Total Solar Energy Transmittance vs. Dynamic Thermal Characteristics of Non-Transparent Building Components

Abstract

Occasionally, there are suggestions from professional public to use the total solar energy transmittance coefficient, g (solar factor), to describe not only transparent, but also opaque structures, particularly with regard to overheating of the under-roof spaces. The standard EN 410:1998 (Glass in building - Determination of luminous and solar characteristics of glazing) introduces the g-value as the sum of primary solar heat gain, g1, due to the transparency of the glazing and the secondary solar heat gain, g2, due to the absorption of solar radiation and its conversion into heat conduction and radiation over the total incident solar heat flux, φe. Nevertheless the value of g1 may have zero or nearly zero value, e.g. in case of non-transparent glass. In addition to it, the standard ISO 15099:2003 (Thermal performance of windows, doors and shading devices - Detailed calculations) introduces equation for calculation of the frame g-value (actually the frame total solar energy transmittance), where window frames are clearly opaque components. What is then the difference between glass and "standard" opaque wall or roof? Why is in the latter case always introduced zero and in the first one some value different from zero? Won't it be practical, especially in time of large existing opportunities of computer use, to implement the use of g-values also in case of ordinary opaque structures and express their resistance to the absorption and conversion of solar radiation and thus overheating the adjacent interior spaces? This paper attempts, using EN ISO 13786 (Thermal performance of building components - Dynamic thermal characteristics - Calculation methods) and computer-aided models of transient heat transfer, to explain why the suggestion of using of the g-value in case of opaque components is not entirely correct and, why priority should be given to the dynamic thermal characteristics specified in this standard.