When a customer points to water on a window, they're usually blaming the window. Building science tells a different story. Windows don't create condensation. They simply reveal what's happening inside the house.
Window condensation is one of the most misunderstood issues in housing. It’s the visible symptom that appears on the glass, but the cause always lives somewhere else entirely. That's why Energy Advisors, builders, professional and DIY renovators, and homeowners looking to hire someone need some building science savvy to understand the problems that are showing up as condensation on windows.
We've seen this repeatedly in the field. A homeowner replaces every window in the house, only to discover the condensation actually gets worse. The windows didn't fail. They’re telling the story of the house.
CONDENSATION:
Liquid water that forms when warm, humid indoor air contacts glass that's cold enough to reach the dew point.
What Causes Window Condensation?
Window condensation forms when warm, humid indoor air touches glass that's cold enough to reach its dew point. It isn't caused by the window itself, but by the relationship between:
Indoor humidity – more moisture in the air means more water available to condense.
Cold glass – windows are usually the coldest interior surface during winter.
The dew point – when the glass temperature falls below this point, water vapour changes into liquid water.
In other words, windows don't create condensation—they reveal the balance between heat, air, and moisture inside the house. Let's look at why.
Windows Don't Sweat. Air Does.
The first thing to understand is that glass doesn't produce water.
Condensation happens whenever warm, moisture-laden air contacts a surface that's cold enough to reach the dew point. At that temperature, the air can no longer hold all of its moisture, so water changes from vapour to liquid. That’s physics.
The window is simply the place where we get to see physics in action.
Think about an iced drink on a hot summer day. The glass isn't leaking but it's wet on the outside. That’s because water from the surrounding warm air condenses onto the cold surface.
Exactly the same thing happens indoors during winter.
Why Does It Happen More in Winter?
In Canadian winters, the outside temperature drops dramatically (well, maybe not so much in Victoria or the Lower Mainland of BC). Winter means batten down the hatches. The temperature difference between the indoor and outdoor air becomes greater. There is more heat flow to the exterior, and we shut the windows for the season to keep the warm, humid air inside.
Keep in mind that windows are always the coldest surface in a building in the winter. Even the best-performing window is still colder than the surrounding insulated wall because glass has a lower insulating value than an insulated wall assembly. As outdoor temperatures drop, the interior surface of the glass approaches the dew point more easily.
Just like that iced drink in the summer, the bigger the difference in temperature, the colder the window glass becomes. Eventually the interior surface of the glass becomes cool enough for moisture to condense. Remember that it’s not just temperature that causes condensation, it’s the combination of temperature difference and the relative humidity inside the house. That’s how windows that are often perfectly dry during mild weather begin collecting moisture during a cold snap.
It's a House Problem, Not a Window Problem
"What's wrong with this window?"
The window often tells the story, but it rarely writes it. One of the biggest mistakes people make is treating condensation as a window problem. The mold that could lead to rotting sills is definitely a window problem, the condensation that is causing the mold growth is a whole-house problem.
Using A House as a System/Envelope First perspective, we ask:
"How is this window, the coldest visible surface in the system, being affected by the conditions inside the house?"
| House Condition | Effect on Condensation |
| High indoor humidity | More moisture available to condense |
| Poor ventilation | Moisture stays indoors longer |
| Oversized humidifiers | Indoor RH remains too high |
| Air leakage around windows | Cold air causes drafts |
| Thermal bridging | Lowers surface temperatures |
| Poor air circulation | Cold pockets develop near glazing |
New Windows Can Mean More Condensation
New high-performance windows are supposed to be better than old windows. So why do people sometimes notice more condensation after replacing them?
This surprises many homeowners who have windows replaced. But here’s the thing: Colder air holds less moisture, and older houses often leak so much air that indoor humidity stays relatively low throughout the winter.
So what’s causing condensation on new windows?
Depending on the age of the house, and the number of windows replaced, there are two things that are directly related to replacement windows that can go a long way to reducing air leakage.
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Window units sold in Canada must meet a standard for air tightness as well as thermal performance. Older units typically have air leakage around the perimeter of each piece of glazing, the subframe and the frame as well as at the hinge or lock area of each operator (the part of the window that opens).
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Installing new windows includes measures that help to reduce uncontrolled air leakage between the window unit and the opening it fits into. Older units typically have air leakage between the unit itself and the wall opening it sits in.
