Evaluation of reflective insulation systems in wall application by guarded hot box apparatus, and comparative investigation with ASHRAE and ISO 15099

Abstract

Abstract In view of discrepancies in former studies on reflective airspaces and reflective insulation systems, two innovative guarded hot box experiments (as the most credible experimental testing method) were conducted in this investigation. Full-scale testing of innovative wall configurations of reflective insulation systems with guarded hot box apparatus were performed according to the ASTM C1363 in energy laboratory of BHRC. In order to verify the assessment methods, and conduct a comparative survey with the most credible calculation methods, the results were compared with ASHRAE data, and with results derived from ISO 15099. Two EnergyPlus models using zone heat balance method and ISO 15099 (as the interior convection algorithm) were developed according to the specifications of the tested specimens. Simulations results showed 56.1% and 60.1% differences from the first and second guarded hot box experiment results respectively. Comparisons proved that in spite of good agreement between results derived from ASHRAE and ISO 15099, these calculation methods significantly overestimate the airspace R-values (up to 85.1%). Simplifications resulting from presumptions related to the computation methods, such as one-directional heat transfer by negligence of thermal bridges inside the cavities, and isothermal assumption of bounding surfaces of the airspace, are the main reasons of incompatibilities (the guarded hot box tests showed average 10.1% temperature variations in each interior surfaces of the assembly). The greatest temperature variations is seen in the surfaces of the reflective insulation layers, which shows the influences of the surface properties on the interior convection currents, which is not considered in ASHRAE and ISO 15099 correlations. The guarded hot box investigations also proved that the thermal conductivity of bubble foil insulation materials highly depends on temperature conditions, which can be possibly due to the enclosed air gaps inside the insulation material, and needs complementary studies. Moreover, imperfect airtightness of interior layers, and non-uniform thickness of interior air cavities increase the influences of inevitable thermal bridges in common real application of reflective insulation systems.