The comparison of a numerical and empirical calculation of thermal transmittance of ventilated facade with different heat-conductive connections
The energy efficient buildings are priority these days. The building energy efficiency requirements are increasing and thicker thermal insulation layer is used for building envelopes in order to achieve these high requirements. Ventilated facades are often used for the construction of public buildings, whose thermal insulating layers are crossed with connections made from various materials. Previously, steel and aluminium connections were used, whose influence on the heat transfer through the construction was high. When increasing the thickness of thermal insulation, the amount of connections should be increased, therefore the heat transfer will increase even more and such connections are no longer suitable for energy efficient buildings. Looking for less heat-conductive materials for connections, it has been observed that different calculation methods give different results of heat transfer, depending on characteristics of materials for thermal insulation and connections. In order to find out the reasons for these differences, calculations of ventilated walls with various connections were carried out.
The heat transfer through the wall with connections crossing thermal insulating layer could be calculated in two ways. The first way is simpler and was often used earlier - this is the calculation of the heat transfer coefficient increase depending on type and geometry of connection using the formulas presented in the standards. A more precise calculation method is the calculation of the heat transfer coefficient of wall with connections using two - dimensional and three - dimensional heat transfer modelling computer software (heat flow analyses). The use of these two calculations methods shows the differences of results of the heat transfer through ventilated walls.
For both calculations, the fragments of ventilated walls with connections made from aluminium alloy (λ - 160 W/(m∙K)), zinc coated steel (λ - 50 W/(m∙K)), stainless steel (λ - 17 W/(m∙K)) and glass fiber reinforced plastic (λ – 0,23 W/(m∙K)) were created. Calculations have shown that the heat transfer coefficient calculated for walls with zinc coated and stainless steel connections using different methods varies by 29 %, for walls with glass fiber reinforced plastic connections – by 0,4 %, and for walls with aluminium alloy connections - by 130 %.
The main aim of this paper is the investigation of reasons of different calculations results according to numerical and empirical calculations methods. When the causes of discrepancies are found, empirical formulas can be corrected.