Monitoring of the building envelope of a heritage house. A case study.

The paper describes the long-term monitoring of the hygrothermal performance of the building envelope of a heritage house located in Ottawa. The house, once the residence of two of Canada's Prime Ministers, now serves as a museum. To preserve the historical artifacts within the building, the specified temperature and relative humidity for the indoor air are 21°C and 35% to 50% respectively. As the house must also be preserved, there was concern about the effect of the high indoor relative humidity (moisture) on the durability of the building structure. The main objective of the monitoring was to assess the effect of the conditioned air on the building envelope. Selected wall sections and a window were continuously monitored from March 1995 to August 1996. The monitoring included indoor and outdoor conditions and the attic environment. Temperature, relative humidity, surface wetting-drying cycles (from precipitation or condensation), and air pressure differential were monitored. This paper describes the monitoring approach and results. The results indicated that the brick walls are unlikely to experience internal condensation problems as long as they are subjected to negative air pressure difference. However, because the building is quite leaky, the negative pressure introduced too much cold dry air from the exterior. It caused localized cold spots with condensation and ice formation on interior of walls and ceiling. Negative air pressures difference are not a solution unless the leakage paths are reduced.