Abstract In order to obtain fundamental information on techniques for the optimum use of solar energy in a semi-underground house, a twin-type test house was constructed on the campus of Tohoku University in September 1984. The test house has two rooms, with a south-facing window above the ground surface and a corridor situated between the two rooms. Insulation 10 cm deep and 1.35 m wide was installed horizontally 30 cm below the ground surface, surrounding one room of the house. The other room is not insulated. During the first year, all windows were insulated with weather shutters to avoid disturbing the heat gain due to solar radiation. As a result, the yearly room temperature swing varied °C less than the temperature swing of outdoor air. The air temperature in the insulated room was 1.2°C higher in the winter and 0.5°C lower in the summer than in the room without insulation. The influence of various thermal insulation formations in the earth around the house on room temperature fluctuation and heating and cooling loads was calculated using the two-dimensional finite element method. Calculations were performed hourly for a year under standard climate conditions in Sendai, Japan.