Abstract
In the National Museum of Western Art which aims at world heritage registration, the restorations of an original design and the building facilities have been an urgent subject. On site observation was performed to evaluate current condition of indoor environment and to develop proper retrofit scheme. Through the observation in a busy period, internal CO_2 concentrations reached around 1500ppm. Pressure of all of the rooms is negative compared to outdoor pressure. The room age was longest in the exhibition space and shortest in the restaurant. The risk of condensation in the skylight gallery was found fairly high in winter season.