We found many houses have condensation in the crawl space in summer. These houses have enough openings on the foundation to ventilate and vapor retarder at the ground surface for the recommend in the building code. The temperature and humidity in the crawl space have great influences to durability of the house. We made clear the annual variations of hygro-thermal environment of the crawl space by the field measurements and numerical analysis. We monitored the temperature and humidity variations of 36 houses in Japan for 2 years. The numerical calculations based on the vertical one dimension heat transfer model represented the monitored results. The results show the houses in Japan normally have condensation in crawl space in summer. The condensation term is from one week to one month. The daily average of crawl space's vapor pressure is nearly equal to that of the outdoors. The difference of 2 years results is so great that we think the main factor is outdoor condition. The thermal resistance of the floor and moisture of the ground do not have great effect on the crawl space humidity.
