Method of Measuring Nighttime U-Values Using the Mobile Window Thermal Test (MoWiTT) Facility

Although primarily designed for studying the dynamic net energy flows through fenestration systems over the full diurnal cycle, the Mobile Window Thermal Test (MoWiTT) Facility is also frequently used to measure nighttime U-values. 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 number of potential error sources is correspondingly larger. The method of deriving the nighttime U-value from directly measured data and the effect of random and systematic errors are discussed.