This study has three parts. The one is for the making model of wood rotting. This is based on the experiences of laboratory tests and observations for wood rotting process. We set huge number of wood pieces with various water contents. Then we wait the starts of rotting, measuring the weight and water content of the pieces every 2 weeks. In the second part of our study, we make numerical model to represent the rotting process with various conditions. The model which we made is based on the growth of fungi hypha in the wood. As the third part, we calculated the humidity variation in the crawl space of dwelling houses in Japan. According to this and the damage function of wood rotting as mentioned above, we estimate the durability of the crawl space.