Why Should I Worry About Moisture Problems?

Moisture problems in Canadian housing are detrimental to our health and to the durability and resale value of our homes. This fact sheet shows you how to solve moisture problems, improve energy efficiency and reduce greenhouse gas emissions that contribute to climate change.

“I can’t see through these windows!”

Winter-long condensation and frost on windows is annoying and can damage the window frame and wall below.

“Our basement smells like … a basement.”

Musty odours are a symptom of dampness and mould growth. A flood, sewer backup or burst plumbing can lead to many hidden moisture problems, even after the mess has been cleaned up.

“Wheeze …”

Large-scale health surveys in Canada and other countries have confirmed a strong correlation between dampness and mould and respiratory disease in children.

“But I just painted this wall!”

Warm, moist air leaking out through walls can condense inside the wall. Symptoms of deterioration can include peeling paint, spalling bricks and buckled or rotting siding. Repairs can be costly and ongoing.

“Can’t we lower these energy bills?”

Moisture problems are often associated with cold surfaces and air leakage, both of which mean excessive heat loss and high energy bills.

“Why aren’t we getting any offers?”

Home buyers will shy away from a house with moisture problems, even if they are minor. Resale value will be lower.

What is Condensation?

Condensation is the passing of a substance from a lighter to a denser physical state. In this instance, water passes from a gaseous (vapour) state to the denser liquid one. Because it is caused by too much humidity, condensation is also the symptom of a more serious problem – excessive moisture at work in your home’s atmosphere.

In seeking cooler, drier outside air, water vapour exerts pressure and forces its way through most building materials. Wood, brick, and drywall are too porous to stop moisture from migrating to the outside. Because window glass is denser and its surface colder than the surrounding walls, vapour is stopped by glass and condenses on it. While glass – the coldest, least porous (and most visible) material in a building – may show condensation first, it may also be noticed on exposed nail heads and other metal surfaces.

Why Does Condensation Occur?

Condensation occurs when warm moist air comes into contact with a cold surface. The incidence of condensation is increased when a room is not properly ventilated, the temperature is not moderated, or if excessive moisture is being produced. The cold weather is usually worse for causing condensation because windows are opened less freaquently and more moist air is trapped indoors for longer.

Where Does Condensation Occur?

In practice, condensation will occur first over the lower part of the window because glass surface temperatures are not uniform, being lower at the bottom than at the top. Windows with a metal sash usually have more severe problems at the base and sides. Occasionally, it may occur on cold spots, such as nail heads and in corners of outside walls and closets where insulation value is reduced.

Sources of Moisture

The humidity level in a house during the winter will depend on both the moisture added to the air through family living habits (see chart) and the rate at which this moisture is removed by ventilation or condensation.

Relative Humidity

A ratio between the water vapour in the air compared to the maximum amount that air can hold at a given temperature. For example, 50% means the air is carrying 1/2 of the total water vapour it is capable of holding at that temperature.


A particular temperature where the relative humidity becomes 100%, called the “dew point” (see chart). At the dew point, the air is saturated with moisture and begins to lose it in the form of condensation.

Relative Humidity Dew Point (Celsius)
20% -4˚
30% 2˚
40% 6˚
50% 10˚
* Note: A new home has significant quantities of moisture because of the water used in various areas of construction (700 gallons of water are used in plastering alone for a six room house). This will be eliminated after a period of time.

Quantity of Moisture Added to the Air Through Normal Household Activities

ctivity (for a family of four) Moisture in Litres each week
Cooking (3 meals per day for 1 week) 6.3
Dishwashing (3 times per day for 1 week) 3.2
Bathing (.2 litres per shower)
(.05 litres per bath) 2.4
Clothes washing 1.8
Clothes drying indoors, or using
an unvented dryer 10
Floor mopping (per 9.3m) 1.3
Occupants 38
Total Moisture Production per Week 63
In addition to the above sources of humidity, such things as gas appliances, dryers vented inside, plants (which put out almost as much water as they receive), pets, humidifiers, damp basements, etc., all increase the humidity level in a home.

Solutions to Humidity

  • Turn off all humidifiers, particularly in homes with forced-air heating.
  • Move plants away from windows (water vapour releases directly onto glass).
  • Vent the clothes dryer and gas appliances outside.
  • Do not dry firewood indoors.
  • Ensure kitchens and bathrooms are well ventilated by windows or exhaust fans.
  • Ensure that basement walls are kept as dry as possible since they will act as humidifiers when wet.
  • A de-humidifier may have to be installed (Note: a de-humidifier alone cannot eliminate the problem).
  • Do not cover windows with heavy curtains since this will restrict air flow over glass.
  • Produce less moisture inside the house.


  • Unless indoor humidity is kept below 10%, it is impossible to avoid some condensation.
  • Lower outside temperatures require lower inside humidity levels (see chart).
  • Electrically heated homes are difficult to rectify because there is very little movement of air.
  • Ventilation is generally the most effective means available to reduce humidity and can be done by opening windows, operating exhaust fans and/or installing a ventilator from outside into the cold air return plenum of a forced air heating system.

Moisture Assessment Checklist

Excess condensation on windows is an indication that the relative humidity in a home is too high. The following are the most common causes of high relative humidity.

  • bathroom or kitchen fans not vented to the outdoors
  • exposed soil, plants, or gravel crawlspace
  • damp foundation or basement
  • furnace humidifier set too high
  • heat recovery ventilator (HRV) ventilation rate too low
  • electric heating without (HRV) or other ventilation system
  • clothes dryer not ducted to the outside
  • drying clothes indoors on open racks
  • whirlpool, greenhouse or large aquarium
  • abundance of plants
  • firewood drying indoors.
  • bathroom usage (showers, & other running water)
  • kitchen usage (washing dishes, dish washer & cooking)

Sources for information:

  • Natural Resources Canada (Office of Energy Efficiency)
  • Ontario Ministry of Municipal Affairs and Housing
  • Canadian General Standards Board
  • National Research Council (Division of Building Research)