Use of solar energy through passive heating is numerically evaluated for different passive designs of weekend houses with one wall made completely of masonry and with other walls made of two layers: a masonry layer and a thermal-insulation layer. To access this problem, the heating and cooling load is determined by using a dynamic, thermal, building model newly constructed on the basis of finite volumes and time marching. The investigation is performed for two days: one winter day for the weekend house with the masonry wall facing south, and for one summer day for the weekend house with the masonry wall facing north. The heating and cooling loads are evaluated for different thicknesses of masonry by using two investigation procedures where in the first procedure the insulation thickness is kept constant, and in the second procedure the U valve of the two-layer walls of the house is kept constant. When the thermal-insulation layer faces the outside of the house, this investigation reveals that the use of the passive house instead of the non-passive house for the winter day gives an energy saving of around 1.5%, and for the summer day gives the maximum energy saving of around 4% for a masonry thickness of 30 cm. When the thermal-insulation layer faces the inside of the house, the investigation reveals that passive heating is not possible at all.