Q: I noticed that fog sometimes forms on my windows. Is this something to worry about, and how do I fix foggy windows?

A: Foggy windows are a common occurrence here in New England. You might find moisture on the glass panes on the inside of the window as well as the outside, depending on the time of year. You might even notice that even new replacement windows have foggy glass. But what gives?

What is Condensation and Why Does it Happen?

Foggy windows example image showing mold caused by condensation on the windows overtime.

The first step to understanding foggy windows is to learn about condensation. According to the US Geological Survey, condensation is the process of when water vapor in the air is changed to liquid. It occurs on the glass of windows when the surface is cooler than the surrounding air. You can also experience condensation on your eyeglasses or a cold glass of lemonade in the summer.

Since condensation is a natural part of the water cycle, it’s entirely normal to find fog on your windows from time to time.

When Does Condensation Occur?

The formation of condensation depends on environmental factors and sometimes the time of year. For example, condensation commonly occurs on the outside of the window during the summer. This is due to high outdoor humidity levels with moisture in the air collecting on the glass. It’s essentially the same concept as finding dew on the grass in the morning.

However, in the wintertime, condensation may form inside of the interior glass because the air outside is cold and the interior air is warm. This indicates that the humidity levels in the home are quite high.

It’s important to never ignore window condensation—especially if it doesn’t go away. Knowing why condensation is occurring on the windows is important for energy savings. Condensation can be a major sign that the window is no longer thermally efficient, which directly results in lost energy dollars.

When are Foggy Windows a Problem?

Condensation shown resulting in foggy windows.

Although condensation is a natural yet common culprit for foggy windows, there are other reasons why you’re experiencing condensation—and it can sometimes signal a major problem:  seal failure.

These are a few red flags for problematic condensation (which typically indicates seal failure):

  • When you see fog on the inner panes of glass (doesn’t wipe off), the seal no longer has argon/krypton gas in between the panes of glass. It is filled with standard air which doesn’t have any energy efficient properties. This means that the insulator of the window is gone and no longer working to maintain indoor temperature.
  • Think of this example: With Argon/krypton gas energy (your home’s heating and cooling) moves through the thickness of gas like honey. If your thermostat is set for 70 degrees and its 30 degrees outside, the air starts to blend together since the gas is gone.  Since energy travels through standard air much faster than gas, it results in lost money on energy bills.
  • When you see fog inside the glass panes, you are seeing the barrier that allows energy/heat to leave and come into your home (see above example. It’s essentially the same concept as a cold/warm front, just like a weather system.
  • With seal failures, you’ll want to be careful about finding moisture on wood windows and vinyl If this problem isn’t addressed, it can result in moisture and mold damage. According to the United States Environmental Protection Agency, interior condensation may also result in mold growth. This is most commonly seen in bathrooms. It’s important to note that lesser-grade vinyl that is not UPEC (high quality virgin vinyl windows) is porous and can be prone to mold and mildew if not properly maintained. Low grade vinyl window frames are like the cheap plastic containers with spaghetti sauce.

How Do I Know if Condensation is on the Inside or Outside?

If you suspect that you have condensation inside the window panes, try cleaning the glass first. Sometimes what is thought to be fog is buildup or dirt, especially if this window is old or located in the kitchen.

If the fog doesn’t go away after wiping it, it is most likely between the glass panes and an indication of seal failure.  When this is the case, there is often no choice other than replacing the window. Replacing the entire window can bring extra benefits aside from addressing seal failure, such as lower utility bills and reduced drafts.

Help! My New Replacement Windows are Foggy

With professional window installation, an airtight seal is created around the frame to block out drafts and moisture. If you discover condensation on new windows, this is often a sign that the windows are doing their job and conserving energy in the home.

Look for virgin vinyl windows with warm-edge spacers. This is important for separating the panes of glass that impact edge temperature, which is typically colder in the center of the window. Condensation resistance measurements run between 1-100—the highest numbers are most effective.

Replace Foggy and Outdated Windows with Coastal Windows & Exteriors

Replacing foggy windows with new high quality options.

Coastal Windows & Exteriors services Massachusetts, southern New Hampshire, and Maine with quality lifetime replacement windows. If condensation is a problem with your windows, contact us for some of the most energy efficient windows in New England. Contact John with questions at svanderbilt@mycoastalwindows.com or call 978-304-0495 for a quote.