The effect of flanking element length in thermal bridge calculation and possible simplifications to account for combined thermal bridges in well insulated building envelopes

Abstract

A well-insulated, airtight, thermal bridge free building envelope is a key factor for nearly zero energy buildings (nZEB). However, increased insulation thickness and minimised air leakages increase the effect of thermal bridges on the overall energy efficiency of the nZEBs. Currently, the calculation of linear thermal transmittance follows ISO 10211, which requires the separate numerical assessment of combined thermal bridges, where two or more junctions are positioned in close proximity within delimiting cut-off planes. This poses a practical problem, as the number of different combinations (mainly related to window to wall connections in combination with corners, intermediate ceiling etc) is too large in practice to follow the standard procedure.In this study, a parametric numerical assessment of different thermal bridges in well-insulated constructions showed that with linear thermal transmittance up to 0.2 W/(mK) in lightweight construction and up to 0.1 W/(mK) in heavyweight construction the reduction of flanking element length from 1.440 m to 0.288 m (80% reduction) had no effect on numerically calculated linear thermal transmittance in steady-state conditions. For thermal bridges inside heavyweight construction with linear thermal transmittance below 1.1 W/(mK) the flanking element length could be reduced by 50% without any effect and by 70% with marginal effect (<2%) on linear thermal transmittance.The shorter flanking element length, roughly equal to its thickness, can then be used to minimise the amount of combined thermal bridges in complex building envelopes. Compared to ISO 10211 requirements, the amount of different combined thermal bridges in two exemplary building envelopes was reduced by 35% to 76%, depending on the building type, and allowed deviation of 0.5%, 1.0% or 2.0%, thus significantly reducing the working hours of practitioners.