Solutions to Frosty Windows Panes
Get a Grip on an Icy Problem That Could Worsen Over Time
Nov 2, 2009
Ice appears on windows when indoor moisture makes contact with the cold glass surface, creating condensation. The condensation then freezes when outdoor temperatures drop below zero.
Infrequent, short-lived condensation and frosting generally does not pose a problem. However, excessive amounts that persist over time may result in major damage to the window frame and wall below. It could also be an indication that an indoor moisture problem exists that may lead to mould growth.
Determine Indoor Moisture Content
As a rule of thumb, the colder it is outside, the less humidity should be inside to avoid window frosting.
To determine moisture content, measure your home's relative humidity (RH) with a hygrometer. (RH is the ratio of the amount of moisture in the air, at a certain temperature, to the maximum capacity of the air at that temperature.)
The following table is based on double-glazed windows. Use it as a guideline to determine RH levels at different outdoor temperatures, in order to avoid window condensation and frosting.
Outside Air Temperature degrees C/ Maximum Inside Humidity at 20 C (68 F)
0 to -12 degrees C.......... 40%
-12 to -18 degrees C...... 35%
-18 to -24 degrees C.......25%
-24 to -30 degrees C.......20%
-30 degrees C or below..15%
Possible Frosty WIndow Solutions
When it comes to frosty window solutions, the more severe the problem, the more extensive work may be required. It's best to try the least costly measures first, if you're unsure what is causing the problem.
Free to low-cost
- Provide additional warmth to the frosty window: draw curtains at night and during extremely cold weather; ensure the heat register by the window is fully open; move furniture and rugs away from heat registers, and/or; install temporary window film by use of a hair drier, purchased at the local hardware store.
- Lower indoor humidity levels over winter: lower the humidistat on the furnace; move plants to a different location (plants produce moisture through transpiration); don't hang-dry clothes indoors; turn off room humidifiers; operate bathroom and kitchen vents to exhaust moisture when necessary; don't bring wood inside to dry; wait to steam carpets during warmer weather, and/or; repair or install attic vents.
- Get fuel-burning appliances checked to ensure their are operating properly. Have them checked annually
Potentially expensive
- Install storm windows or new, high performing windows. Double-panes or, better yet, triple-paned glazing won't readily fog up and freeze as opposed to single-glazed windows.
- Install a whole-house heat recover ventilator (HRV) system. An HRV continuously extracts used, moisture-laden air to the outdoors and replaces it with fresh outdoor air. 80% of the old air used is to preheat the incoming air, making an HRVa fairly energy-efficient system.
- Renovate a recently flooded basement that expels large amounts of moisture into the inside air
Before undertaking a potentially costly moisture-control measure, first consult an expert (such as an indoor air quality specialist or energy management consultant) who will visit your home and assess your situation.
Final Thoughts
Knowing what your indoor humidity levels are during colder weather and how to control them is the first step to tackling an icy situation.
The right measure(s) required to eliminate window frosting will depend on the degree of indoor humidity, the thermal performance of the window and outside temperatures. A severe indoor moisture problem and long lasting window ice-buildup will require immediate attention before an even bigger problem develops.
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