Condensation on the inside of windows can make mornings unpleasant, damage the windowsill, and over time encourage mold to form in the window corners. The good news is that in most cases it’s not a “window defect,” but a combination of indoor humidity, glass temperature, and airflow around the window. Once you understand the cause, the solution is often surprisingly simple.
Windows fog up on the inside when warm, humid indoor air touches cold glass and the glass surface temperature drops below the dew point. Most often, this is caused by high humidity (showering, cooking, drying laundry) combined with poor ventilation or cold thermal bridges around the frame. It helps to reduce humidity, ventilate strategically, and improve the flow of warm air near the window.
What condensation actually means and why it happens
Condensation is simply water vapor turning into liquid water. Indoor air always contains some moisture, but how much it can “hold” depends on temperature. Warm air can hold more water vapor than cold air—so when warm air is suddenly cooled on a cold surface, the excess moisture turns into tiny droplets.
Dew point in plain English
The dew point is the temperature at which water vapor starts turning into water. For example, you might have a comfortable 21 °C (70 °F) in the room, but if the inside surface of the glass is much colder, it can easily drop below the dew point and the window will “fog up.” The higher the humidity indoors, the higher the risk of condensation—because the dew point rises.
Why it happens mostly in winter and at night
In winter it’s cold outside, so the glass (especially along the edges and near the frame) has a lower surface temperature. At night, people often ventilate less; blinds and curtains restrict the flow of warm air toward the window; and the room may cool slightly. Add moisture from breathing during sleep, and in the morning the condensation is at its worst.
The most common causes of inside condensation
Some homes “produce” a lot of moisture; others can’t get that moisture out. Condensation on the inside is usually a sign that two things have come together: high humidity and a cold window.
High humidity from everyday activities
Showers, baths, cooking, washing dishes, drying laundry indoors, or lots of houseplants—these all gradually raise humidity. The issue is often not a one-off event, but the day-to-day “cumulative effect” of small things that shows up on windows because they’re typically the coldest surface in the room.
Poor air exchange in an airtight home
After replacing windows, condensation sometimes gets worse even though the windows are “better.” That makes sense: old leaky windows let some moisture escape unintentionally; new ones seal well, so humidity stays indoors unless you deliberately vent it out. This is often compounded by keeping trickle vents or ventilation slots closed, which reduces the supply of fresh (usually drier) air.
Cold spots and thermal bridges around the frame
Condensation often starts at the glass edges, in corners, and near the frame. That’s where the surface is usually coldest—whether because of the frame design, metal spacer bars in the insulated glazing unit, or poorly insulated reveals and lintels. Even small “cold strips” can trigger condensation, even if the center of the glass looks relatively fine.
Curtains, blinds, and furniture that block airflow
If a window is “cut off” from the room’s warm air, the glass cools more and condensation starts sooner. A typical scenario is a radiator under the window, but long curtains are drawn during the day and at night, preventing warm air from rising along the glass. Similarly, furniture pushed tightly against the wall near the window can keep air stagnant in corners.
When condensation forms between the panes
If moisture appears between the panes of double or triple glazing and can’t be wiped off from the inside, it usually indicates a failed seal in the insulated glass unit. In that case, ventilation or dehumidifying won’t help, because the condensation isn’t coming from indoor humidity—it’s happening inside the space between the panes. This typically requires service or replacement of the insulated unit.
When condensation is “normal” and when it’s a problem
Occasional fogging after a shower is normal, especially if your bathroom doesn’t have strong exhaust. The real problem is a long-term condition where droplets form on the windows, run down the glass, and regularly soak the sill or the plaster around the window. Then it’s not worth waiting—damp materials around the window can develop stains, swell, or grow mold over time.
Signs it’s excessive humidity
If your windows fog up almost every day—especially in the morning—and the air feels “heavy,” or your mirror stays fogged for a long time after showering, the humidity is likely higher than ideal. Excess moisture is often also revealed by condensation on colder walls, in room corners, or behind furniture. Windows are just the first place it shows up.
