Simplified Infinite Fin Method to Model the Heat Transfer Associated with Slab Edges and Balconies

Abstract

As building codes become more stringent in terms of thermal performance of building envelopes, and higher insulated wall assemblies are becoming more common, the heat flow due to major thermal bridges can contribute to a significant portion of the total heat transfer through a building façade. Characterizing different thermal bridging elements is essential not only to capture the thermal resistance of wall assemblies and understand the thermal efficiency of buildings, but also in terms of understanding the impact of each thermal bridging element and mitigation strategies that can be used. Numerical simulations are used widely to characterize different thermal bridging elements. However, not all designers have access, technical skills or time to complete numerical simulations to calculate the heat transfer loss through thermal bridges. In this study we propose an analytical method to integrate the effect of adding a slab edge/balcony/eyebrow into a clear-field wall assembly. The additional heat transfer due to the slab edge is calculated by considering the slab edge to be an infinite fin. The additional heat transfer is integrated into the clear-field as a quasi-convective heat transfer coefficient. The overall thermal resistance of the wall assembly is calculated by employing the parallel path method. Comparing the results obtained from this method with the numerical simulations which were benchmarked against guarded hot box results, an overall deviation of 1 to 8 percent was observed.