A calorimetric field facility was developed to measure and compare night U-values of window systems for product evaluation and development. Each of three small, outdoor buildings has a removable window panel with a "portable" type guarded-box calorimeter on the inside of the window, suitable for side-by-side comparison tests on a regular basis~ Clear insulated glass and low-emissivity fixed casements from four manufacturers were tested. U-values and differences in U-values are presented and discussed, with emphasis on the advantages of direct comparison field testing along with an in-depth error analysis and comparison with calculated values. While U-value can be measured with acceptable accuracy, the real advantage of the test method is that of side-by-side testing in which the bias or systematic errors cancel out, so small average differences in U-value can be detected with statistical significance. This method is well suited for field testing for comparison of window systems; for night testing, this approach offers a fairly simple and inexpensive supplement to indoor lab testing and computer modeling.

Units: Dual