Condensation on windows: What causes it and how to manage it

Why is there fog on your windows? Should you be concerned? We’ve got the answers to your frequently asked condensation questions. 
Condensation on glass

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 is the key to understanding if it 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. 

How to prevent condensation on the inside of your windows

To help prevent condensation inside your windows, reduce indoor humidity. Here are a few ways to do that:

  • 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.  

How to get rid of condensation on the outside of your windows?

You don’t need to worry about this! It’s just dew. Eventually the sun will come out and help dry up the condensation outside.  

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. 

How to get rid of condensation in between the glass?

When the seal on your window breaks and condensation starts to form in between the panes of glass, you won’t be able to prevent it from forming. This condensation is an indication that your window is no longer performing well, which means the ideal solution is to replace your windows. Here are a few things to keep in mind when considering replacement:

  1. Select energy-efficient windows: From the type of windows to glass coatings and even details like grilles, you can make choices that will help your new windows perform optimally for your climate. We’ll explain how to pick out energy-efficient windows.
  2. Understand any financial incentives: If you select energy-efficient windows, you may be eligible for a tax credit. In addition, some utility companies offer rebates for installing more energy-efficient windows. We can help you understand the energy-efficient home improvement tax credit.
  3. Find a replacement to match: If you’re only replacing one or a few windows, you might be wondering how to make sure they’ll match your existing windows. To do this, consider sight lines (amount of glass to framing) as well as the more obvious features, like color and grilles. Try our design tool to understand all the ways you can customize your new windows.

Did you know? You can make selections that fool the eye. So let’s say you live in a traditional home full of double-hung windows, but you’d like to replace a few of those windows with casements because of their superior energy efficiency. You could order your casements with grilles applied in such a way that the new windows look like they are double hung. 

How to stop condensation on windows?

In summary: You can help manage condensation on the inside of your windows by adjusting your indoor humidity following the pointers above. You don’t need to do anything if the condensation is forming on the outside of your windows — it’s the result of atmospheric conditions, and it will go away when the sun comes out. Your windows are not at fault in either of these scenarios. It’s only when you notice condensation in between the glass panes that the window is failing and you need to start planning for replacement.

Still have questions about condensation?

Here are some answers to more frequently asked questions.

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