A procedure has been developed for calculating the one-dimensional heat flux through complex windowssystems. A computer program embodying this procedure (WINDOW 2.0) is available to the public. Theswindow may consist of multiple glass or plastic layers, separated by gas-filled spaces. Gases other than airsmay fill the space between solid layers, and the layers may have low-emissivity coatings. Measurementssfrom five different manufacturers and researchers of the overall thermal conductance of various windowssystems confirm the validity of this model for a range of temperatures and layer spacings less than 3 cms(1.2 in). A list of U-values and shading coefficients is given for a representative group of windows.