Frequently Asked Questions about condensation.

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Window condensation is a natural and common occurrence caused by differences in temperature and humidity.

When a surface is cooler than the surrounding air, moisture in the air condenses on that surface, forming small water droplets. This is the same process you see on a cold glass of lemonade on a warm day, on car windows, and on home windows—especially during colder weather.

For homeowners with new windows, outside condensation can be unexpected. However, it is actually a positive sign.

Exterior condensation occurs when the outdoor air is slightly warmer and more humid than the window glass. This often happens during mild weather, when temperatures are neither very hot nor very cold.

Outside condensation indicates that your windows are effectively preventing heat transfer. In other words, your windows are doing a good job insulating your home. If you notice fog on the outside of your windows, there is no cause for concern.

If the windows are easily accessible, you may wipe away the condensation. In most cases, though, it will disappear on its own once the sun reaches the glass.

It is still important to inspect window sills and frames. Gaps between the window and the frame can allow moisture to collect indoors, which may signal moisture inside the walls. Checking for drafts helps protect your home from potential water damage.

If condensation forms on the inside of your windows, it should be taken seriously. While interior condensation is often caused by steam from showers or cooking, it can also indicate high humidity levels and poor ventilation in your home.

Excess moisture can be harmful to both your home and your family’s health. When a home is not properly ventilated, moisture from daily activities—such as bathing, cooking, and normal household water use—builds up in the air. With nowhere to escape, this moisture condenses on windows and can also settle on walls, furniture, carpets, and inside wall cavities.

This leads to two major concerns.

First, excess moisture creates ideal conditions for mold and mildew growth. Mold often appears around or below windows as small black spots or gray patches, but it can also grow on wood, carpet, insulation, paper, and other materials. Mold not only damages your home but can also affect your health. Exposure may cause symptoms such as coughing, nasal congestion, eye or throat irritation, skin irritation, and allergic reactions that resemble asthma or hay fever. In many households, at least one person may be sensitive to mold, leading to ongoing health issues if the problem is not addressed.

Second, trapped moisture can damage the structure of your home. Wood can swell, soften, or rot, while metal components may rust. Over time, this deterioration can weaken insulation and structural elements, potentially resulting in costly repairs.

The best way to reduce mold and mildew is to prevent interior window condensation by controlling indoor humidity and improving ventilation. Taking these steps helps protect your family’s health while keeping your home safe, durable, and energy-efficient.

The most serious type of window condensation occurs between the panes of glass.

When condensation forms inside a double- or triple-pane window, it often indicates a significant problem. While it may seem impossible to address moisture trapped between panes, this type of fogging usually means the window has failed and will need to be replaced.

Most modern windows are double- or triple-paned, meaning they are made of two or three layers of glass sealed together within a single frame. The space between the panes is filled with an insulating gas, such as argon, and sealed to prevent air and moisture from entering. This design helps improve energy efficiency and indoor comfort.

Condensation between the panes indicates that the airtight seal has failed and the insulating gas has escaped. As a result, moisture can enter the space between the glass layers when temperatures drop below the dew point. Once this happens, the window can no longer insulate properly.

This trapped moisture can cause further damage and significantly reduce energy efficiency, allowing heat to escape in winter and enter in summer. Over time, this leads to higher heating and cooling costs.

If you notice condensation between the panes, the window is no longer functioning as intended and should be replaced with a high-quality, energy-efficient window.

High humidity is not the only cause of interior window condensation. If condensation appears on only a few windows, the issue may be related to air leaks or drafts. When moist air enters the home around a specific window, it tends to condense on that window first. In this case, the window should be inspected for gaps, and a professional may need to reseal or replace it, depending on the extent of the issue.

Interior condensation can also indicate a more serious problem if the source of moisture is unclear. If you manage steam properly by using exhaust fans, closing doors, and taking precautions while cooking and showering, yet condensation persists, there may be hidden sources of moisture in your home.

Common but often overlooked sources include firewood, which can retain moisture—especially if stored outdoors or in a garage—and houseplants, which release moisture into the air. During colder months, this moisture can settle on cooler window surfaces.

Condensation on the inside of windows is a concern, and in some cases, it may point to a larger issue that requires further investigation.