In the first phase of the current project, the ventilation rates of occupied houses in the northern region of Japan were investigated from 2001 to 2005. It was revealed that, despite installing mechanical ventilation system, low ventilation rate was found in many houses. The causes were believed attributing to lack of cleaning of grills, filters and insect protection meshes. The second phase of the project was to investigate the causes and resolve the problems. This project was started in 2005 and 12 houses installed with mechanical ventilation system were investigated.