Numerical study of heat losses of building walls containing reflective foils

Abstract

In external building walls, air cavities are occasionally used to reduce heat losses. Closed air cavities improve the thermal resistance of building structures, especially when the inner surfaces of the cavities are covered with low-emissivity materials such as metalized foils. With these structures, the problem of accurate determination of radiative heat arises. In this contribution, radiative heat transfer is determined by the precise algebraic radiosity method, so far not applied in the field of thermal building technology. Radiative heat computed by the radiosity method has been coupled with convective heat evaluated by the correlation functions of the Nusselt numbers. The combined radiative-convective heat transfer through a sandwich wall containing an air cavity with metalized surfaces (aluminium foils) has been analysed. A computer program has been developed to quantify the real insulation effect of reflective aluminium foils. The sandwich building structure has been subjected to real one-month winter conditions as defined by official meteorological data valid for the city of Brno. The software is based on an iterative procedure that solves the combined radiative-convective steady heat transfers by means of one-day average temperatures and wind speeds. Various arrangements of foils in the cavity have been explored to achieve the maximum value of thermal resistance. The corresponding heat savings have been quantified. The best configuration of foils in the cavity has been found to ensure maximum thermal savings. The insulation system that uses foils has been compared to the system that uses mineral wool. The comparison was based on economic considerations.