Window condensation 101

Why is there fog on your windows? Should you be concerned? We’ve got the answers to your frequently asked condensation questions.
 A father dries off his son after a bath in a bathroom with a three-panel sliding glass door.

If you’re noticing condensation on your windows, you might be wondering whether or not you should be concerned. The short answer is: It depends. It depends on where the condensation is forming, i.e., inside, outside, or in between the glass. It also depends on the time of year and even the age of your home — condensation on windows in a newly built home is a common temporary occurrence.

Let’s explore what you need to know about condensation on your windows.

What is condensation? Why does it form on windows?

When warm air comes in contact with a cooler surface, condensation forms. The same way your glass of ice water “sweats” on a hot summer’s day, your window might fog up on a cold winter’s day.

Condensation happens because the air can only hold so much water vapor (moisture) before it changes into liquid water droplets. The amount of moisture the air can hold varies by temperature. Because cold air can hold less moisture than warm air, window condensation happens most often in the wintertime when your windows, warmed from the heat inside your house, meet the cold air outside.

Where the condensation forms on your window is the key to understanding whether or not the condensation is a problem.

Condensation on the inside of your windows

When condensation forms on the inside of your windows, it can block your view, drip on the floor, or freeze on the glass. It can be annoying, and it can cause problems if left unchecked. The reason condensation forms inside the window is because the humidity level is too high indoors, not because there is something wrong with the window.

Humidity is the moisture in the air. It forms when we breathe, when we cook, when we shower, when we do laundry, and other everyday activities. It’s always present in air, and it’s not an inherently bad thing — as you’re probably aware if you’ve ever bought a humidifier. However, humidity can cause damage to your home if it’s too high for too long — for example: cracking, peeling or blistering paint; mildew and mold; and more.

If you’re noticing condensation on the inside of your windows in the winter, it’s a warning sign and you should take action. Here are a few ways to reduce indoor humidity:

  • Vent all gas appliances, clothes dryers, and exhaust fans to the outside. Your attic and crawl space should also be ventilated. Cover the earth in the crawl space with a good vapor barrier.
  • When you cook, make sure to run the exhaust fans in the kitchen. When you bathe or shower, run the fans in the bathroom until your mirror is clear. Be careful not to overheat exhaust fans by running them too long.
  • Open your drapes or shades during the day so warm air can reach your windows.
  • Use ceiling fans to promote air movement.
  • Avoid storing firewood in your house or basement.
  • If you have a forced air furnace, make sure your home is properly ventilated by installing a fresh air intake. If your home is extremely “tight,” it may be helpful to install an air-to-air heat exchanger.
Tip: In the winter months, you should aim for 30 to 35 percent humidity in your home. Some thermostats measure humidity. If yours doesn’t, you can buy an inexpensive tool called a hygrometer to measure humidity.

Condensation on the outside of your windows

When condensation forms on the outside of your windows, it’s really just dew — just like wet grass on a summer morning. Dew forms on windows when the glass is cooler than the dew point (the point when water vapor turns into liquid water droplets.)

It’s most common to see condensation outside your windows on spring and fall mornings when humidity levels are higher and cool nights are followed by warm days. You may only notice this a few times a year. When you do, rest assured, it’s the result of conditions outside — not a sign your windows are failing.

In fact, condensation on the outside of the window is a sign of energy efficiency. It means the outside pane of glass is thoroughly insulated from the heat indoors.

Condensation in between the glass

Most windows these days are dual pane, which means there are two panes of glass with an air space in between. If you ever notice condensation in between the two panes of glass, it’s a sign that the seal on your window has failed. Unfortunately, this is an indication that your window needs to be replaced. Older windows can lose their seal over time, making them less energy efficient.

So, in summary:

Condensation on the inside of your window means humidity is too high indoors, and condensation on the outside of your window means conditions are humid outdoors. Your windows are not at fault in either of these scenarios. It’s only when you notice fog in between the glass panes that the window is failing.

Still have questions about condensation?

Here are some answers to more frequently asked questions.
What does excess condensation do to my windows?

Too much condensation can cause the paint to peel on your window’s sash and damage the window frame.

Why does condensation sometimes form on windows in new construction homes?

All the building materials used in a new construction or remodeling project can produce a great deal of moisture. All that moisture can cause condensation to form on the windows. When heating season starts, the excess moisture will dissipate. It should disappear during this first heating season and not cause any further issues.

I’ve noticed condensation since I turned on my heat. What’s going on?

Your home probably absorbed moisture during the humid summer months. So, when this humid indoor air meets the window glass chilled from the colder weather, condensation forms. After a couple of weeks, the moisture in your house should dissipate and the condensation should disappear.

Is condensation worse in some areas than in others?

Yes, condensation is more common in climates where the average January temperature is 35°F or colder.

What time of year is condensation the worst?

Condensation is most common in the winter, but it can occur whenever your window’s temperature is lower than the dew point (temperature when air vapor, or moisture, turns into water droplets). That’s why condensation sometimes appears on the outside of windows in the spring or fall (and occasionally on hot, humid summer days). This is usually an indication that windows are energy efficient.

Why is there condensation on my bay window?

Since bay and bow windows project out from the insulated house, they’re usually a few degrees cooler in temperature. To help prevent excessive condensation, insulate between the window head and platform. Adding extra insulation between the window head and platform can also help in very cold climates. As a secondary measure, try placing a fan near the window to help promote air circulation and reduce condensation.

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 A father dries off his son after a bath in a bathroom with a three-panel sliding glass door.