Condensation is the formation of moisture on the inside surface of a window. The effects of window condensation can range from minor inconvenience of having obstructed visibility and reduced natural lighting, to major concerns of water stains, peeling interior finishes, rotting and the formation of mold.

Causes of window condensation include:

  • Day-to-day living: 4 occupants will create 1 – 1.5 gallons of moisture daily
  • New home construction: drying of building materials, concrete, drywall, paint, etc
  • Frequent / excessive use of humidifiers
  • Hanging clothes to dry inside your house
  • Use of gas appliances (stoves, fireplaces, dryers)
  • Boiling water from a pot
  • Use of a hot tub or spa
  • Bathing / showering without a fan
  • Large number of plants inside the house
  • Moisture from dishwashers
  • Heavy curtains or shutters restricting airflow around windows

One of the best steps in preventing window condensation is to control the humidity levels inside your home. When the outside temperature dips to -17º C, the relative humidity should not exceed 25%. Anything higher will result in condensation forming on the glass frame surface, which over time may lead to fungus growth.

Ways to control moisture and prevent window condensation include:

  • Improve air circulation in your home to keep the warm air moving
  • Open windows slightly throughout the home for a short period during the winter months to allow humid air to escape
  • Heat registers in front of windows should remain unobstructed and clear
  • Add ventilation fans in high humidity areas such as laundry, kitchen and bathrooms and leave running
  • Open blinds, draperies, interior shutters during the nigh and/or day to allow air to circulate near glass surface
  • Turn off humidifiers
  • Run a dehumidifier if necessary
  • Remove interior screens on windows during winter months
  • Install an in-house air exchanger system (HRV)
  • Turn on furnace fan only to circulate air in the home

Below is a chart to guide you on maximum humidity levels based on the outside temperature

Outside Temperature

Relative Humidity

-28º C

Not over 15%

-28º C to -23º C

Not over 20%

-23º C to -17º C

Not over 25%

-17º C to -12º C

Not over 30%

-12ºC to -6º C

Not over 35%

-6ºC to 4ºC

Not over 40%

If you’re still experiencing condensation and moisture on your windows after taking the steps listed above to control it, it may be time to replace your windows.