Field measurements of the indoor climate have been carried out in 70 detached houses with heavyweight external walls in Finland during the years 2005-2007. The temperature and relative humidity (RH) were continuously measured in bedrooms for each house at 1-h intervals over a 1-year period. Indoor climate was measured and analysed in detached houses and results were compared according to envelope construction types. The studied houses had different types of external walls: autoclaved aerated concrete, lightweight expanded clay aggregate, bricks, concrete blocks, concrete elements, and timber logs. To study the stability of RH and absolute humidity, the amplitudes of 24 hours, i.e., the difference between daily maximum and minimum values, were calculated. The studied houses were divided into different groups according to wall material: timber log, autoclaved aerated concrete, and other materials. Results of this study show how structures influence the indoor climate under real use in houses.