You wake up on a cold Ontario morning and notice water collecting along the bottom of your windows. Or perhaps one window looks permanently cloudy, even after you clean both sides of the glass. While both problems may look like “foggy windows,” they can have very different causes and solutions.
For homeowners dealing with foggy windows in London, Ontario, the first step is determining where the moisture is actually forming. Condensation on the room-side surface is often related to indoor humidity and cold glass temperatures, while condensation between window panes can indicate a failed sealed glass unit.
For homeowners in London, Kitchener-Waterloo, Oxford County, and Chatham-Kent, this guide explains why windows sweat or fog, how to identify the type of condensation you have, what you can do to reduce it, and when window repair or replacement may be the better long-term solution.
1. Why Your Entrance Door Matters
Window condensation happens when moisture in the air comes into contact with a surface that is cold enough for the water vapour to turn into liquid droplets.
This is similar to the moisture that forms on the outside of a cold drink on a warm day.
During an Ontario winter, the difference between your indoor temperature and the outdoor temperature can be significant. The interior surface of a window may become much colder than the surrounding room air.
If warm indoor air contains enough moisture, condensation can develop when that air reaches the colder glass.
However, not every foggy window is caused by indoor humidity.
Where the condensation appears is one of the biggest clues to what is happening.
2. The Three Types of Window Condensation
Before choosing a new entry door, it helps to understand what matters most for your home and lifestyle.
2.1 Condensation on the Inside Surface
This moisture appears on the glass surface facing the inside of your home.
You can usually wipe it away from inside the room.
It is often related to:
- Indoor humidity
- Cold outdoor temperatures
- Poor air circulation around the window
- Cooler window surface temperatures
- Everyday household moisture
This type of condensation does not automatically mean that the window seal has failed.
2.2 Condensation Between Window Panes
If the fog or moisture appears trapped inside a double or triple glazed window and cannot be wiped away from either side, the issue is different.
Condensation between window panes can indicate window seal failure within the insulated glass unit.
That usually requires repair or replacement of the sealed glass unit, depending on the condition, age, warranty, and construction of the window.
2.3 Condensation on the Outside Surface
Sometimes moisture forms on the exterior side of the glass, particularly during cool, calm, and humid conditions.
Exterior condensation does not necessarily indicate a problem.
In some situations, highly energy-efficient windows can experience exterior condensation because less indoor heat is passing through the glass to warm the exterior surface.
Understanding which of these three situations you have can prevent unnecessary repairs or replacement.
3. Why Is There Condensation on the Inside of My Windows?
Interior window condensation is particularly common during Ontario winters.
Your household creates moisture every day through activities such as:
- Cooking
- Showering
- Bathing
- Doing laundry
- Washing dishes
- Breathing
- Using humidifiers
- Keeping many houseplants
- Drying clothing indoors
When warm, moisture-containing air reaches cold window glass, some of that moisture can condense.
The colder it becomes outside, the colder the window surface can become, which means condensation may develop at a lower indoor humidity level.
3.1 High Indoor Humidity
If condensation appears regularly across multiple windows, indoor humidity may be too high for the outdoor temperature.
A hygrometer can help you monitor your home’s relative humidity instead of guessing.
Humidity that feels comfortable during mild weather may become too high when outside temperatures fall significantly.
3.2. Limited Air Circulation
Window coverings and furniture can also influence condensation.
Heavy curtains, tightly closed blinds, deep window sills, or furniture positioned directly in front of a window can restrict warm indoor air from reaching the glass.
Without enough air circulation, the glass can remain cooler and condensation may become more noticeable.
3.3. Window Performance
Older or less energy-efficient windows may have colder interior glass surfaces during winter.
That can make condensation more likely compared with higher-performance windows that maintain warmer interior surface temperatures.
4. How to Fix Sweating Windows
If your goal is to fix sweating windows, start by addressing moisture and airflow before assuming the windows need replacement.
4.1. Monitor Indoor Humidity
Use a hygrometer to understand the actual humidity level inside your home.
If condensation increases when temperatures drop significantly, reducing indoor humidity may help.
4.2. Use Bathroom and Kitchen Exhaust Fans
Activities such as showering and cooking release significant moisture into the air.
Run properly vented exhaust fans during these activities and for an appropriate period afterward to help remove excess moisture.
4.3. Use Your HRV or Ventilation System
If your home has a heat recovery ventilator, or HRV, make sure it is operating correctly.
Proper ventilation helps remove moisture-laden indoor air and replace it with outdoor air in a controlled way.
4.4. Improve Airflow Around Windows
Allow warm indoor air to reach the glass.
You may be able to improve circulation by:
- Opening blinds or curtains periodically
- Avoiding furniture directly against windows
- Keeping heating vents unobstructed
- Making sure air can move around the window area
4.5. Review Humidifier Settings
A humidifier that is comfortable in milder weather may produce too much moisture during very cold conditions.
If condensation becomes heavy or frost develops, review the humidity setting.
4.6. Remove Standing Water
Do not allow persistent condensation to sit on window sills, frames, trim, or surrounding finishes.
