The 80 detached houses in Kanto area, which were built by using Government Housing Loan in 2000 Fiscal Year, were measured of their equivalent leakage area per unit floor area. The samples consisted mainly of the houses, which did not comply with the 1999 national energy code for residential buildings to improve the thermal performance and the air tightness. The following findings were obtained, although further survey is necessary in order to collect more accurate and broad information on the air tightness of recent houses. 1. For the post-and-beam construction (ZAIRAI) without using plywood panel for the exterior walls, mean equivalent leakage area per unit floor area is 5.5 cm^2/m^2, while it is 2.2 cm^2/m^2 for the two-by-four construction. 2. By analyzing the relationship between the air tightness and the specifications of insulation technique, the usage of plywood panels on the exterior wall and the floor, the usage of water proof sheet on the exterior wall and the existence of vented cavity on the exterior wall were proved to be significantly related with the air tightness. 3. Air leakage rate in summer and winter was estimated by using the existing predictive equation, which has been developed by the author. For summer, the air leakage is estimated to be 0.1 ACH for the post-and-beam construction and 0.05 ACH for the two-by-four construction, respectively. For winter, it is estimated to be 0.24 ACH for the post-and-beam construction and 0.1 ACH for the two-by-four construction, respectively.
