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Community and Q&A

Frost on outside of triple pane window

Myrtleboone | Posted in General Questions on

Every so often I notice frost/water (depending on outside temp) developing on the outside pane of my triple pane windows. It’s infrequent but will mostly notice it early mornings. Actually, as I sit here this evening (68F indoors, 28F outdoors) the exterior of my windows are frosting up a bit. Interior pane is warm to touch. No moisture problems inside. Air exchange keeps interior moisture around 35-40% in the winter. Moisture (minimal-in bottom corners) only forms on interior windows when it’s -10F and below. Thoughts? Thanks.

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  1. GBA Editor
    Martin Holladay | | #1

    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.

  2. charlie_sullivan | | #2

    Assuming this is on the outside surface of the outside pane, it's not a problem, and is an indication that your windows in face have very low heat loss, which is a good thing.

  3. Myrtleboone | | #3

    Yes. The frost is on the outside of the outer pane. And yes, I live on a hill with no tree cover.

  4. GBA Editor
    Martin Holladay | | #4

    "Yes, I live on a hill with no tree cover."

    Very occasionally, I get to feel like Sherlock Holmes.

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