BA-92-10-1 -- Method of Measuring Nighttime U-Values Using the Mobile Window Thermal Test Facility PDF

States that although the Mobile Window Thermal Test Facility (MoWiTT) is designed primarily for studying the dynamic net energy flows through fenestration systems over the full diurnal cycle, it is also frequently used to measure nighttime U-values. Notes that these measurements have the advantage of incorporating the exterior film coefficient resulting from the true ambient conditions at a particular time and location, rather than relying on a laboratory simulation of some assumed average or extreme condition. On the other hand, the MoWiTT is a much more complicated facility than a laboratory hot box, and the sources of potential error are correspondingly greater. Discusses the method of deriving the nighttime U-value from directly measured data and the effect of random and systematic errors.

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