A detailed, three-dimensional numerical model of the thermal performance of conditioned and unconditioned residential basements is described. The model has the ability to simulate a basement’s geometry, construction, insulation treatment, and environmental control in detail. Hourly weather data are taken from standard Typical Meteorological Year (TMY) files. The transient foundation and soil temperature distributions are calculated using an ADI finite-difference numerical technique for a one-hour time step. A ground surface heat balance accounts for conduction, radiation, convection, and evapotranspiration. On the portion of the foundation wall extending above grade and along interior basement surfaces, both convective and radiative effects are modeled. Unconditioned basement air temperature is computed at each time step by a heat balance algorithm. Capabilities of the model are illustrated through representative case studies.