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.