Why Do Windows Fog Up

Whether it’s in your house or car, you’ve surely noticed times when your glass windows have looked foggy. You may have found yourself asking, “Why do windows fog up?” That’s a good question. You’ve probably also wondered if it’s a serious problem and how you can prevent it from happening. First, you need to understand the source of the fog. There are a few potential problems but also some potential fixes. Let’s take a look at the issue as a whole.

What is causing the fog?

While your instinct may be to blame your windows for fogging up, the truth is that it’s most likely due to the humidity inside your home or car. Humidity causes condensation, which causes your windows to fog.

If you’ve ever stepped out of a hot shower and tried to check your reflection in the bathroom mirror, only to wind up staring straight into a fog, then you know what we’re talking about. You’ve also surely seen your car windows fogged up in the morning as you are about to take your kids to school or embark on your morning commute. Both your bathroom and the interior of your car have higher humidity, which causes the fog.

Why does humidity cause windows to fog up?

So if it’s humidity causing the fog, then what’s the root of the issue?

The primary cause of this is humid air contacting a cold surface. This causes the air to cool and condense its moisture. If this happens on a glass surface, the condensation becomes visually foggy. You’re most likely to see this on windows or a mirror as the result of warm indoor air cooling itself on cold glass. Indoor air is especially humid because of activities such as cooking and bathing and due to breathing, all of which result in gallons of moisture per day being released into the air.

New technology in homes and cars has been able to use insulation to better regulate the humidity and help control heating and energy costs, but some moisture is often kept in the home by the same forces that keep air from the outside out of your home or vehicle. HVAC companies like Entek usually offer several options for managing humidity in your home.

Is humidity-caused fog bad?

Is all this foggy condensation a problem? Not necessarily. Sometimes it honestly doesn’t matter at all. However, the condensation can certainly create health problems if it’s not managed. Mildew and mold can survive and thrive in very moist areas that contain metal, plaster, and wood. If ingested, these mildews and molds can be responsible for sinus and nasal problems, respiratory issues, nasty allergic reactions, dizziness, asthma attacks, and overall weakness.

It should be noted that dehumidifiers were once thought to help with these problems, but research has shown that dehumidifiers can also cause health issues. Consulting an HVAC contractor is the best route to ensuring you’re fixing the problem and not causing more.

It’s worth noting that too much condensation fog caused by humidity can damage your home or vehicle. Indoor air can push its way through wood, concrete, and plaster, which can cause the deterioration of insulation, not to mention floor rotting, stains on walls and ceilings, foundation damage, peeling paint, and damage to apholstry.

Is condensation more likely in certain climates, types of homes or windows, or times of the year?

Yes, there are certain conditions that are far more likely to lead to fogging up, such as:

  • Car and home windows where the temperature is abnormally cold (below freezing) in the winter. This happens because the cold air meets the warmer air inside and turns to vapor, resulting in foggy windows.
  • Windows in warmer months when the hot air meets the cool glass and causes water vapor. This phenomenon can be exacerbated by the presence of plants, which give off an abundance of warm oxygen gas.
  • Quick, sharp increases or decreases in temperature. This creates a contrast between the air temperature and window temperature, producing a foggy condensation.
  • Building materials can cause a high level of condensation, so a site where any kind of new construction or remodeling is being done may see a spike in fogged-up windows.
  • Windows in areas with restricted circulation can be several degrees cooler, so any glass windows in such areas could see increased condensation.
  • Drapes and shades that are insulated and pulled down can restrict the flow of air to glass windows and cause a significant amount of condensation.

Can inside humidity be decreased?

Yes, inside humidity can be decreased, and there are a few ways to achieve this. First, you can increase ventilation by opening windows and allowing humid air to escape, while simultaneously letting dry air inside. The loss of heat won’t be noticeable, but the decrease in humidity could be significant. Using exhaust fans for kitchen and bathroom appliances will help. So will placing vents by energy-generating devices such as clothes dryers and gas burners.

You’ll also want to do your best to control the amount of moisture you allow inside. You can purchase a humidity monitor or use a humidity thermostat. Simple models can be acquired at a home center or building supply store, while more advanced thermostats are available through an HVAC company—and they can set the system up for you.

In addition, if you’re a person who has a lot of plants, do your best to confine all of your plants to one room with proper sunlight and do everything in your power not to overwater them, because extra water is a recipe for condensation.

Waterproofed floors and walls in your basement will be helpful to the cause because less water means less potential vapor and condensation. Also, be certain to properly insulate as many windows as you can. Proper insulation is an excellent safeguard against unwanted condensation because it keeps air flowing in and out in a healthy way. This helps avoid the issue of too much moisture, which will, in turn, lead to an overabundance of foggy condensation on your windows.