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Community and Q&A

Is there a way to solve the problem of condensation on the outside of windows?

user-6809777 | Posted in General Questions on

Hi there, 
We have just built an almost passive house on Canada’s west coast.  Our windows are triple-paned and passive house certified.  Because they are so well insulated, on many clear, cold days in the late fall, winter, early spring, condensation will form on the outside of the windows.   
Many people have suggested that we are lucky, because that means the windows are doing a great job insulating.  However, we would love to see our view on those clear winter mornings. 
Has anyone had a similar problem and has any solution worked for them?

Thank you!
Beth C

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  1. Yupster | | #1

    To quote Martin: "The phenomenon you observe is the same phenomenon that causes dew or frost to form on blades of grass or your car windshield.

    "The outdoor air contains moisture. At the end of a clear, cold night, the temperature of some surfaces drop below the dew point -- these surfaces get cold enough for condensation or frost to form. At that point, the moisture in the outdoor air condenses (or is deposited as frost) on the cold surfaces.

    "Surfaces exposed to the sky (not protected by tree cover or cloud cover), especially horizontal surfaces, are usually the first to get cold enough for this to happen, due to the phenomenon called nighttime radiative cooling. Here's what happens: the car windshield or blades of grass radiate their heat to space. Nothing stops this radiation on a clear night, so the car windshield or blades of grass can get colder than the outdoor air temperature.

    "When this phenomenon occurs on a vertical surface, as apparently happens on your triple-glazed windows, it's usually a safe bet to declare that your windows have a good view of the sky. This typically happens when a house is on a slope or on the top of a hill, and the windows face the downhill view, without intervening trees.

    "The phenomenon is more likely to happen with triple-glazed windows than single-glazed windows, because a single-glazed window is kept above the dew point by heat leaking through the window. (The panes are warm, all night long, as long as the house is heated.) With triple-glazed windows, the innermost pane is warm, while the outermost pane is cold."

    So, block their view of the sky? Put in worse windows? Hire a neighbours kid to standby with a rag? Blow a hair dryer on the outside surface? Sorry, don't have a useful suggestion but at least now you know why! Maybe someone else has run into this before and solved it...

  2. Expert Member
    Akos | | #2

    There are anti fog films for boat windows. Don't know how well they work, but have seen them at the marine stores.

  3. GBA Editor
    Martin Holladay | | #3

    Yupster is quoting from an answer I posted on a Q&A thread on the topic ("Frost on outside of triple-pane window").

    For more information on this topic, see this article: "Night Sky Radiation."

    In that article, I wrote, "Night sky radiation also explains why triple-glazed windows sometimes get condensation or frost on the outside of the window. This happens during certain mornings after a clear night. The phenomenon is most common on windows facing downhill on homes perched on a steep hill: these are the windows that are most likely to 'see' the sky."

  4. user-6809777 | | #4

    Hi there! Thank you for your quick reply. You pretty much exactly described how our house is situated and where the windows are located. I like your suggestion of hiring a neighbourhood kid with a rag on standby! We are wondering if some kind of coating might repel the condensation somewhat. Or perhaps some sort of drip irrigation system, although maybe that's not worth the effort. I only now thought to ask the Green Builder community because of Martin's most recent article on condensation on swim masks/mirrors.

    1. Expert Member
      MALCOLM TAYLOR | | #9


      I'm quite close by on southern Vancouver Island. I'm happy to send you one of my neighbour's kids and will throw in a supply of rags and free shipping. His name is Daniel and I don't want him back.

  5. Aedi | | #5

    One temporary solution is to try a water-repellent windshield treatment like Rain-X. It will not prevent condensation from occurring, but accumulated water will bead and slip off the window quicker, improving visibility. I have not personally tried this though, so I don't know how well it works. You'll probably have to re-apply every year.

    Insulated exterior shutters are a more permanent solution, but are costly and inconvenient. If night sky radiation is the problem, as Martin suggests, then ordinary shutters, or other outdoor shades, might work.

  6. Expert Member
    BILL WICHERS | | #6

    I was about to suggest trying Rain-X. Rain-X coats the glass with a film that allows the water droplets to slide off easier (it’s really cool to watch when used on a car). It’ll help the condensation to run off too, but the very small droplets that makeup the condensation will likely not slide off as dramatically as rain drops would.

    You could use some mini sprinkler sprayers to “hose down” the windows on command if you really want to clean it quick. Orbit makes some cheap ones (similar to rain drip, but less expensive). Sprinklers combined with rain-X would be a quick way to clear your windows.


  7. user-6809777 | | #7

    Yes, we were thinking of Rain-X. Thank you, Bill, for the recommendation of Orbit as a source for a sprinkler system. We may attempt that.
    Thank you everyone else for all your swift and helpful replies!
    Beth C.

  8. Jon_R | | #8

    Windows awnings would block much of the sky view.

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