OT-88-07-2 -- A Summary of Tested Glazing U-Values and the Case for an Industry-Wide Testing Program PDF

Since April 1986, the Seattle Energy Code and the Washington State Energy Code have required that all glazing installed in residential buildings comply with maximum tested U-values. Five hundred fifty test reports have been submitted to the Seattle Department of Construction and Land Use as of June 1988. From this information, it appears that tentative conclusions can be drawn about the effects of product size, product type, frame type, airspace width, and the use of screens on overall U-value. Yet, the test results also show a much greater variation in U-values than expected both for the same type of window made by different manufacturers as well as for different types of windows made by the same manufacturers. Coupled with the increasing use of a range of low-emittance coatings and various types of gas-filled airspaces, the calculation of U-values is necessarily a complex procedure with results that may well be questionable. To provide accurate information, a common test procedure and certification program should be developed and implemented.

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