations, and the air tightness of the buildings. The performance requirements of envelope parameters depend on factors such as cooling and heating demand, the mode of the building, i.e., whether the building is open, closed or hybrid. In combination with high levels of air tightness, reduced thermal bridging as well as high performance windows, savings in heating energy of up to 75-90% are possible in cold climates. In regions with mild winters, the need for winter heating may be eliminated altogether. Cooling loads can also be reduced by about half in hot climates and eliminated in regions with mild summers, if internal loads and solar gains are minimised.
The building envelope is the shell or skin of the building, through which the heat transfer, between the outside and the inside of the building, takes place. Building envelopes should minimise the negative impact on internal thermal comfort conditions through the influence of the external climatic conditions. In cool climates when internal spaces are heated, it is very important to make sure that the heat cannot escape through the building’s envelope. In hot climates, it is equally important to make sure that the outside heat cannot penetrate too easily into the internal spaces, thus increasing the cooling load. Energy savings due to optimized building envelope are more pronounced in buildings with considerable heat transfer through envelope compared to internal heat gains. Theoretical and practical application of efficient building envelopes has been proven a success especially in North American and European residential buildings.
Key parameters that influence the performance of building envelope are the thermal properties of opaque elements of building’s outer shell such as exterior walls, roof, on grade floor slab, doors; transparent elements such as windows, skylights; weak elements of heat transfer called thermal bridges and the reduction of infiltration by improving the air tightness of the buildings. Although these parameters need to be optimized, their design and exact specifications and cost effectiveness are highly dependent on the buildings function, mode and climate.
The heat flow through the building’s envelope can be improved or controlled by adding a layer of thermal insulation to the exterior of the building components such as walls, roof, and floor. These materials are of low density and lightweight. Some insulation materials are filled with special gases, which have higher a higher resistance to heat flow than air. Insulation materials can come in various forms whether mineral synthetic / petrochemical or natural materials. Mineral wool is a common insulation material (can be clicked upon for more information, see text box below) used for buildings and a number of other typical insulation materials exist, including aerated concrete, cellular glass, expanded and extruded polystyrene (EPS and XPS), polyurethane, cellulose fibres and fibre boards. More innovative insulation concepts include the dynamic insulation that has been in use in Scandinavia and Austria for some time as well as transparent and translucent insulation and vacuum insulation panels and nano fibres.
Thermal bridging occurs as a result of the weakly or discontinuously insulated joinery in an otherwise highly insulated surroundings. Breaking thermal bridges to a maximum possible extent is very crucial to achieve ultra low energy and plus/zero energy buildings. Special joinery details to break thermal bridges of all potential thermal bridges are now available in the market. Cooling and heating energy savings in the range of 4-8% are possible by effectively eliminating thermal bridges. In addition, eliminating thermal bridges also helps in reducing the risk of condensation arising due to uneven internal surface temperatures.
Windows are the least insulating element of the building shell. At the same time, they represent an important part of the building envelope. Windows let in daylight and admit passive solar heat to the internal spaces or indeed, lose heat to the outside. Larger windows allow increased amounts of daylight in to the space albeit at the cost of unwanted heat transfer. There is an optimum design for windows, which attempts to provide a balance between these energy flows. This balance is based on the orientation, location, obstructions and user requirements. Generally, between 25% and 45% glazing ratios are regarded as being the optimum. This means that no more than between 25 and 45% of the total facade area of a building should be glazed. Double or triple layered glazing systems with low e coatings offer cooling or heating energy savings in the range of 3 % to 10% compared to typical single glazing systems.
Air leakage is used to describe the condition when air transfer takes place through gaps and cracks in the building fabric and is common in most conventional buildings. In cold climates, air leakage can contribute 40% or more of the total heating load during of the building winter. In summer or in warm climates in general, this effect is a little more moderate. This is because the air exchange between inside and outside is increased by larger temperature differences between inside and outside which is the case in cold winters. The pressure difference between the top and the bottom of the building is greater here than in hot conditions. In winter the temperature difference between inside and outside might be 21K (+20 °C inside and -1 °C outside) and in summer or any other warm climate the difference might only be 12K (inside 25 °C and outside 37 °C). Controlling air leakage leads to savings in cooling and heating energy. For example, in the dry climate of Tehran, the cooling load can be reduced by about 12% with an envelope that is twice as tight as the average. Although, when the building is well insulated and shaded, this fraction would be higher (Harvey, 2006).