Computer simulation is being widely used to evaluate the thermal performance of building components including windows, walls, and doors. Canadian standards (CSA A440.2 and A453) and U.S. standards (National Fenestration Rating Council 100-97) require that the U-factor of windows and doors be determined with one-dimensional heat transfer programs for the glazing and a two-dimensional program for the edge-of-glazing and opaque portions. In these standards, the total U-factor is determined by area-weighting the glazing and frame portions. Thus, the total of the three-dimensional heat flow is approximated as a series of one- and two-dimensional cases. For most building components, this is a reasonable assumption. There are, however, some cases where three-dimensional effects may be significant and must be accounted for in order to obtain an accurate value for the total product U-factor. The purpose of the study presented in this paper is to assess the magnitude of common three-dimensional effects, thereby determining whether they can be ignored, and to develop techniques that can be used with two-dimensional computer programs to approximate the three-dimensional effect. It is hoped that this work can improve the accuracy of two-dimensional computer simulation without increasing the level of effort or cost.

Units: SI