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A Proud Canadian HVAC Company
Noticing foggy windows or water droplets collecting on your windows during the winter? You’re not alone. Window condensation is common in Canadian homes when cold outdoor temperatures meet warm, humid indoor air. While a small amount of condensation can be normal, excessive moisture may indicate that your home’s humidity, ventilation or insulation needs attention.
Condensation is caused by moisture in the air: Warm, humid indoor air meeting cold window surfaces creates water droplets.
Indoor humidity matters: Maintaining appropriate humidity levels can help reduce condensation.
Ventilation helps control moisture: Proper airflow and exhaust ventilation help remove excess humidity.
Insulation makes a difference: Older windows, cold surfaces and air leaks can increase condensation.
Persistent moisture deserves attention: Excessive condensation can contribute to mould growth and moisture damage.
During an Ontario winter, the difference between indoor and outdoor temperatures can create the perfect conditions for condensation to form. When warm indoor air comes into contact with cold window glass, the air near the surface cools. If it cools below its dew point, moisture in the air turns into tiny water droplets that collect on the glass.
This is particularly noticeable during colder months when windows and exterior walls are exposed to freezing temperatures. Everyday household activities, including cooking, showering and doing laundry, also introduce moisture into the air. When that moisture has nowhere to escape, indoor humidity can increase and condensation becomes more noticeable.
Although occasional condensation isn’t necessarily a concern, persistent moisture can lead to problems if left unaddressed. Excessive water around windows can damage window frames, deteriorate surrounding materials and contribute to mould growth. Understanding why condensation occurs and how your home’s heating and ventilation systems influence indoor humidity can help you maintain a healthier and more comfortable indoor environment.
Window condensation occurs when warm, moisture-filled indoor air comes into contact with a colder surface, such as a window. As the air cools against the glass, it can no longer hold the same amount of water vapour. Once it reaches its dew point, the excess moisture becomes visible as fog, droplets or a thin layer of water.
This is especially common during Canadian winters because outdoor temperatures can drop well below freezing while indoor heating keeps homes warm. The greater the temperature difference, the colder the interior surface of the window may become, increasing the likelihood of condensation.
Condensation can also vary throughout your home. Bedrooms may experience more moisture overnight due to breathing and limited ventilation, while kitchens and bathrooms often have higher humidity because of cooking, showering and other everyday activities. Windows with poor insulation or older glazing may also be more susceptible because their interior glass surfaces become colder.
A little condensation on particularly cold mornings isn’t always a reason for concern. However, if you regularly notice water collecting along window frames, running down the glass or remaining throughout the day, it may be worth investigating your home’s indoor humidity and ventilation.
Indoor humidity is one of the biggest factors influencing how much condensation develops on your windows. Every day, normal household activities release moisture into the air. Cooking, showering, washing dishes, drying clothes indoors and even breathing contribute to indoor humidity.
During the winter, homes are often kept closed to retain heat. While this helps maintain comfortable temperatures, it can also limit the amount of fresh air entering the home and allow moisture to accumulate. When indoor humidity becomes too high, condensation is more likely to develop on cold windows, exterior walls and other surfaces.
Maintaining an appropriate indoor humidity level can help reduce condensation and improve overall comfort. A general winter target is approximately 30% to 40% relative humidity, although the ideal level depends on outdoor temperatures, window performance and the construction of your home. During particularly cold weather, some homes may need lower humidity levels to prevent moisture from collecting on windows.
A simple digital hygrometer can help you monitor indoor humidity and understand whether moisture levels are contributing to the problem. If readings remain consistently high, consider reducing moisture-producing activities, using bathroom and kitchen exhaust fans, and improving ventilation throughout your home.
Ventilation plays an important role in controlling indoor moisture, particularly during the winter when windows and doors tend to remain closed. Without adequate airflow, moisture produced by everyday activities can become trapped inside your home, increasing humidity and creating conditions where condensation is more likely to develop.
Bathrooms and kitchens are two of the biggest sources of indoor moisture. Running exhaust fans while showering, bathing or cooking helps remove humid air before it spreads throughout the house. It’s also important to make sure exhaust fans are functioning properly and that their exterior vents are not obstructed.
