Why are my windows fogging on the outside?

It usually happens on a morning when you’re already behind schedule: As you approach your car, you can see that the windows are covered with frost or condensation, and you have to spend time — almost always time you don’t have — clearing it off to drive safely. No one enjoys clearing frost or condensation on cold mornings, but turning on the heat often takes a while to solve the problem. Unfortunately, driving with foggy windows isn’t an option.

Related: Why Do My Car’s Windows Fog Up?

In another of life’s mysteries, at times only some of a car’s outside windows may have frost or condensation, not all. What’s up with that? Why does moisture sometimes only seem to settle on certain spots while others remain clear?

Having condensation on some of your windows while others remain clear is a common — though frustrating — problem. It can make driving dangerous and throw your day off schedule. What causes this inconvenience, though?

What Causes Some Car Windows to Fog Up?

If you’ve guessed that the air temperature is part of the issue, you’re right. The temperature of the air outside and even inside your vehicle plays a major role in the development of condensation.

The main cause of condensation is a difference between the temperature of the air and the temperature of the glass. Condensation on the outside of a car is caused when warm, moist air (often present in early morning hours) condenses when it hits the car’s colder window glass. During cold weather, turning the heat off inside your car may also cause moisture to build up on your windshield or windows. That’s also why on warm, humid days, condensation may form at the base of the windshield, where outside air encounters cooled air from the car’s air conditioner. Generally speaking, when warm air meets cold air, moisture develops on surfaces. When the surface is a window, it fogs up.

Mats Selen, a University of Illinois physics professor, explained that when warm, moist air cools, it eventually gets to a temperature at which it can no longer support all the water and starts to condense. That’s the dew point, or the point at which moisture begins to settle on cold surfaces.

“When relative humidity is 100 percent, it means that if the temperature goes down any more, then the water has to come out somewhere,” Selen said. “On a cool morning, when there’s dew [or fog] on the ground, that’s because you have warm, moist air that comes in contact with the cool ground” — the same goes for condensation on a car that’s been sitting outside.

When it’s cold enough, the condensation turns to frost.

But, why would only some car windows get condensation? Selen suggested that wind could be a factor, but another theory focuses on whether the windows are vertical (like side windows) or at an angle (like a windshield or the rear window on some cars). It’s possible that slanted surfaces are more likely to collect condensation because they provide a more horizontal landing spot.

During the colder months, you may start to notice fog on your windows at home. Here’s how to fix foggy windows and prevent the fog from returning.

Run the bathroom and kitchen exhaust fans when you shower and cook.

• Crack the windows open when it’s relatively warm and dry outside to air out your home.
• Make sure your clothes dryer vents to the exterior.
• Turn off portable and whole-house humidifiers. If necessary, run a dehumidifier.
• Fix plumbing leaks as soon as you notice them.
• Keep your heat set to the daytime temperature overnight to help keep windows from fogging up in the morning.
• Keep curtains open and run the ceiling fan to increase the circulation of warm air around the windows and help condensation evaporate faster.
• Seal exposed soil in the basement or crawlspace with a vapor barrier.
• Consider installing a whole-house heat recovery ventilator to exhaust moist, stale air to the exterior and replace it with clean, pre-heated air from outside.
• Install storm windows or films over single-pane windows to increase their insulating ability. Storm windows help keep the glass warmer, while window films block humid indoor air from reaching the glass. Both techniques work to prevent water from condensing on your windows.

What Causes Foggy Windows in a House?

The causes of foggy windows all relate to condensation. Picture an ice-cold drink with water beading up on the outside of the glass. This process occurs because the air cools immediately next to the glass. Since cold air can’t hold as much water vapor as warm air, the moisture condenses on the cool surface.

The same thing happens to the windows in your home when it’s cold. A combination of high indoor humidity and a cool outdoor surface collects condensation on the inside of the glass. High outdoor humidity (especially in the morning) and rain can also cause windows to fog up on the outside.

Humidity and Foggy Windows in Your Home

The average family of four releases about 1.3 gallons of water into the air every day from showering, washing, cooking, keeping houseplants, breathing, sweating, and so on. Seeing a regular window fogging in your home could simply mean that your home is too humid. By reducing the relative humidity, you should be able to keep your windows from fogging up.

How to Fix Foggy Windows that Just Won’t De-Fog

If your windows still fog up despite your best efforts to reduce indoor humidity, you might have a ventilation problem or old, single-pane windows. These windows are notorious for fogging up when it’s cold outside.

If you have double-pane windows and the fog appears between the glass, the window seal is likely broken, and moisture has gotten inside. Old age, improper drainage around the window, and years of exposure to harsh sunlight can cause the seal to break. The only way to address this problem is to replace the glass in the window or, sometimes, the entire window.

How to Prevent Foggy Windows with Help from Glass Doctor

Glass Doctor is here to help you prevent your foggy windows from leading to bigger problems such as water damage, mold growth and floor stains. Our expertly trained service professionals can repair, replace or install high-efficiency double-pane windows to keep your home safe and free of fog. Give us a call at (833) 365-2927 or schedule an appointment online today to get started.

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