In most whole building thermal modeling computer programs like DOE-2, BLAST, or ENERGY PLUS simplified, one-dimensional, parallel path, descriptions of building envelope are used, For several structural and material configurations of building envelope components containing high thermal mass and/or two-and three-dimensional thermal bridges, one-dimensional analysis may generate serious errors in building loads estimation. The method of coupling three-dimensional heat transfer modeling and dynamic hot-box tests for complex wall systems with the whole building thermal simulations is presented in this paper. This procedure can increase the accuracy of the whole building thermal modeling. Current thermal modeling and calculation procedures tend to overestimate the actual field thermal performance of today's popular building envelope designs, which utilize modem building technologies (sometimes highly conductive structural materials) and feature large fenestration areas and floor plans with many exterior wall corners. Some widely used computer codes were calibrated using field data obtained from light weight wood frame buildings. The same codes are used now for thermal modeling of high mass buildings with significant heat accumulation effects. Also. the effects of extensive thermal shorts on the whole building thermal performance is not accurately reflected by the commonly used one-dimensional energy simulations that are the current bases for building envelopes and systems designing.