WINDOW 5 scheduled for release in 1996 is the latest version of a tool now widely used by industry, the NFRC, and researchers, to design and rate windows. It will provide increased accuracy, a flexible and state-of-the-art user interface, and the capabilities to handle more product types. WINDOW 5 includes THERM, a new 2D finite element thermal model with the capabilities to define and model the thermal performance of frames/dividers and their associated edge effects. WINDOW 5 also will include a built-in version of RESFEN that calculates the orientation-dependent annual energy impacts of a given window in a typical residence in various U.S. climates. Other technical additions include an improved angular/spectral model for coated and uncoated glazings, the ability to analyze the optical properties of nonhomogeneous layers, and the ability to model the effects of laminated glazing layers.