Dynamic modelling of thermal bridges: Impact on the energy performance of low-energy buildings

Currently, scarcity and cost of fossil fuels increase and we are facing more and more problems due to pollutants emissions. In response, the European Union has set energy savings and greenhouse gases reduction targets of 20% by 2020, from the 1990 levels. As the building sector represents about 40% of the total energy consumption of the European Union, many energy efficiency measures are taken in this domain. Simulations are performed to evaluate, predict or improve energy performance of new buildings thanks to numerical tools. Their accuracy must be improved as the building energy consumption becomes lower and lower.
This thesis focuses on the dynamic modelling of thermal bridges and multidimensional details of low-energy buildings. Thermal bridges may be responsible for 4% up to 39% of the heat losses of a building. In most of the building energy software packages, the heat flux is considered as being 1-D and the real dynamic and multidimensional effects of the thermal bridges are not considered. The aim of this work is to develop a simple model accurately evaluating the impact of these effects on the building energy performance and easy to integrate into existing building energy software.
To meet these specifications, the equivalent wall principle is used: the multidimensional geometry is replaced by a 1-D three-layer equivalent wall, having a similar thermal behaviour. A new method is proposed, using the transfer functions in the frequency domain in addition to the total thermal resistance, the total heat capacity and the structure factors, to determine the thermo-physical properties of each fictitious layer. These properties are thereafter introduced into the building energy software.
This equivalent wall method is validated on a 3-D corner and five particular 2-D thermal bridges in light or heavy construction for hourly meteorological data (temperature and solar heat flux) of four different climates and for a constant or variable indoor temperature. Moreover, a technique of geometry reduction to lower the calculation time without losing accuracy and a procedure to solve thermal bridges with three zones of temperature are also validated.
Finally, six thermal bridges are selected in a passive wooden-structure house, their equivalent walls are validated and the impact of the use of this method on the energy behaviour of this building is evaluated, in comparison with a classic static consideration of the thermal bridges.