How to quickly check the cause at home
The fastest way is to stop guessing and start measuring. Within a few days you can tell whether the main issue is humidity, cold spots around the window, or a combination of both.
Measure humidity and track when condensation forms
Start with a hygrometer (humidity meter) and the simple habit of noting when the windows fog up. Compare days when you dried laundry indoors, cooked without a range hood, or took longer showers with days when you ventilated intensively. If condensation varies a lot with household routines, the cause is almost always humidity and ventilation.
Check exactly where the water forms
If condensation forms mainly in corners and near the frame, there’s a good chance you have a colder surface there (a thermal bridge, weaker insulation in the reveals, or poorer airflow). If the sill and the bottom edge of the frame are wet too, check whether curtains are preventing heat from the radiator from rising toward the window. Sometimes adjusting window coverings and your ventilation routine is enough to significantly reduce the problem without any building work.
Practical solutions that work
You can eliminate condensation in two ways: either reduce the humidity in the air or raise the surface temperature of the glass (or both). In practice, that means better ventilation, better moisture extraction at the source, and proper warm-air circulation near the window.
Ventilation that works—even in winter
Short, intensive airing is often more effective than leaving a window tilted open all day. When you open windows wide for a few minutes, you replace humid air with drier air and the walls don’t have time to cool down too much. If you have heat recovery ventilation (HRV) or mechanical ventilation, stick to settings that ensure regular air exchange at night too—when condensation most often forms.
Remove moisture where it’s generated
In the bathroom, an exhaust fan helps—especially if it runs after the shower until the steam has fully cleared. In the kitchen, a range hood vented outdoors (or at least intensive ventilation while cooking) matters, because cooking can release a surprising amount of moisture into the air. When drying laundry indoors, it helps to keep it in a separate room with the door closed and ensure moisture is vented out; otherwise the humidity spreads throughout the home and ends up on the coldest windows.
Improve warm-air circulation near the window
Even a good window can fog up if the air near the glass is stagnant. Pull curtains aside so heat from the radiator can rise along the glass, and set blinds so they don’t block airflow completely. If there’s furniture by the window, leave at least a small gap so corners don’t turn into “cold pockets” of still, humid air.
Long-term fixes: thermal bridges, insulating the reveals, window servicing
If condensation keeps recurring at the edges and in corners even with reasonable humidity, the problem may be construction-related—cold reveals, a lintel, incorrect installation, or weaker insulation around the frame. In that case, insulating the problem areas, improving the details around the window, or upgrading the insulated glazing unit (where relevant) can help.
Video: a clear explanation of why windows fog up on the inside
If you want to see the dew-point principle and practical tips in action, this video illustrates the topic nicely on real windows:
Summary
Condensation on the inside of windows is most often a combination of higher indoor humidity and cold glass that drops below the dew point. Start by measuring humidity and tracking when condensation gets worse—that will tell you the most. In most households, targeted ventilation, extracting moisture during showers and cooking, and allowing warm air to flow along the window will help; but if condensation repeatedly appears in corners, it’s worth addressing thermal bridges and the construction details around the frame as well.
Sources
- U.S. Department of Energy – Moisture Control. Available at: https://www.energy.gov/energysaver/moisture-control(The Department of Energy’s Energy.gov)
- U.S. Environmental Protection Agency – What are the main ways to control moisture in your home? Available at: https://www.epa.gov/mold/what-are-main-ways-control-moisture-your-home (US EPA)
- University of Minnesota Extension – Controlling moisture problems in your home. Available at: https://extension.umn.edu/moisture-and-mold-indoors/controlling-moisture-problems-your-home(extension.umn.edu)
- Building Science Corporation – Controlling Cold-Weather Condensation Using Insulation. Available at: https://buildingscience.com/documents/digests/bsd-controlling-cold-weather-condensation-using-insulation(Building Science)
Jana
I like turning curiosity into words, and writing articles is my way of capturing ideas before they slip away — and sharing them with anyone who feels like reading.