Repeated moisture exposure can damage some materials and create conditions where mold or deterioration may develop.
5. What Does Condensation Between Window Panes Mean?
Moisture between panes of sealed double or triple glazed windows is different from condensation on the room-side surface.
Modern insulating glass units contain multiple panes of glass separated by a sealed space.
The space may contain an inert gas such as argon and typically includes a spacer system around the perimeter.
When the edge seal of the insulating glass unit fails, moisture can enter the sealed space.
You may notice:
- A cloudy appearance
- Haze that cannot be cleaned away
- Water droplets between panes
- A streaky appearance inside the glass
- Fogging that comes and goes with temperature changes
Because the moisture is trapped inside the insulated glass unit, cleaning the inside and outside surfaces will not remove it.
6. What Causes Window Seal Failure?
A sealed glass unit is exposed to years of changing temperatures and weather conditions.
Over time, several factors can contribute to seal failure.
6.1. Age
Window components can deteriorate as they age.
The repeated expansion and contraction that occurs through Ontario’s seasonal temperature changes can place stress on seals.
6.2. Temperature Changes
Homes throughout London, Kitchener-Waterloo, Oxford County, and Chatham-Kent experience hot summers and freezing winters.
Those temperature cycles can cause window components to expand and contract repeatedly.
6.3. Installation Issues
Window performance depends on proper installation.
If the frame is improperly supported, distorted, or otherwise installed incorrectly, unnecessary stress may be placed on window components.
6.4. Physical Damage
Damage to the glazing unit, frame, or surrounding components can also affect the integrity of the window.
6.5. Normal Wear Over Time
Even a properly installed window is exposed to sunlight, moisture, temperature changes, and weather for years.
Eventually, some components may reach the end of their service life.
7. Can a Failed Window Seal Be Repaired?
If condensation is trapped inside a sealed insulating glass unit, reducing indoor humidity will not correct the failed seal.
The solution may involve replacing the insulated glass unit rather than replacing the entire window, depending on the age and condition of the window system.
Before deciding, consider:
- How old the window is
- Whether the frame is still in good condition
- Whether the window opens and closes correctly
- Whether the hardware works properly
- Whether the problem affects one window or several
- Whether the window is still under warranty
- Whether there are additional drafts or performance issues
If the frame, sash, and hardware remain in good condition, replacing only the sealed glass unit may sometimes be reasonable.
If the window also has deteriorated frames, operating problems, air leakage, or several other issues, complete replacement may offer better long-term value.
8. What Does Condensation on the Outside of a Window Mean?
Seeing condensation outside your window can seem alarming, but it is not necessarily a sign of failure.
Exterior condensation may develop when:
- The outside air is humid
- The night has been cool
- The exterior glass surface becomes cold
- There is little wind
- The window is preventing indoor heat from warming the outside pane
This can occur with high-performance windows because improved insulation reduces the amount of indoor heat reaching the exterior glass.
Exterior condensation often disappears as outdoor conditions change or sunlight warms the glass.
The key difference is location.
If you can wipe the moisture from the outside surface, it is not the same problem as condensation trapped between the panes.
9. Does Window Condensation Mean You Need New Windows?
Not necessarily.
This is one of the most important distinctions for homeowners.
9.1. Interior Condensation
If condensation is forming on the room-side glass, first investigate:
- Indoor humidity
- Ventilation
- Air circulation
- Outdoor temperature
- Window coverings
You may be able to reduce the problem without replacing the window.
9.2. Condensation Between Panes
If moisture is trapped between panes of a sealed insulated glass unit, the seal may have failed.
The glass unit or window should be assessed to determine the appropriate solution.
9.3. Exterior Condensation
Exterior condensation alone generally does not indicate that the window needs replacement.
Where the condensation is located matters more than simply whether moisture is visible.
10. Can Energy-Efficient Windows Reduce Condensation?
Higher-performance windows can help reduce the likelihood of interior condensation because their interior glass surfaces can remain warmer during cold weather.
Features that can contribute to improved performance include:
10.1. Double or Triple Glazing
Multiple layers of glass create insulating spaces that help reduce heat transfer.
Triple glazed windows can provide stronger insulation than comparable double glazed systems.
10.2. Low-E Coatings
Low-emissivity coatings are designed to reduce unwanted heat transfer through the glass.
10.3. Inert Gas Fills
Insulating glass units may contain gases such as argon between panes to improve thermal performance.
10.4. Improved Spacer Systems
The spacer around the edge of an insulating glass unit can influence temperatures around the perimeter of the glass.
10.5. Insulated Frames
The window frame and sash also contribute to overall thermal performance.
10.6. Airtight Construction and Installation
Air leakage can affect both comfort and energy efficiency, making proper manufacturing and installation important parts of overall window performance.
Douglas Window & Door’s vinyl window solutions are ENERGY STAR certified, giving homeowners energy-efficient options when replacement is appropriate.
However, even an efficient new window can experience interior condensation if indoor humidity is too high for the outdoor conditions.
