The thermal performance of windows with alternative glazing systems was compared by field testing in a side-by-side arrangement using portable calorimeters. Existing double-hung windows installed in an NBS test building were replaced with new sash and insulating glazings provided by the original window manufacturer. Two of the new glazing units had insulating glass with a low-emittance coating applied to one glass surface. The other two glazing units were identical, except there was no coating present.

Portable calorimeters were placed on the interior of the two north-facing windows, which included a low-emittance and an uncoated control window. Comparative measurements of air leakage, beat loss, and solar transmittance were made for these windows. Nighttime heat loss for the uncoated window was approximately 23% greater then that of the low-emittance window, although the U-values for both windows were greater than indicated by the manufacturer’s laboratory test results. Possible causes for the discrepancy between laboratory and field test data are evaluated, and the potential use of portable calorimeters for both comparative and absolute measurements of windows is discussed.

Units: Dual