How are window U-Factors determined?
Before the advent of finite element computer thermal modeling tools, the only reliable way to determine thermal performance characteristics of complex frame-glass assemblies was in a “guarded hot box” test facility, constructed per ASTM C236. A “guarded hot box” is essentially a large, highly insulated room divided into a “cold side” and a “warm side.” An insulated wall separates the two, and it is into this wall that the test specimen is mounted. On the cold side, a large fan simulates a 15 mph wind directed at the exterior face of the test specimen. Cold side temperature is held constant at about 0°F, per ASHRAE standards. On the warm side, temperature is held constant at 70°F by a small electric heater. By measuring the energy necessary to heat the warm side, an indirect measurement of heat loss through the specimen (in BTUs per hour) is inferred. Because both the insulating wall surrounding the specimen and the warm side’s chamber walls are highly insulated against extraneous heat
