ACMO Blog
Why Do Windows Get Foggy?
Why Do Windows Get Foggy?
As a homeowner, you probably don’t spend much time thinking about your windows. Beyond occasionally opening them to let in fresh air or cleaning them for a clear view outside, they tend to fade into the background of daily life.
That all changes, however, when your windows start to fog up. Persistent window fogging can be frustrating and may even raise concerns about potential underlying issues. So, what’s really going on when your windows fog up, and what can you do about it? Let’s dive in.
At the heart of foggy windows is a simple scientific principle: condensation. What you perceive as fogginess is actually moisture condensing on the glass, forming a layer of water that clouds the surface. Condensation occurs when warm, humid air comes into contact with a cold surface, like your window. Warm air can hold more moisture than cold air, so when it hits the cooler glass, some of that moisture is left behind, creating condensation. Windows are often among the coldest surfaces in your home, making them prime spots for this phenomenon.
Where Is the Condensation on Your Window?
Condensation on the Inside Pane
If your windows are fogging up on the inside, it’s likely because warm, humid indoor air is meeting cold outdoor temperatures. In most cases, this doesn’t mean anything is wrong with your windows. It’s often just a result of natural conditions in and around your home. However, it’s worth investigating further to rule out any potential issues.
Condensation on the Outside Pane
In the winter, the warm air inside your home can cause condensation on the inner surface of the window. Conversely, in the summer, the warm, humid air outside can condense on the cooler, air-conditioned glass. This exterior condensation is typically nothing to worry about and should clear up as the sun warms the window during the day.
Condensation Between the Panes
Modern double-pane windows are designed to insulate better than older single-pane versions by trapping air between the two panes. When condensation appears between the panes, it’s often due to a broken seal, which allows moisture to enter where only air is supposed to be. However, don’t jump to conclusions just yet—it’s possible the fogging is on the inside or outside, not between the panes.
Quick Test to Check for Window Condensation
Before you start assuming there’s a seal issue, here’s a quick test: Are multiple windows in your home fogging up? If so, it’s probably not a seal problem. The odds of seals breaking in several windows at once are low, meaning the issue is more likely weather-related.
To confirm, run your finger along the inside of the window. If it gets wet, the moisture is likely condensation on the inside surface, not between the panes, indicating the seal is still intact. If the moisture isn’t on the inside, go outside and check the exterior. If the condensation is there, your seals are probably fine.
If you have old windows (20 years or above), that have continual condensation, replacing the windows or in some cases changing the broken thermos will reduce significantly the amount of condensation if you opt for high energy-efficient windows1. The good news is that new window technology offers improved insulating value and can greatly reduce condensation. High-performance windows, which have special “low-E” coatings and are filled with inert gases such as argon can provide energy savings that are considerable1.
Homeowners might experience even with their new energy-efficient windows outdoor condensation. This occurs when moisture from the air outside the building condenses on the surface of the windows, creating a foggy or cloudy appearance. While this can be a cause for concern for some homeowners, it is typically not a sign of a problem with the windows themselves.
The windows are designed to prevent heat from escaping the building, which means that the exterior surface of the window stays cooler than it would with traditional windows. When warm, humid air outside comes into contact with the cooler surface of the window, the moisture in the air condenses on the glass, creating the appearance of fog or cloudiness. Moisture condenses out of the air onto a cold surface that is said to be below the dew point. The dew point varies with the air temperature and the amount of moisture it contains. Particularly in spring and autumn, the glass temperature can fall to a low level during the night and the dew point can be comparatively high in these seasons. The glass temperature can be below the dew point under these conditions and moisture can condense onto the surface. This condensation is a sign that the windows are performing well and keeping the heat in, making them superior insulating glass products.
Efficient windows have special “low-E” coatings and multiple glass panes with inert gas between them to reflect heat back into the home's interior. Under specific conditions, such as quick temperature changes and high humidity, the exterior glass surface may cool below dew point temperature and collect moisture from the outside air, resulting in condensation. This is a natural occurrence and does not indicate any problems with the window.
With modern low emissivity glass, more heat is kept inside, resulting in a lower outer pane temperature and greater risk of condensation on the external surface. While this may seem like a problem, it is actually a sign that the windows are functioning as intended. The condensation is a result of the windows creating a barrier between the inside and outside of the building, preventing heat from escaping and saving energy. In fact, if there is no condensation on the windows, it may be a sign that the windows are not functioning properly and heat is escaping.
There are steps that can be taken to reduce or eliminate outdoor condensation on energy-efficient windows:
- Use HVAC equipment to manage indoor humidity. Lowering the humidity levels in your home can significantly reduce wintertime window fogging. Adding a dehumidifier to your HVAC system is a great investment that not only reduces condensation but also makes your home more comfortable year-round.
- Use a dehumidifier to remove excess moisture from the air. This can be particularly effective in areas with high humidity levels, such as bathrooms or kitchens. Reducing the amount of moisture in the air, will reduce the likelihood of condensation on the windows
- Clean your windows regularly. A build-up of grime can create a rough surface where condensation is more likely to form. By keeping your windows clean and smooth, it becomes harder for moisture to accumulate.
- Use curtains or shades. Closing curtains adds an extra layer of insulation between your windows and the warm indoor air, reducing the likelihood of condensation. On cold winter days, keeping them closed can make a big difference.
- Reduce the number of plants in your home and water them less. They release water vapours;
- Use the bathroom exhaust fan when showering or bathing; and
- Remove interior screens on windows during cold months.
Lastly, ensure that the windows are properly installed and sealed. If there are gaps or leaks around the windows, warm, humid air can enter the building and condense on the surface of the windows. Proper installation and sealing can help to prevent this from happening. New windows should also be much more air-tight due to the improved construction of the window and better, more efficient installation, sealing and caulking of your windows. Because new windows are more air-tight, the moist air in your home is less likely to escape. That means homeowners need to pay more attention to humidity levels their homes.
Homeowners can enjoy the energy savings and other benefits of their energy-efficient windows, while minimizing the inconvenience of outdoor condensation. Ultimately, the benefits of energy-efficient windows far outweigh the minor inconvenience of outdoor condensation, making them a smart and responsible investment for any homeowner. We hope you find these tips helpful in managing window condensation and keeping your home comfortable!
1. Moisture Problems – Why Should I Worry About Moisture Problems? Natural Resources Canada’s Office of Energy Efficiency, 2007 Cat. No. M144-59/2007E or ISBN 978-0-662-45438-0, pages 5-7
2. https://oee.nrcan.gc.ca/pml-lmp/index.cfm?action=app.formHandler&nr=1#searchResults
3. file:///C:/Users/Verdun/Downloads/Strengthening%20Canada's%20Building%20Code%20Process%20to%20Achieve%20Net-Zero%20Emissions.pdf Strengthening Canada’s Building Code Process to Achieve Net-Zero Emissions Kevin Lockhart and Brendan Haley October 2020
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