What Causes Window Condensation?

On a regular basis I find either condensation staining or excessive condensation on and “in” windows. In our harsh Wisconsin climate we get extreme weather. Large swings in temperature and humidity can cause certain conditions that may harm your home and/or your home’s indoor air quality. In this post I wanted to discuss window condensation, what causes it, and steps homeowners can take to control it.

What Causes Condensation?

All air contains water vapor. Warm air can hold much more moisture than cold air since the water vapor particles are far apart. As that air cools, the water particles compress and at just the right temperature (the dew point) the water particles come together in a process that is known as condensation.

In our winters it is not uncommon to find condensation on our windows when the outdoor temperature reaches single digits. In some cases extreme temperatures and high indoor humidity can cause condensation so severe that water may pool on window sashes. The balance of indoor humidity to outdoor temperature is essential to preventing condensation from forming on your windows. Generally speaking acceptable indoor humidity would fall between 40-60% but I can tell you from experience, if you have a new more sealed up home and if your humidity is more than 50% you are likely to get excessive condensation at certain cooler times. It is not likely you will be able to completely eliminate it unless you left your windows open.

I recently was called to a friends home to analyze his situation. The condensation was so great that there were puddles on his window sashes, and even large amounts of condensation in basement wall cavities. This was certainly an extreme case. What we found was that the indoor humidity was likely way too high in relationship to the 10 degree outdoor temperature. This was a newly constructed home and all of the materials had yet to dry out, which was causing high indoor humidity levels. I would guess that in a years time he won’t experience such an extreme case because the humidity will naturally be brought down.

What do I do to prevent/limit condensation?

There are a few steps that can be taken to limit the amount of condensation that forms.

  • Run ceiling fans. Keeping ceiling fans on to move air during periods of cold weather will promote airflow and will help to dry the glass.
  • Open curtains and blinds. During these times keeping window coverings open is essential to allow air to flow to the glass.
  • Wipe any excess moisture off the window with a cloth or paper towel. Keeping your window sashes as dry as possible is necessary in preventing deterioration and/or moisture staining.

Depending on how tightly sealed your home is as well as your humidity levels will determine the amount of condensation that forms.

What about condensation in-between the layers of glass?

Double pane windows have a layer of gas, usually Argon or Air, in-between two layers of glass. This extra space acts as insulation reducing heat (energy) loss. If your multiple pane window appears foggy or misty, or you notice condensation inside the glass, your window seal that holds the gas in place has failed and has lost it’s insulation value. Silica desiccant is a material that is used to absorb excess moisture. This material is used in the aluminum perimeter to absorb condensation that forms on a regular basis. Over time the silica will become saturated and no longer be able to absorb moisture. At this time the sash will need to be repaired or replaced as condensation will become visible.

The good news is that if your windows appear foggy or have condensation present, replacement of the sash or the glass assembly alone is generally possible without the need to replace the entire window. Some companies claim that the window can be repaired through a process called “de-fogging” The jury is out on the effectiveness of this process. Basically, this process only cleans the stained area between the glass so it appears aesthetically pleasing. The insulation value is lost and condensation is likely to return during certain conditions.

To summarize, it is not uncommon to have condensation form on your windows in the dead of winter, it is probably normal in-fact. The steps you take to control that condensation is the important part so more damage and possible mold growth doesn’t occur.

As always, I hope you took some value from this post and if you have any home related questions, please let me know at the email address below.



Aaron M. Zuehlke is the owner and inspector at Zuehlke Inspection Service, LLC, a full-service home inspection company serving Southern Wisconsin. Specializing in Home inspection, Radon Testing, Mold Testing/inspection, Residential Thermal Imaging, and Manufactured Home Foundation Certifications. He also manages several rental properties through Zuehlke Properties, LLC. He can be reached by email at Aaron@Zuehlkeinspections.com or by calling the office at 608-931-7485.

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