A study of the walls of a medieval church shows that porous, water absorbent materials such as lime plaster and porous limestone will provide effective short- and medium-term humidity buffering in a room with about one quarter of an air change per hour. The church also has natural humidification. The source of the water vapour is probably ground water evaporating from the saturated lower part of the wall. The porous brick floor also contributes to the humidification. Adchitects can adapt some aspects of church architecture to improve climate stability in museums. -- CAL