Assessing the Effect of Air Intrusion on Reflective Insulations Performance with Horizontal Heat Flow

Abstract

The determination of thermal resistances (R-values) for enclosed airspaces, including those with one or more low-emittance surfaces, has advanced from one-dimensional heat transfer between large parallel planes to multi-dimensional heat flow in a wide variety of physical configurations. The key elements in this advancement, however, are the evaluation of the heat flow due to conduction-convection and the solution for radiation that includes all surfaces bounding the region of interest. The model used in this study has been validated against several sets of laboratory test data, including the data from the U.S. National Bureau of Standards for the thermal resistance of airspaces, which has been the basis for handbook values for reflective airspaces for five decades. In addition, this model has been previously used to determine the reductions in the R-values of reflective insulations assemblies due to imperfect installations and internal defects in multilayer reflective systems with cross-airflow between the layers. In this study, the model is used to examine the impact on R-value of air intrusion of different air changes per hour (ACH) at various exterior air temperatures into reflective insulation assemblies with a range of effective emittance from 0 to 0.82. Two cases are considered in this study. In the first case, called “infiltration”, exterior air enters the assembly through an opening located in the hot side of the assembly and exits through another opening located in the cold side of the assembly. In the second case, called “wind washing”, the exterior air enters the assembly through an opening located in the hot side of the assembly and exits through another opening located in the same side of the assembly. To quantify the reductions in the R-values due to infiltration and wind washing conditions, a reference case is included in this study for the case in which no air intrusion occurs in the reflective insulation assemblies. Finally, consideration is given to investigating the effect of the aspect ratio on the R-values of reflective insulation assemblies without air intrusion and with air intrusion of different ACH at various exterior air temperatures. The results show that the aspect ratio has a significant impact on the R-value of reflective insulation assemblies with and without air intrusion. Additionally, the results show that the impact of infiltration and wind washing on R-values of reflective insulation assemblies increase as the difference between the exterior air temperature and the undisturbed temperature of the cavity increases.