For homes with mechanical ventilation systems, such as Heat Recovery Ventilators (HRVs), regular maintenance can also make a difference. HRVs help bring fresh outdoor air into your home while exhausting stale indoor air. They also transfer heat between incoming and outgoing air streams, helping reduce heat loss during ventilation. When properly maintained and operated, these systems can support better indoor air quality and help manage excess moisture.
If condensation continues despite using exhaust fans and maintaining reasonable humidity levels, your home’s ventilation may need further assessment.
Not all windows perform equally when it comes to preventing condensation. Older windows, single-pane glazing and poorly sealed frames are generally more susceptible because their interior surfaces can become much colder during winter.
When the inside surface of a window becomes particularly cold, moisture in the surrounding air is more likely to reach its dew point and collect on the glass. This is why condensation often appears around window edges, corners and frames first.
Poor insulation and air leaks can contribute to the problem as well. Gaps around windows and doors allow cold outdoor air to enter, potentially creating colder surfaces where moisture can accumulate. Insufficient insulation around exterior walls can also contribute to cold spots and, in some cases, hidden moisture problems.
If you consistently notice condensation around the same windows, check for damaged weatherstripping, deteriorating seals or noticeable drafts. Persistent condensation between the panes of a double- or triple-glazed window may indicate a failed window seal, which is different from ordinary indoor surface condensation and may require window repair or replacement.
Improving insulation, sealing air leaks and upgrading older windows can help improve overall thermal comfort and reduce the likelihood of condensation.
While you may not be able to eliminate condensation entirely during extreme cold weather, there are several practical steps you can take to manage indoor moisture and reduce how frequently it develops.
Here are some ways to help keep your windows clearer and your indoor environment more comfortable:
Monitor indoor humidity: Use a hygrometer to keep track of humidity levels and adjust them when necessary, particularly during colder weather.
Use kitchen and bathroom exhaust fans: Run exhaust fans during and after cooking or showering to remove excess moisture from the air.
Maintain consistent indoor temperatures: Avoid significant temperature fluctuations that can contribute to colder window surfaces.
Keep air circulating: Make sure furniture and curtains aren’t blocking heating vents or preventing warm air from reaching windows.
Avoid drying clothes indoors: Whenever possible, dry laundry outside or use a properly vented dryer to limit additional indoor moisture.
Check windows and weatherstripping: Look for damaged seals, gaps and drafts that may allow cold air to enter.
Maintain your ventilation system: If your home has an HRV or other mechanical ventilation equipment, follow the manufacturer’s recommendations for filter cleaning and regular servicing.
If condensation remains excessive despite these steps, it may be worth having your home’s ventilation and HVAC systems assessed. A professional can help determine whether airflow, ventilation performance or another issue is contributing to the moisture buildup.
Yes. Some condensation is normal during colder months, particularly when outdoor temperatures are significantly lower than indoor temperatures. However, frequent or excessive condensation may indicate high indoor humidity, poor ventilation or inadequate window insulation.
A relative humidity level of approximately 30% to 40% is a reasonable starting point for many homes during winter. During extremely cold weather, lower humidity levels may be necessary to prevent condensation, depending on your windows and insulation.
Yes. If water droplets accumulate on your windows or window sills, wiping them away can help prevent moisture damage and reduce the risk of mould developing around the frames. However, it’s also important to address the underlying cause of excessive condensation.
It can. When moisture repeatedly collects around window frames, sills or nearby walls, it creates conditions that may encourage mould growth. Keeping surfaces dry, controlling indoor humidity and maintaining proper ventilation can help reduce this risk.
Your furnace helps circulate heated air throughout your home, but a standard furnace does not actively remove moisture from indoor air. Heating can lower relative humidity as air temperature increases, but it does not remove the actual water vapour. Ventilation, exhaust fans and dedicated humidity-control equipment play a more direct role in managing indoor moisture.
You should investigate condensation that persists throughout the day, regularly pools on window sills or is accompanied by mould, musty odours or deteriorating window materials. These signs may indicate an underlying moisture or ventilation issue that needs attention.
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