If other air sealing work was done at the same time, that means the way the envelope of the house interacts with heat, air, and moisture flows is very different than it was originally. It doesn’t have as much air exchange between the inside and outside.
As a result, the ‘better windows’ mean more indoor humidity, which means visible condensation. The windows aren't creating the moisture problem—they are exposing an imbalance elsewhere in the house. And that is the key to the whole story: unless ventilation and humidity management are addressed, indoor moisture levels often increase, leading to condensation on windows.
Relative Humidity Matters More Than Most People Think
To keep the house and the people living in it healthy, the indoor relative humidity should change with outdoor temperature. As it gets colder outside, indoor humidity usually needs to decrease. Too much humidity during very cold weather increases the likelihood of:
- window condensation
- mould growth
- damage to wood trim
- paint failure
- moisture accumulation inside wall assemblies
Comfort is a result of balancing temperature, air movement, radiant temperatures, and humidity. That, dear reader, is the core of applied building science. This balancing act is what the energy efficiency sections of the National Building Code, the BC Energy Step Code, and the Ontario Building Code’s Supplementary Standard 12 are based on.
Not All Condensation Means the Same Thing
Understanding where condensation occurs helps identify the likely cause. Each location tells a slightly different story.
| Location | Likely Cause |
| Centre of interior glass | Indoor humidity too high for conditions |
| Lower edge of glass | Cooler air pooling near window |
| Corners of glazing | Spacer edge thermal losses |
| Around window frame | Air leakage or thermal bridge |
| Between panes | Failed insulated glazing seal |
| Exterior surface | High-performance glazing stays cooler than outdoor air |
Why This Matters to Builders
Window condensation isn't just a homeowner annoyance. Builders, renovators and contractors who understand the relationship between humidity, temperature, and air movement, and how that relationship is affected by air sealing/air leakage, occupant behaviour, and ventilation are much more likely to solve the real problem instead of replacing perfectly good products.
That builds confidence with clients and reduces unnecessary warranty claims.
Common Myths About Sweaty Windows
Myth: Condensation means the window is defective.
Usually false. Most interior condensation is related to indoor humidity and surface temperature.
Myth: Triple-pane windows eliminate condensation.
They reduce the likelihood because interior glass stays warmer, but extremely high indoor humidity as a result of a tighter envelope and inadequate mechanical ventilation can still produce condensation.
Myth: Lowering humidity always makes homes uncomfortable.
Not if ventilation, temperature, and air movement are properly balanced.
Myth: Opening windows is the only solution.
Sometimes. But better ventilation control, moisture management, and understanding the whole house often provide a more effective long-term solution.
The House Has the Last Say
One of the most important ideas in building science is that every change affects something else.
Install better windows → Humidity may rise.
Improve airtightness → Ventilation becomes more important.
Increase insulation → Surface temperatures change.
Every upgrade influences the rest of the system. You can’t diagnose the problem of condensation by looking at the window alone, you have to look at the whole house. Condensation is the result of other conditions in the house.
Learn More
If you're serious about understanding why comfort, moisture, air leakage, and condensation are all connected, explore:
The more you understand the system, the fewer "mystery" condensation problems you'll encounter.
Free Download
Download: Winter Window Condensation Troubleshooting Checklist
A one-page guide covering:
- Where is the condensation?
- Is it inside, outside, or between panes?
- Indoor humidity checklist
- Ventilation quick check
- When to suspect installation problem
- When to suspect glazing failure
faqs
Why do my windows only sweat in winter?
Cold outdoor temperatures lower the temperature of the interior glass. If indoor humidity remains high, the glass can reach the dew point and condensation forms.
Does window condensation mean my windows are bad?
Not usually. Most interior condensation is caused by the relationship between indoor humidity and surface temperature rather than a defective window.
Can new windows increase condensation?
They can make existing moisture problems more noticeable. Better windows reduce air leakage, allowing indoor humidity to increase if ventilation isn't adjusted.
Is condensation harmful?
Occasional light condensation isn't unusual - a hot shower or a boiling pot of pasta can overwhelm your ventilation for a short period. Persistent moisture can damage trim, encourage mould growth, and signal that indoor humidity should be reduced.
Should I replace my windows if they sweat?
Not until you've identified the cause. Often condensation issues are solved by improving ventilation, managing humidity, reducing air leakage, or addressing thermal bridges rather than replacing the window itself.
Want to understand why houses behave the way they do—not just memorize symptoms?
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