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What are Sources of Moisture in the Home?

Even if your house has no leaks in the basement or roof and is apparently dry, it can have moisture problems. Where does all the moisture come from? There are a number of major sources that are not always obvious:

  • 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.

Despite all this water produced each day, most older houses have “dry” air in winter to the point where they have to have humidifiers installed. Why?

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