Measurement of Fenestration Net Energy Performance: Considerations Leading to Development of the Mobile Window Thermal Test (MoWiTT) Facility

The need for information about fenestration net energy performance under realistic conditions is discussed and methods of measuring it by means of a net energy balance are reviewed. A detailed consideration of the energy flows entering the energy balance on a building space and the effect of random measurement errors on the determination of fenestration performance is presented. Estimates of the error magnitudes are made for several hypothetical measurement situations and it is shown that a specialized, highly accurate test facility is necessary for reliable measurements on fenestration systems with thermal resistance in the range 2-10 times that of single glazing or shading coefficient less than 0.7. A test facility of this type, the MoWiTT, which has been built at Lawrence Berkeley Laboratory, is described. The effect of random errors in the MoWiTT is discussed and computer calculations of its performance are presented. The discussion shows that, for any measurement facility, random errors are most serious for nighttime measurements, while systematic errors are most important for daytime measurements. It is concluded that, for the MoWiTT, errors from both sources are expected to be small.