As the envelope is tightened, household humidity levels rise. Condensation and moisture problems can occur, less fresh air is circulated through the house and less air is available for combustion appliances. Therefore, an important part of comprehensive air leakage control is attention to controllable whole-house ventilation and combustion-air supply.

Each house will respond to comprehensive air sealing in its own unique way, so monitoring is important. Older houses may require remedial measures before comprehensive air sealing.

For example, moisture can accumulate in the walls over time, resulting in mould buildup. This situation may require cleanup and replacement of the affected materials and installation of an improved air and vapour barrier. The best way to avoid problems is to understand how they occur and to take steps to control humidity and ventilation.

Furnaces, water heaters, fireplaces, woodstoves and any other fuel-burning appliances require air for combustion and for exhausting the products of combustion out of the house. If there is not enough air, the chimney or flue could spill dangerous gases into the house.

Humidity, ventilation and combustion air are discussed in more detail in "Operating your house."