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Moisture Problems: Humidity vs. Ventilation
By Gil Strachan

Condensation will occur on any surface that is below the dew point of the air in contact with it (temperature at which condensation will occur at the existing humidity level). In most houses in winter, the inner surfaces of the windows are the coolest surfaces and condensation will usually occur there before it appears on other surfaces. It seldom appears on walls because they are normally warmer, although occasionally condensation may occur on cold spots such as nail heads and in the corners of outside walls where the insulation is reduced, or in confined spaces where the circulation of warm room air is restricted. In some cases this can lead to mildew and the growth of mould.

When condensation becomes excessive the moisture load must be decreased or the ventilation rate increased. To reduce the moisture load, humidfiers should be shut off, automatic dryers vented outdoors, clothes drying indoors avoided, and the structure checked for other possible moisture sources (ventilation, insulation and moisture barriers in the attic, basement or crawlspaces; open cisterns; seepage in the basement etc.)


Thoroughly dry a damp spot on an exposed foundation wall or floor with a hair dryer. Tape a 12 inch square of heavy plastic over the spot, and check it in one or two days. Seepage will cause the underside of the plastic to be wet, but condensation will form on the top side!

Ventilation is generally the most effective means available to the homeowner for reducing humidity and should be obtained by opening windows or operating an exhaust fan (or the furnace fan) as soon as significant window condensation appears.

If only a moderate or minimal amount of condensation is experienced, switching the fan on a forced-air furnace to the "continuous" or "summer" setting can help reduce humidity in moist areas, by circulating and blending the air throughout the house.

Attic ventilation is normally provided by vent openings totalling approximately one square foot for every 200 to 300 square feet of ceiling area (not more than 50% through the roof, and no less than 50% through the gables or soffits).


Activities Moisture Produced (pounds) (litres)
Cooking 3 meals a day 2 1.0 litre
Dishwashing 1 0.5 litre
- shower 5 2.5 litres
- tub 2 1.0 litre
Clothes washing (per week) 4 2.0 litres
Drying indoors or with unvented dryer (per week) 26 13.0 litres
Floor mopping (per 100 sq.ft/) 3 1.5 litres
Occupants (family of four per day) 12 6.0 litres

Dehumidifiers, depending on the capacity of the model chosen, are typically capable of removing 7.5 to 20 litres of water from the air in the house, per 24 hour day. The age, type and style of home will affect the degree of moisture retention, and therefore have a bearing on the size of dehumidifier required. Controlled air exchange, using an HRV (heat-recovery ventilator) is usually a requirement in newer homes, especially those built to R2000 standards. In these homes, uncontrolled infiltration/exfiltration of air is reduced to almost zero, and therefore moisture will quickly accumulate causing condensation on windows and other cool surfaces.

Copyright Gil Strachan - All rights reserved.

Gil Strachan is a professional home inspector, representing Electrospec Home Inspection Services in east-central Ontario, Canada since 1994. Visit http://www.allaroundthehouse.com to learn more about home inspections.

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