Frost on Windows


The Why, How, and What to Do About It.

Window Arens

We’ve had a lot of question this winter about failing windows. The temperature drops and clients call us in a panic when brand-new windows end up coated in frost – inside the house! The good news is that your windows are probably working just as they should, and there are things you can to to mitigate the frost. We’ll go over why this happens and when you should worry.

Specify the Problem

Let’s address the most pressing question – does the frost on the windows mean the units themselves are itself failing? This is usually the assumption by homeowners and it causes needless worry and panic. It’s easy enough to figure out if this is the case, just answer the two questions below.

  • How many windows and where is it happening?
  • Can you easily clean it off?

If you can easily clean off the condensation or frost and if this is something that is occuring on multiple windows at the same time, your windows aren’t failing. Keep reading to learn why it happens and what you can do about it.

Frost on Windows

This window has a broken seal that allows moisture between the dual panes of glass. The sash will need to be replaced.

If it’s only one window and you cannot clean the fog from either the inside or outside, this means that the seals have failed. Moisture is getting between the panes of glass and causing it to look dirty. Your best bet is to contact your builder or window rep for a sash replacement, and if these are very new windows, make sure you’ve got your warranty info on hand.

If the Seals Aren’t Failing, What’s Happening to My Windows?

Frost on Windows

Condensation on new double-hung windows.

Think about taking a shower. A ton of steam is produced and without a bath fan running, the steam collects on the coldest surface available. In your shower, it’s the mirror. In your home, it’s your windows. When the temperature of the glass drops below the dew point of the air in your home, you start to see visible condensation on the coldest areas.  We see this condensation problem most frequently on homes built in the 80’ and 90’s. Unlike earlier homes, these were built with an eye for energy conservation and the house-wrap used to eliminate heat loss also keeps all of the warm, yet moist air inside the home. Older homes have gaps, cracks, and other little fissures that foster air movement and help to prevent this effect. Water still condenses on the windows, but the air movement from aged and cracking seals wicks it away before it has time to freeze.


Keep this in mind once the air warms, too. This same effect will cause condensation on the exterior of your windows when the AC is blasting, and is an indicator that the windows are tightly sealed and performing as they should.

So What Can I Do About It?

The key is finding a balance between the level of humidity needed for your comfort, and the level of dryness needed to avoid frost on windows.

  •  Run the bath fan during showers and the exhaust fan during cooking to send that moisture right outside.
  • Simply opening a window for ten minutes will allow your stale moist air to be replaced with dry, fresh air. If there window blow, open the window on the opposite side of the home and let nature do all of the work.
  • Keep the overhead fan moving or put a fan in the room if you’re not keen on opening your home to the elements in winter. This will help prevent the moisture from settling on the windows.
  • Blinds and heavy coverings should be opened daily to avoid trapping cold and moisture between the window and the curtain.
  • Adjust the humidity levels on your furnace, if you have AprilAir. You’ll probably have to play to find a setting that is comfortable for you but not so intense that it’s leaving moisture on the windows.
  • If you don’t have a humidifier integrated with your furnace and you’re still seeing this problem, setting up a dehumidifier in the space will further eliminate excess water in the air.
Frost on Windows

Condensation combined with constantly closed curtains is a recipe for mold.

On more thing – while you’re questing to find the perfect balance of warm and dry, make sure you’re cleaning off that excess moisture. We have seen perfectly sealed, well-heated, gorgeous homes with mold on every windowsill. Wipe off the condensation when you see it. Rubbing alcohol on a cloth will get rid of frost quickly so there’s no worry about melting, but do what you must to avoid water buildup on your wood.

Confusion around this phenomenon is understandable, but you’ll save yourself a lot of time and worry by educating yourself on the how and why. Far from being a problem caused by cracking or imperfect seals, frost on windows actually happens because the seals are intact, tight, and functioning. The long and short of it is that building technologies are always evolving and as soon as we solve an old problem, a new one pops up on the horizon. Eliminate or lessen the effect by limited the amount of moisture in your home, and your gorgeous and functioning units will give you great views for years.

Frost on Windows