Publication date: Available online 19 April 2013
Source:Building and Environment
Author(s): Hamed H. Saber
A table is provided in the 2009 ASHRAE Handbook of Fundamentals (Chapter 26) that contains the thermal resistances (R-values) of enclosed airspaces. The ASHRAE table provides the R-values for enclosed airspaces of different thicknesses, effective emittances, mean airspace temperatures, and temperature differences across the airspaces. This table is extensively used by modellers, architects and building designers in the design for the R-values of building enclosures. The effect of the airspace aspect ratio (length/thickness) on the R-value is not accounted for in the ASHRAE table. However, previous studies showed that the aspect ratio of the airspace can affect its R-value. In this paper, the previous studies that focused on determining the R-values for vertical enclosed airspaces and horizontal enclosed airspaces with upward and downward heat flow are extended to investigate the effect of the aspect ratio on the R-values of 45o sloped enclosed airspaces under downward heat flow for different airspace thicknesses and having a wide range of values for the effective emittance, mean temperature, and temperature differences across the airspaces. The predicted R-values are compared with those provided in the ASHRAE table. Considerations are also given to investigate the potential increase in the R-values of enclosed airspaces when a thin sheet is placed in the middle of the airspace and whose surfaces have different values of emissivity. Thereafter, practical correlation is developed for determining the R-values of 45o sloped enclosed airspaces for future use by modellers, architects and building designers. The simplicity of this correlation for the sloped enclosed airspaces along with those that were previously developed for vertical and horizontal airspaces suggests that these correlations could be included in the ASHRAE Handbook of Fundamentals.
Source:Building and Environment
Author(s): Hamed H. Saber