Replacing windows should not be used as a substitute for addressing a significant indoor moisture problem.
11. How Ontario Winters Affect Window Condensation
Ontario homes experience large seasonal changes in temperature and humidity.
During winter, outdoor temperatures in Southwestern Ontario can fall well below freezing while indoor temperatures remain comfortable.
That creates a large temperature difference across your windows.
The colder the outdoor temperature becomes, the greater the chance that the interior glass surface may reach a temperature where moisture in the indoor air condenses.
This is why a window that looks perfectly clear during fall may begin sweating during a cold January morning.
For homeowners in London, Kitchener-Waterloo, Oxford County, and Chatham-Kent, winter condensation should therefore be evaluated alongside:
- Indoor humidity
- Outdoor temperature
- Window performance
- Ventilation
- Air circulation
- Window age and condition
Looking at all of these factors provides a much clearer picture than assuming every foggy window has failed.
12. When Should You Consider Window Replacement?
Replacement may be worth exploring when condensation is only one of several problems with the window.
Consider having your windows professionally assessed if you notice:
- Persistent condensation between sealed panes
- Multiple failed glass units
- Drafts around the window
- Windows that no longer close correctly
- Locks or hardware that no longer operate properly
- Warped or deteriorated frames
- Significant air leakage
- Moisture damage around the window
- Older windows with several recurring problems
- Rooms that consistently feel uncomfortable near the windows
The decision should be based on the condition of the complete window, not condensation alone.
13. How to Prevent Window Condensation
While you cannot control Ontario’s outdoor weather, you can take steps to manage indoor moisture and improve conditions around your windows.
13.1. Monitor Humidity
Use a hygrometer to track relative humidity.
If condensation becomes excessive during colder weather, reducing indoor moisture may help.
13.2. Use Exhaust Ventilation
Use properly vented bathroom and kitchen exhaust fans when creating moisture.
13.3. Keep Air Moving
Avoid completely blocking windows with furniture or tightly closed coverings for long periods.
13.4. Operate Your HRV
If your home has an HRV or similar ventilation system, use and maintain it according to its operating requirements.
13.5. Check Window Condition
Inspect:
- Weatherstripping
- Frames
- Locks
- Seals
- Caulking
- Glass units
Identifying small issues early can help prevent larger problems.
13.6. Deal With Persistent Moisture
Occasional light condensation during extreme cold may occur, but persistent heavy condensation, frost, or water collecting around surrounding materials should not be ignored.
The moisture source and window condition should be investigated.
14. Ready to Find Out Why Your Windows Are Foggy?
If your windows are sweating, cloudy, or developing condensation, start by identifying exactly where the moisture is located.
Condensation on the interior glass may be related to humidity, ventilation, airflow, or window surface temperature. Moisture trapped between sealed panes can indicate window seal failure, while exterior condensation may simply be the result of outdoor weather conditions and efficient glazing.
Douglas Window & Door Inc. helps homeowners throughout London, Kitchener-Waterloo, Oxford County, Chatham-Kent, and surrounding Southwestern Ontario communities understand whether their window problem calls for better moisture control, glass replacement, or a complete window upgrade.
Request a free no obligation quote, book a consultation, or call the team today to have your windows assessed and understand the right next step for your home.
15. FAQ: Foggy and Sweating Windows in Ontario
15.1. Why are my windows sweating on the inside during winter?
Interior window condensation usually develops when warm, moisture-containing indoor air reaches cold window glass. Indoor humidity, outdoor temperature, ventilation, air circulation, and window performance can all affect how much condensation forms.
15.2. Does condensation between window panes mean the seal is broken?
If moisture is trapped inside a sealed double or triple glazed unit and cannot be wiped from either surface, it can indicate that the insulating glass seal has failed. The sealed glass unit should be assessed for repair or replacement.
15.3. Can you fix condensation between window panes?
If condensation is inside a sealed insulating glass unit, lowering indoor humidity will not repair the failed seal. Depending on the window, replacing the sealed glass unit may be possible. If the entire window has additional damage or performance problems, full replacement may be worth considering.
15.4. How do I fix sweating windows in winter?
Start by monitoring indoor humidity, using bathroom and kitchen exhaust fans, operating your HRV if you have one, improving airflow around the windows, reviewing humidifier settings, and removing standing condensation. If the moisture is between the panes rather than on the room-side surface, the solution is different.
15.5. Is exterior window condensation a problem?
Usually not. Exterior condensation can occur during cool, humid conditions and may even appear on high-performance windows because less indoor heat reaches the exterior pane. It normally disappears when outdoor conditions change.
15.6. Will replacing my windows stop condensation?
Energy-efficient windows can have warmer interior glass surfaces and may reduce the likelihood of condensation, but replacement cannot correct excessive indoor humidity on its own. Moisture levels and ventilation still need to be properly managed.
15.7. When should foggy windows be replaced?
Replacement may be worth considering when fogging between panes occurs alongside other problems such as drafts, damaged frames, poor operation, air leakage, failed hardware, or several aging windows with recurring issues.