Improved model for the thermal performance calculation of non-planar window frames for building simulation programs

摘要

Windows are a key factor for designing energy-efficient buildings, particularly the frame area that can produce high thermal bridging. This paper deals with accurately estimating heat transfer through window frames under fluctuating film coefficients. The one-dimensional frame conductance model traditionally used by building simulation programs is analysed and an alternative model is proposed, which takes into account the non-planar morphology of studied frames. This model shows a positive agreement with the results obtained from a two-dimensional heat-transfer simulation program, demonstrating that the thermal performance of high-conductance non-planar frames strongly depends on the ratio between the boundary surface area and the projected frame area. According to the results, the traditional conductance model seems to be suitable for all frames with a thermal transmittance lower than 5W/m(2)K; however, frames with a U-factor higher than 6.2W/m(2)K need an alternative conductance model that better reflects the 2D nature of frame sections.