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What Causes Condensation on My Windows?

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The Case of the Condensation Concern



Dear Glass Detective,

I have new replacement triple-pane windows and I live in central New York. The windows were installed in July. Since then, the windows have had heavy condensation on the outside on several occasions. The windows they replaced never had that problem. What could cause this?


James M.

Holland Patent, NY

insulating glass, triple glazing, double glazing



Dear James,

Thank you for making contact with the Glass Detective with your concern over why your new triple-pane insulating glass windows are getting condensation on the outside. You are specifically concerned about why your new windows are experiencing condensation when your old windows apparently did not. This is actually a case that, with some variation, the Glass Detective solves often. You don’t say what kind of windows you had before you installed the new triple-pane windows and I am wondering if they were single-glazed or double-pane insulating glass units prior to the new ones being installed. I would also be interested in knowing if the new windows are fixed, double-hung or casement type windows.

Regardless of the new window type, I would think that, based on where you live, that you must have experienced some of this condensation issue previously, especially at this time of year. I would be quite surprised if you had not had this happen before with the original windows. Assuming that your new windows were installed by a competent company and that the job was done correctly, the condensation forming on the outside of your new windows should not be of concern. Without getting too deep into the theories and proven conclusions regarding condensation, let’s take a quick look at what causes condensation.

Condensation forms when the air cannot handle any more water vapor (moisture). The air becomes saturated (theoretically 100% humidity) and when it comes in contact with a solid cooler surface, the moisture in the air forms/collects on the cooler surface. The moist air has reached its “dew point”, which is the temperature at which condensation occurs. Think of a glass of ice water—when it is put into a room with high relative humidity, moisture starts to condensate on the surface of the glass. The dew point (around and on the glass) has been reached. We know that warm air can hold more water vapor (moisture) than cold air so the room air temperature is much higher than the temperature on and around the glass of ice water. So as the warm moist air circulates around the glass of ice water, it has to relieve itself of some of its moisture. A simplistic explanation for sure but an accurate one nonetheless. For more on dew points, relative humidity and condensation, you can poke around Google for a while or maybe borrow a sixth-grade science book.

The point is, this is a natural phenomenon and probably nothing to worry about whatsoever. I believe you have been experiencing high relative humidity in your area and as the air temperature cools at dusk, you are reaching a dew point that allows the moisture in the air to deposit itself on your new windows. Try listening to the nightly weather forecast and you will probably hear the forecaster stating the relative humidity and the dew point. Again, the dew point is the temperature at which relative humidity reaches 100% and therefore is at a point when condensation will form on solid surfaces.

Now, if you start getting a bunch of moisture accumulation when the dew point is not being reached, you might have a different problem altogether, but I am fairly confident that you will not. I really hope this response has been of some value to you and I again thank you for contacting the Glass Detective.

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Lyle Hill

Lyle Hill has been in the glass and metal industry for more than 40 years. In this time he has managed glass retail, contract glazing, mirror, architectural window, window film, and automotive glass businesses throughout America. He obtained an MBA from IIT with a focus on Technology and Engineering Management. Hill is also a columnist for glass industry trade magazines and often called the “face” of the glass industry. He has also authored books including “The Broken Tomato and Other Business Parables,” which is available through Amazon. Find out more about Lyle on Linkedin.

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2 Responses

  1. If that is the case, then why don’t all the other windows in the house do it? I have been tracking the relative humidity everyday and do agree that is what is causing it. If what you say is true then all windows everywhere would do this.
    Isn’t it possible that the chemical composition or the construction of the window unit be partly to blame?

    1. You stated that the condensation was on the “outside” of the new windows. We take this to mean the farthest exterior glass surface … not a surface within the triple pane insulating glass unit. So our initial summary/report is still accurate if this is the only place the condensation is being seen.

      You question why the condensation is not appearing on other windows. Are the other windows on the same side of the house, next to the ones that are having condensation.? Moving air, sun, or other factors could be contributors to uneven condensation.

      You ask if there is a possible defect in the window construction that allows some to condensate and others to not. If the windows are on the same elevation of the house and have exact interior and exterior conditions and some condensate and others do not, you could have a defective window (insulating glass problem). All things being equal, all (equivalent) windows should react the same way.

      The composition of the windows (the desiccant/absorbent) used in the manufacturing process could cause interior (within the air cavity BETWEEN the glass panes) to fog if it was defective or inadequate, but would not significantly contribute to any exterior condensation which is what I believe you are reporting.

      We hope this helps explain the situation. Have you asked the installer/manufacturer for comments or perhaps a site visit to help you with this? That maybe should be your next step if the above does not clear up any remaining doubts or confusion.


      The Glass Detective attempts to answer all questions accurately but cannot be held liable for any information provided or omitted. You should always work with a licensed, insured and reputable glass shop that can asses your specific needs and local building codes and offer professional services. Never attempt to cut, install or otherwise work with glass yourself. The Glass Detective answers questions on an informational basis only.

      (C) 2019 Inc. All rights Reserved. No reproduction without expressed written permission. Questions? Contact

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