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To reduce fogging on the room-side of household windows, lower indoor moisture and keep warm room air moving across the glass. Use kitchen and bathroom exhaust fans, control humidifiers and indoor clothes-drying, and open curtains or blinds. First check whether the moisture is indoors, outdoors, or between the panes: each points to a different cause.
Why windows fog up in cold, rainy weather
Indoor window fog is condensation: moisture in the air turns to liquid when it meets glass that is colder than the indoor air. Cold weather chills the glass, while showers, cooking, plants, humidifiers, and drying clothes indoors add moisture to the home. The U.S. Environmental Protection Agency explains that in cold weather moisture condenses on cold surfaces, including the inside of a window (EPA moisture guidance).
Rain alone does not establish that water is leaking through a window. In rainy weather, outdoor air may also be humid, so opening a window is not automatically a good way to dry out the home. Focus first on moisture sources and ventilation that exhausts air outdoors.
Identify where the condensation is
Wipe the glass, frame, and sill dry, then check where moisture returns. The location helps distinguish indoor humidity from an exterior dew effect or a window problem.
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- Room-side glass: Indoor moisture is meeting a cold pane. Reduce moisture sources and improve air circulation at the glass.
- Outside glass: Dew can form on exterior glass, including on high-performance windows during cool, calm, humid mornings. Exterior condensation by itself is not evidence of a failed window (Natural Resources Canada: Condensation).
- Between panes: Moisture between the panes of sealed insulating glass is consistent with a failed seal. Wiping the room-side surface will not resolve it; have the insulating glass unit assessed for replacement.
Reduce moisture at its source
Exhaust bathroom and kitchen air outdoors
Run the bathroom exhaust fan during showers and afterward, and use the kitchen exhaust fan while cooking. Confirm that these fans vent outdoors rather than into an attic or another interior space. Vent clothes dryers outdoors as well. These steps remove moisture where it is produced (EPA moisture guidance; Health Canada: Fight mould).
Review humidifiers and indoor drying
If you use a humidifier and condensation is appearing, lower its setting or stop it temporarily and observe whether the windows improve. Avoid drying clothes indoors when possible, and consider whether other moisture sources—such as a leak—are keeping a room damp.
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Use outdoor ventilation selectively
Opening a window can help in some conditions, but not when the outdoor air is humid. The EPA recommends opening windows to reduce moisture only if it is not humid outdoors (EPA moisture guidance).
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Use humidity readings as a guide, not a guarantee
A digital hygrometer can show whether indoor relative humidity (RH) is high and help you track changes after using exhaust fans or reducing humidifier use. The EPA says an ideal indoor RH range is 30%–50% where possible and advises keeping it below 60% if possible. Health Canada also recommends maintaining home RH at 30%–50% (EPA; Health Canada).
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- If you want to open your windows in the warmer months, you'll need to take it off. Otherwise, sunlight and normal stress can also make the tape fail eventually. For a good seal, it's probably best to re-apply them every year.
Those ranges are general guidance, not a promise that condensation will stop. The temperature of the glass, window construction, and airflow near the pane also matter. Natural Resources Canada’s table gives different maximum RH values at an indoor temperature of 21°C (70°F), depending on outdoor temperature and window type:
| Outdoor temperature | Older-generation double-glazed low-E window | Double-glazed window | Double-glazed low-E window | Triple-glazed low-E window |
|---|---|---|---|---|
| 0°C (32°F) | 50% | 58% | 70% | 81% |
| -10°C (14°F) | 34% | 44% | 59% | 74% |
| -20°C (-4°F) | 24% | 34% | 50% | 67% |
| -30°C (-22°F) | 15% | 25% | 42% | 60% |
| -40°C (-31°F) | 10% | 18% | 35% | 54% |
These are Natural Resources Canada’s listed maximum values for the specified indoor temperature and window categories, not universal targets for every home (Natural Resources Canada: Condensation).
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- Save on energy costs during cold weather months. This rolled window kit creates an airtight seal inside your home to block drafts and cold weather
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- Tools needed: scissors and hair dryer. For best results apply window films indoors on clean and dry surfaces, including painted or finished wood, aluminum or vinyl
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Keep air moving along the glass
Warm room air at the pane helps keep the glass surface warmer and reduces the chance of condensation. Natural Resources Canada notes that proper air circulation near windows is necessary to minimize it (Natural Resources Canada: Condensation).
- Open curtains and blinds enough for room air to reach the glass.
- Move furniture away from the window if it blocks airflow.
- Keep baseboard heaters and HVAC vents clear so they can circulate air toward the window.
- Dry water on frames and sills; persistent moisture can damage finishes.
When a dehumidifier or window repair makes sense
Consider a dehumidifier for ongoing dampness
If source control and appropriate ventilation do not sufficiently reduce dampness, a dehumidifier may help lower indoor moisture. Continue investigating leaks and other moisture sources: a dehumidifier does not fix a leak or repair a failed window seal.
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- Save on energy costs during cold weather months. Duck Max Strength shrink window film is puncture-resistant and two times thicker than standard window kits to create an airtight seal inside your home to block drafts and cold weather
- Easy-to-install roll of shrink film means no measuring needed - once applied, cut film to size
- Tools needed: scissors and hair dryer. For best results apply window films indoors on clean and dry surfaces, including painted or finished wood, aluminum or vinyl
- After installation, crystal clear and transparent window film is easy to see through. Once season is over, the window kit removes easily
- Window Kit includes 2, 62" x 210" roll of shrink film and 2, 0.5" x 54' foot rolls of tape; Can insulate up to 10 standard sized 3' x 5' windows
Assess persistent or damaging condensation
Repeated heavy condensation, wet frames or sills, or damage to finishes calls for more than repeated wiping. Look for unresolved indoor moisture and consider whether the window itself is performing properly. If condensation is between sealed panes, ask a window professional to assess the insulating glass unit rather than assuming room humidity is the only issue.
Think of window upgrades as a longer-term measure
For a cold pane that remains prone to condensation, window construction can matter. Natural Resources Canada identifies features associated with better energy efficiency and less condensation than ordinary models: triple glazing, low-emissivity coatings, inert gas between panes, thermally improved spacer bars, insulated frames, and greater airtightness. Such upgrades address the window’s thermal performance; they do not replace moisture-source control or leak repair (Natural Resources Canada: Condensation).
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