Optimal thickness determination of insulating air layers in building envelopes

Abstract

In building envelopes, using enclosed air layer becomes a popular way of thermal insulation, since the air has a much lower conductivity coefficient, and is much cheaper than other building materials. The motivation of this research is to investigate the coupled convective and radiative heat transfer in the air layer of building envelopes, in order to find the optimal geometrical parameters of the insulating air layers. Based on CFD technology, a Ra number judgment basis is summarized for flow pattern judge in insulation air layers, and the coupled convective and radiative heat transfer characteristics across the air layer is analyzed. The results indicated that a larger layer height results in a weaker convective heat transfer; when the thickness is below 20mm, an increasing thickness leads to a considerable decrease in the heat transfer; but when thickness exceeds 20mm, the heat transfer is slightly influenced by the thickness. The optimal thickness of the insulation air layer is 20-30mm depending on the climate condition.