Assessing the accuracy of predictive thermal bridge heat flow methodologies

Abstract

Heading towards more energy-efficient buildings, legislative requirements are becoming increasingly demanding. A decade ago, the thermal bridge effect was a known deficiency of building facades but, beyond design guidelines, no qualitative or quantitative requirements occurred in order to minimize its impact. The implementation of international standards in many national building codes has shown that the thermal bridge effect may significantly contribute to heat flows through building envelopes. Due to the multifaceted nature of heat flow through thermal bridges, its magnitude is usually estimated on the basis of predefined values or as a surcharge of the overall heat transmission loss. This study aims to investigate the differentiation and inaccuracies derived by the use of such simplified approaches with respect to the determination of the exact solution. In the context of this study, a multifamily building is assumed as a base case and its thermal bridge typologies are identified and analysed. By using an advanced numerical tool, the magnitude of thermal bridges heat flows is calculated for different building envelope thermal transmittance requirements (ranging from slightly to highly thermally insulated elements), representing the envelope thermal transmittance range currently found in most European countries. Results are compared to the respective ones derived from four prevailing methodologies highlighted in modern building codes aiming at calculating the heat flow through thermal bridges. This study reveals that existing approaches for implementing thermal bridges effect have not followed the progress achieved in thermal insulation requirements and the relevant framework should be updated to better support the effort for more rational and precise results.