This paper describes the development of a detailed 2D analysis tool for determining slab-on-grade F-2 factors for heat loss load calculations. The results for one sample case are presented and an enhanced simplified design heat loss equation is suggested. In the related literature, there are significant variations in the approaches to and assumptions made for this problem. This study sought to expand the application of the existing simplified load calculation method to wood-framed buildings while also beginning to investigate improved simplified design models. Opinions are requested from readers on what is needed in future slab-on-grade heat loss load calculations models.