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 relative strength index 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 relative strength index of enclosed reflective air spaces is the reciprocal of the overall heat transfer coefficient for the air space. This air space relative strength index is especially useful as input for the calculation of U-values for ceiling–roof assemblies located in hot climates.
Abstract:The objective of this research was to obtain thermal performance data for reflective insulations in a South East Asia environment. Thermal resistance data (RSI, m 2 · K/W) for reflective insulations are well established from 1-D steady-state tests, but thermal data for reflective insulation in structures like those found in South East Asia are scarce. Data for reflective insulations in South East Asia will add to the worldwide database for this type of energyconserving material. RSI were obtained from heat flux and temperature data of three identical structures in the same location. One unit did not have insulation above the ceiling, while the second and third units were insulated with reflective insulation with emittance less than 0.05. RSI for the uninsulated test unit varied from 0.37 to 0.40 m 2 ·K/W. RSI for a single-sheet reflective insulation (woven foil) varied from 2.15 to 2.26 m 2 · K/W, while bubble-foil insulation varied from 2.69 to 3.09 m 2 · K/W. The range of RSI values resulted from differences in the spacing between the reflective insulation and the roof. In addition, the reflective insulation below the roof lowered attic temperatures by as much as 9.7 ∘ C. Reductions in ceiling heat flux of 80 to 90% relative to the uninsulated structure, due to the reflective insulation, were observed.
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