7 Surprising Causes of Summer Window Condensation—and How to Stop It

Condensation on windows typically occurs when warm, moist air meets a cooler surface. The air cools rapidly in response to this contact, preventing it from holding as much moisture. The moisture that is lost from the air, remains behind on the cool surface.

While this is common in winter, it can also happen in summer too. Find out why you have condensation on your windows in summer, and when it’s a sign of a bigger problem.

High Indoor Humidity

For condensation to occur, the air needs to be moist enough for water to be left behind on the cool glass surface when the air comes into contact with the window. Having high indoor humidity increases the risk that condensation will form on the windows and other cool surfaces around the home.

“On a humid morning, window glass is cooler than the outdoor air, and water may condense on the outside,” says Mark Ginzburg, owner of EcoTech Windows & Doors. “This is harmless and will evaporate as the temperature rises through the day. If you are experiencing summer condensation inside, it is again likely to be a sign of high indoor humidity (potentially from cooking, a damp basement, or poor ventilation).”

Want more home reno project tips and inspiration? Sign up for our free daily newsletter for the latest how-tos, reno guides, and more!

Poorly Insulated Windows

When windows are not insulated properly, they don’t provide appropriate protection from outdoor heat and moisture, increasing the risk that condensation will form on the inside of the windows, especially in the morning when the sun if first rising into the sky.

“Inefficient windows, especially single-pane or aluminum-framed windows, transfer outdoor heat and don’t insulate well,” says John Parks, Certified Mold Remediation Professional and COO of Advanced Insulation System.

Parks says you can fix this issue by upgrading your windows to double- or triple-pane windows with Low-E coatings. This upgrade will provide better insulation, improving your home’s temperature and humidity.

Air Leaks and Gaps Around the Windows

Windows are designed to keep moisture out of the home and to prevent the transfer of heat, but if there are air leaks and gaps around the windows, warm, moist summer air will be able to flow into the home, even when the windows are closed.

“Air leaks or poor air sealing allow warm, moist air to infiltrate and condense on cooler surfaces, but you can seal air leaks around windows, doors, and attic hatches to prevent warm air from entering and condensing,” Parks says.

Don’t let condensation gather on your windows when you can take steps to properly seal the windows and other possible areas around the home where warm, moist air may be entering.

Blocked or Closed HVAC Vents Near the Windows

In some cases, the windows are not the cause behind the condensation formation.

“Blocked or closed HVAC vents near windows create cold spots that attract moisture,” Parks says.

Since condensation is formed when warm, moist air comes into contact with a cold surface, these cold spots create a perfect location for the moisture to condense.

To prevent this issue, keep the HVAC vents open and if you find that the vents are blocked, either clear them yourself or have an HVAC pro come to clear the vents and restore proper airflow.

Broken or Compromised Window Seal

If a window seal is broken or compromised, this can increase the risk of condensation forming both inside the home and between double- or triple-paned windows.

“In general, condensation happens when warm, moist air meets cooler surfaces (like the window glass) and it is usually a sign of excessive indoor humidity with insufficient ventilation,” Ginzburg says. “If the fog is between panes of glass, especially in double- or triple-glazed units, that usually means the seal is broken or compromised.”

Poor Air Circulation and Ventilation

Since many of the issues that contribute to indoor window condensation seem to be due to damaged windows, gaps, or compromised seals, it may seem odd that poor ventilation can also be a cause of condensation.

However, when the air inside the home is not properly ventilated, it can become saturated with moisture from indoor moisture sources, like the shower, dishwasher, clothes washer, or bathtub.

“Homeowners can improve air circulation by using ceiling fans, running bathroom [or] kitchen exhaust fans, and keeping doors open,” Parks says.

Old or Damaged Windows

The age and condition of your windows should always be considered if you are experiencing condensation problems or any other issues with your windows, like leaks or drafts.

Old or damaged windows can allow moisture and heat to enter the home, leading to higher indoor humidity levels, which when coupled with cooler indoor temperatures can increase the risk of summer window condensation. If this occurs, it may be time for a window replacement.

“Some of the signs that you may need a window replacement would be seal failure (noticing the fogging between the panes of your window), there are cold drafts or other frame damage, you have single-pane windows or other older styles, or if your energy bills are climbing,” Ginzburg says.

When Is It Time to Replace Your Windows?

Knowing how long windows last and when to switch out your windows is important for maintaining the security and condition of your home.

“Quality windows typically last 15 to 30 years, depending on the material, the quality of installation, and environmental conditions,” Parks says.

If you notice any of the following signs, you should consider window replacement:

• Visible condensation between panes indicating a failed seal.
• Warped, damaged, or rotted window frames.
• Noticeable drafts and rising energy bills despite having an efficient HVAC system.
• Poor noise insulation from the outdoors.
• You have single-pane windows, which don’t provide as much insulation, comfort, or moisture control as double-pane, or triple-pane windows.