This study examines the thermal mass effects in a code-traceable DOE-2 simulation of the 2000 IECC (International Energy Conservation Code) for a single-family residence in Texas. This report is composed of two major simulations: 1) the simulation according to the location of the insulation of IECC2000, and 2) the simulation according to the types of real brick and block walls which are practically used at the residential house. In this study, the 2000 IECC was used to develop the base case simulation model in Houston, Texas. The DOE-2 energy simulation program was used to analyze changes to the annual energy use caused by changing various building materials. The best energy conservative material layout was then chosen that contained reduced annual energy use, peak cooling and heating loads, and peak day electricity use.