Steady-state and time-dependent heat fluxes through building envelope walls: A quantitative analysis to determine their relative significance all year round

Abstract

The selection of the best type of building envelope wall for a given location and the calculation of the heat flux exchanged with the exterior environment are key issues in avoiding undesirable heat losses and gains, thus contributing to energy saving. The total heat flux transmitted by conduction through opaque envelopes, driven by daily and seasonal variations in the outdoor air temperature and solar irradiance, can be regarded as the sum of steady-state and time-dependent contributions. This paper presents a novel analytical method to calculate and compare both heat fluxes for a full year and the most significant south and north wall orientations, considering the materials used and the location of the construction. The purpose is to provide builders with objective information to optimise the choice of the building envelope materials during the design stage, taking advantage of both steady-state and dynamic features of the walls. The quantitative analysis is applied to three types of envelope walls of buildings located at an intermediate latitude in the southern hemisphere and low altitude above sea level, but it can be extended to any other type of building envelope and geographical site. The effects of solar irradiance on heat conduction are considered using the equivalent sol-air temperature. The results show how the relative significance of each heat flux depends on the type and orientation of the wall and, consequently, the early comparison between the steady-state and dynamic heat approximations ensures the most suitable building envelope for the specific construction purposes.