Determining the Thermal Resistance of Enclosed Reflective Airspace

Abstract

Enclosed airspaces of various effective emittances exist in the building envelope in walls, roofs, and double/triple glazing windows, curtain walls and skylight devices. Assessing the energy performance of a building component with enclosed reflective airspaces requires evaluation of all modes of heat transport inside the airspace. The thermal resistance (R-value) of the enclosed airspaces depends on the dimensions and orientation of the airspace, and the emittance and temperature of all surfaces that bound the airspace. To the best of our knowledge, the existing methods used around the world to evaluate the airspace thermal performance (e.g., ISO 6946, AUS/NZ 4859, and methods based on the U.S. NBS data) are one-dimensional and assume isothermal conditions on the hot and cold surfaces. In actual applications, however, the temperatures of both the hot and cold surfaces vary (i.e., they are non-isothermal), and the heat transfer takes place via conduction, convection and surface-to-surface radiation. In some cases, convection is absent or negligible, as is the case for downward heat flow or situations with a small temperature difference between the hot and cold surfaces. Radiation transport is significantly reduced by the presence of a low-emittance surface in the heat flow path. One of the goals of this study is to answer the question “is it a good assumption to use the isothermal conditions on both the hot and cold surfaces for determining/reporting the R-value of enclosed airspaces for different building applications?” A complete evaluation of the thermal performance of enclosed airspaces that includes the impact of bounding materials can now be undertaken in multiple dimensions with convective transport described by computational fluid dynamics. In addition to providing a relatively complete evaluation of the thermal performance of the enclosed airspaces, the adequacy of popular simplifying assumptions can be evaluated. This paper describes the computational process used, along with examples of the variation in thermal resistance with the airspace aspect ratio, realistic thermal boundary conditions and radiative heat transport on all surfaces that bound the airspace. The analysis completed in this research shows that the assumption of isothermal hot and cold surfaces affects calculated R-values for enclosed reflected airspaces by less than 3%. This was demonstrated for the five conventionally considered heat-flow directions and effective emittances from 0 to 0.82.