Detailed Thermal Performance Data on Conventional and Highly Insulating Window Systems

Data on window heat-transfer properties (U-value and shading coefficient [SC]) are usually presented only for a few window designs at specific environmental conditions. With the introduction of many new window glazing configurations (using low-emissivity coatings and gas fills) and the interest in their annual energy performance, it is important to understand the effects of window design parameters and environmental conditions on U and SC. This paper discusses the effects of outdoor temperature, wind speed, insulation, surface emittance, and gap width on the thermal performance of both conventional and highly insulating windows. Some of these data have been incorporated into the fenestration chapter of the ASHRAE Handbook - 1985 Fundamentals.

The heat-transfer properties of multiglazed insulating window designs are also presented. These window systems include those having (1) one or more low-emittance coatings, (2) low-conductivity gas-fill or evacuated cavities, (3) a layer of transparent silica aerogel, a highly insulating microporous material, or (4) combinations of the above. Using the detailed building energy analysis program, DOE 2.1B, we show that these systems, which all maintain high solar transmittance, can add more useful thermal energy to a space than they lose, even in a northern climate. Thus, in terms of seasonal energy flows, these fenestration systems out-perform insulated walls or roofs.