This paper discusses a study that measured and analyzed the indoor climate conditions in 46 lightweight timber-framed detached houses occupied by single families. This survey was conducted during 2002-2003 in Finland. The main objectives were to analyze the indoor temperature and moisture conditions as well as to determine the internal moisture supply and moisture production. Additionally, the impact of HVAC systems and envelope solutions on the indoor climate were analyzed. The temperature and relative humidity were continuously measured in bedrooms, living rooms, and outdoors. The results showed high indoor temperatures during the summer period and rather unstable temperature conditions during winter, which exceeded acceptable comfort levels in most of the studied houses for a long period of time. During the summer, ventilation systems had significant influence on the indoor temperature levels, humidity levels, and the average daily amplitude of temperature and humidity. The vaporpermeable envelope caused low relative humidity in the winter, and the indoor air was drier for a longer period than in houses with a vapor-tight envelope. The hygroscopic mass showed effect on the average daily amplitude of humidity. Based on the measured indoor and outdoor humidity values, the moisture supply values were calculated. Using those calculated values along with measured air change rate, the moisture production rates are specified