Radiant heat and thermal comfort in vehicles

Abstract

Two layers of an infrared-reflective (IRR) film were used in alternating combinations (no treatment, one layer, and two layers) over the driver-side window of a stationary car. In Phase 1 of the experiment, cabin air temperature was held constant at one of two levels (75° F and 90° F [24° C and 32° C]) while subjects rated their thermal comfort. In Phase 2, air temperature was adjusted by fixed steps according to the subjects’ responses (“too hot” or “too cold”). In Phase 1, the IRR treatment significantly improved localized thermal comfort (on the left forearm, which was exposed to direct solar irradiance), but not whole body thermal comfort (although ratings for whole body comfort followed the expected trend). In Phase 2, IRR treatment resulted in subjects indicating that they were comfortable at an average air temperature of 2.5° F (1.4° C) higher than in the untreated condition. The results indicate that reducing radiant heat by the application of an IRR treatment affects subjective assessments of thermal comfort and allows occupants to maintain the same level of comfort in a warmer vehicle cabin. This may imply greater fuel economy savings than have previously been estimated because less cooling of the cabin is required in a vehicle with IRR-treated glazing. The range of conditions investigated in this study was limited, and the results should therefore be considered preliminary. Future research should examine how the following factors influence the relationships observed in this study: the percentage of IR rejection provided by the treatment, the total surface area of the treatment, the duration and amount of the subjects’ irradiant exposure, and a wider range of ambient weather conditions.