Reflective insulation assemblies for above-ceiling applications

Previously published hot-box data have been used to construct equations for the thermal resistance of enclosed reflective air spaces (reflective insulation assemblies) for a wide range of temperatures, air gap dimensions, thermal emittances, and heat flow directions. The thermal resistances or R-values (RSI) calculated with the equations compare favorably with previously published thermal resistances. Significant differences from RSI values (m² K/W) calculated using ISO 6946 were observed. Equations for calculating heat transfer coefficients for conduction–convection with constants for the heat flow directions up, 45° up, horizontal, 45° down, and down are contained in this article. The conduction–convection coefficient for planar air spaces oriented at any angle and heated above can be obtained by interpolation between heat flow down and heat flow at a downward angle of 45° or heat flow down at an angle of 45° and horizontal heat flow. The overall heat transfer coefficient is obtained by adding the thermal radiation contribution to the conduction–convection contribution. The RSI of enclosed reflective air spaces is the reciprocal of the overall heat transfer coefficient for the air space. This air space RSI is especially useful as input for the calculation of U-values for ceiling–roof assemblies located in hot climates.