- Occupants and their activities: An average family of four will generate about 63 litres (20 gallons) of water a week through normal household activities.
- Wind-blown rain in walls: Where basement damp-proofing is inadequate, ground water in the soil can migrate through the foundation by capillary action and evaporate on the surface of the wall or floor.
- Damp basements
- Moisture stored in building materials and furnishings: Building materials and furnishings absorb moisture from the air during damp, humid weather and then expel it during the heating season.
Cold outdoor air cannot carry much water vapour. In older homes, uncontrolled airflow brings colder, drier air indoors and forces the warm, moist household air out through openings in the upper walls and attic. The air quickly escapes through the un-insulated envelope without cooling down enough to cause condensation.
When insulation is added, the building exterior becomes much colder. Unless additional protection is provided, water can condense in the building structure.
How? Remember that cold air is able to hold much less moisture than warm air. 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.
Quantity of Moisture Added to the Air Through Various Household Activities
| Activity (for a family of four) | Moisture (litres per week) |
| Cooking – three meals daily for one week | 6.3 |
| Dishwashing – 3 times daily for one week | 3.2 |
Bathing
| 2.4 |
| Clothes washing (per week) | 1.8 |
| Clothes drying indoors or using an un-vented dryer (per week) | 10.0 |
| Floor mopping per 9.3 m2 (100 sq. ft.) | 1.3 |
| Normal respiration and skin evaporation from occupants | 38.0 |
| Total moisture production per week | 63.0 |
