2875 (RP-332) -- Simplified Window Energy Calculation Procedure

BEEM (Building Energy Estimation Module) is a simple hand-calculation procedure for calculating window-related building energy impacts, including daylighting electric lighting, cooling, and heating energy. It was developed under a previous ASHRAE research project. Reported here are subsequent derivations and successful validation using the DOE 2.1b energy-prediction computer program.

The fill-in-the-blanks procedure, also adapted to microcomputers, was created for use in the schematic design stage by architects and engineers to make early, quantitative design decisions regarding glazing types and sizes and on daylighting controls. Included are the calculation of interior daylighting levels due to a given window, as well as the yearly lighting, cooling, and heating energy costs due to the window relative to a solid wall. Workplace daylighting level and lighting-savings-fraction are calculated for both June 21 and December 21. These two illumination values are used, with appropriate corrections for overcast days and lighting control type, to predict annual lighting savings. Yearly energy cost is calculated so that various fenestration alternatives can be compared for optimizing the total economic impact.

The procedure requires about 30 minutes for reading instructions and completing an energy calculation for one window situation. Subsequent calculations take 10 to 15 minutes; changing one parameter and recalculating requires about 5 minutes. The procedure is easily coded on a microcomputer, and such a code has been prepared. Both a long form (with helpful narrative) and a short form (for subsequent faster calculations) are available.