Calculation of thermal bridges in (Nordic) building envelopes – Risk of performance failure due to inconsistent use of methodology

Abstract

Reduction of energy use in buildings is an important measure to achieve climate change mitigation. It is essential to minimize heat losses when designing and building energy efficient buildings. For an energy-efficient building in a cold climate, a large part of the space heating demand is caused by transmission losses through the building envelope. Therefore, calculations of these must be carried out in a correct way to ensure a properly sized heating system and a good indoor climate. There is today a risk of misunderstanding and inconsistent use of methodology when transmission heat transfer is calculated. To investigate the state of knowledge among Swedish consultants a survey was conducted regarding thermal bridges and calculations of transmission heat transfer. Furthermore, the impact of thermal bridges was studied by comparative calculations for a case study building with different building systems and different amounts of insulation. The study shows that the relevant standards and the building code in Sweden are interpreted in many different ways regarding calculation of transmission heat transfer and energy performance. There is a lack of understanding regarding the impact of different measuring methods on thermal bridges. When more insulation is used the relative impact of thermal bridges increases. It is therefore not suitable to use a single predefined percentage factor, increasing the transmission heat transfer through building elements, to account for the effect of thermal bridges. If values for normalized thermal bridges are to be used, they need to be differentiated by building system and different amounts of insulation