Thermal Resistance of 30° Sloped, Enclosed Airspaces Subjected to Upward Heat Flow

Abstract

Heat transmission across an enclosed space is determined by the type of filling gas, the temperatures and long-wave emissivities of all surfaces that define the space, and the heat flow direction. The ASHRAE Handbook of Fundamentals provides the thermal resistances (R-values) of enclosed airspaces with only five effective emittance values (E) for vertical, horizontal, and 45° airspaces. ASHRAE R-values do not include the case of 30° sloped airspaces. In addition, ASHRAE R-values ignore the impact of the airspace aspect ratio (A) on R-values. However, many previous studies, as well as this study, have shown that A can have a significant effect on the R-value. Previously, correlations were developed for determining the R-values for vertical (90°) airspaces subjected to horizontal heat flow, horizontal (0°) airspaces subjected to up and down heat flow, 45° airspaces subjected to up and down heat flow, and 30° airspaces subjected to downward heat flow. To the authors’ knowledge, no such correlation existed for determining the R-value of 30° airspaces subjected to upward heat flow, which is developed in this paper. The potential increase in R-value by placing a thin layer of varied emittance on both sides in the middle of the airspace was also considered. Architects and building designers can use the developed correlation to compute the R-values of airspaces of various values for A and E and various operating conditions. This correlation along with the previous correlations can be included in the current energy models (e.g., EnergyPlus, ESP-r, DOE, and Design Builder).