Window fog or condensation is normal and isn’t always a cause for concern. It will happen at one point or another and can be observed at any time of the year, especially when there are drastic changes in temperature and humidity.

HOW CONDENSATION FORMS

Fog will appear as a thin coating of water or frost on window glass when its surface temperature is lower than its dew point. That’s the basic condition required for condensation to form.

What’s the dew point? The dew point is the maximum temperature at which water can exist in vapor form. In other words, it is the saturation temperature for water in air. Below that, water vapor begins to condense or form dew, hence the name.

WHY IT HAPPENS WHEN IT’S COLD

Normal household activities—like baths, showers, cooking, or laundry—all contribute to indoor humidity, so you can expect that there’s a decent amount of water vapor in the air in your home at any time of the year.

When it’s cold outside, the surface temperature of even the most energy-efficient window glass will drop to a certain extent. If its temperature dips below its dew point, condensation will form on the interior surface of your window glass.

Part One: Why Windows Fog Up

DEW POINT50 F

WARM MOIST AIR70 F

COLD GLASS45 F

WHY IT HAPPENS WHEN IT’S HOT

Condensation can form on both interior and exterior window glass surfaces when it’s hot:

• Interior. Provided the ambient temperature remains more or less constant, an increase in indoor relative humidity will result in a corresponding rise in the dew point. Once it gets to the point where there’s so much moisture in the air that the dew point goes above the ambient temperature, condensation will form.

This is why fogging is a common occurrence in bathrooms and kitchens.

• Exterior. The effect here is the reverse of what happens in winter. During hot, humid summers, condensation can form on the outer surface of your window glass when your air conditioning cools it to below the dew point.

This is common in areas where the humidity is high (e.g. near a lake or river) or when you have a lot of plants just outside your window.

Part One: Why Windows Fog Up

WHY IT HAPPENS WHEN IT SHOULDN’T

If there haven’t been any big changes in temperature or humidity to account for your foggy windows, that may mean that something is wrong.

Generally speaking, condensation outside of the instances we described above is a sign of physical damage to or around the window, infiltration of moisture-laden air, or window failure. In any of these cases, you’ll want to stop and take a better look.

Part One: Why Windows Fog Up

Part Two: Why Windows Fail

Windows put up with a fair amount of abuse to keep you protected from the elements. Their seals and frames contract and expand with dips and spikes in temperature. Their glazings are tested by wind, rain, hail, and harsh sunlight. And then there are those who subject them to their own brand of abuse by constantly banging them shut or taking a laissez-faire attitude to maintenance.

Over time, this damage accumulates, leading to eventual window failure—and fog.

The most common reasons behind window failure include:

Repeated and prolonged exposure to waterWithout the right safeguards, puddles can form along the perimeter of a window. Having this happen too often can quickly compromise the seal.

Excessive heat or coldCycles in temperature cause window glass to expand and contract over and over, softening and weakening window seals. Not only does this reduce a window’s insulating properties but also lead to more air leakage and condensation.

Old ageSometimes a window is just too old to function properly. You can extend its service life with religious maintenance, but time will inevitably get the better of even a well-cared-for window.

CAN’T WINDOW FAILURE BE PREVENTED?

Ultimately, no. Strictly speaking, the best anyone can do is delay window failure, but it is possible for windows to last multiple generations with the right amount of care. Here’s what else you can do:

• Choose windows with generous warranties. Most windows these days are covered for 10 to 20 years. Make sure you go for products that have the best warranties within your budget. Should premature failure happen, the manufacturer or contractor may be able to take care of the problem for you.

BONUS: Warranties give you a pretty good clue as to how good products really are. The more confident manufacturers are about their products’ quality, the more willing they are to back them up with generous warranties.

• Make sure your windows are properly installed. Don’t waste your investment by scrimping on installation. Even the best window products can fail prematurely in the hands of a careless contractor.

• Be proactive about window maintenance. It’s hackneyed advice, but prevention does trump cure. Do a visual inspection of your windows. Keep an eye out for drips, gaps, and rot. Do an incense test if you suspect air leaks. Spotting problems early on can and will save your windows.

Part Two: Why Windows Fail

Part Three: What You Can Do about It

There are three ways by which you can reduce or eliminate window condensation: keep water vapor production down, get rid of moisture-laden air, or improve your windows’ thermal performance.

TO KEEP WATER VAPOR PRODUCTION DOWN:

- Don’t use a humidifier when the weather is cold. Cold air has a lower vapor capacity than warm air so any moisture added to the air can quickly condense. If you must use a humidifier, follow the guidelines recommended by window and humidifier manufacturers:

- Take shorter showers.

- Water your house plants only as needed.

- Make sure your house has proper drainage. When water ponds around your house, moisture can seep through, increasing your home’s indoor humidity.

TO REMOVE MOISTURE-LADEN AIR:

- Increase the ventilation in a room or throughout your house. Even simply keeping a door open can help keep the air flowing.

- Open up your window treatments during the day. In the colder months, it helps to allow the warm room air slightly heat up your windows to keep their temperature from dropping below dew point.

- Open up your window treatments during the day. In the colder months, it helps to allow the warm room air slightly heat up your windows to keep their temperature from dropping below dew point.

TO IMPROVE WINDOWS’ THERMAL PERFORMANCE:

- Add another layer of glazing. Short of replacing your windows, you can do this by adding storm windows.

- Re-seal your windows. Fresh weatherstripping will cover any cracks and gaps that may have been causing your window to fog up.

- Stuff desiccant between layers of window glazing. While this works, do take note that this is only a band-aid solution. It solves the fogging issue but will not take care of the cause of the condensation.

- Caulk the exterior wall beside your windows. In some cases, the window itself isn’t the one to blame. Moisture may be passing through small gaps and openings around it.

Depending on the degree of fogging you have to deal with, it may be possible for you to fix the problem using one or more of the means described above. Your best bet is a combination of all three strategies, which will cover both the symptoms and the causes of condensation.

Good luck!

Part Three: What You Can Do about It