Even houses that are apparently dry, with no leaks in the basement or roof, can have moisture problems. Where does all the moisture come from?

A family of four generates about 50 litres (L) (17 gallons) of water a week through normal household activities. Where basement waterproofing is inadequate, groundwater in the soil can migrate through the foundation by capillary action and evaporate on the surface of the wall or floor. A small plumbing leak can produce a lot of moisture. Finally, during humid weather, building materials and furnishings absorb moisture from the air and then expel it during the heating season.

Despite all this water produced each day, most older and even newer, poorly air sealed houses have dry air in winter to the point that they have to have humidifiers installed. Why?

Cold outdoor air cannot carry much water vapour. In poorly air sealed homes, uncontrolled airflow brings colder, drier air indoors and forces the warm, moist air out through openings in the upper walls and attic. The moist air condenses and causes mould and structural damage.

When insulation is added, the building exterior becomes much colder. Unless there is additional protection, such as an air and vapour barrier, water can condense in parts of the building structure, for example, in the insulation.

Table 2-2 Moisture added to the house through various household activities (for a family of four)

As the warm, moist air cools in the cold outer layers of the building, the water vapour it holds may condense as liquid or, if it is cold enough, as frost. This can reduce the effectiveness of insulation and even cause rot, peeling paint, buckled siding, mould growth and other problems.