Three different types of slab-on-grade heat-loss calculating procedures are discussed and com­pared with each other. The procedures discussed are the Green's function type solution, Delsante's Fourier Transform type solution, and the Mitalas procedure derived from finite element analysis. Although the Green's function and Delsante type solutions agree very well with each other, the Mitalas solution showed a larger time-lag effect, resulting in lower winter heat loss and higher summer floor heat loss than those determined by the other two methods.