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Building Science

Choosing Windows: Condensation Resistance

Learn how to use information from two performance rating systems to select windows that will stay dry in given environmental conditions

The condensation on this window occurred at precise conditions of surface temperature and concentration of water vapor in the surrounding air. Photo: Energy Vanguard

Any reasonably energy-aware designer, builder, or homeowner is likely to look at the U-factor, solar heat gain coefficient (SHGC), and air infiltration of any window system prior to specifying or purchasing a particular product. And while this does make sense when defining a window system’s energy performance, it doesn’t suggest how much consideration should be given to other areas of window performance such as condensation resistance, design pressure, or sound transmission performance.

Energy considerations aside, any homeowner who tries to look out of their windows, but can’t because the outer glass is covered in fog, or who has had to trust in the structural strength of their windows while watching and hearing wind and rain buffeting the glass during a violent thunderstorm, or being woken at 7 a.m. on an otherwise quiet Sunday morning to the sound of their neighbor’s lawn mower, might not be worried about energy performance at that moment. It is for such times that condensation resistance, window structural strength, or noise blocking, respectively, should be given some serious consideration.

Let’s start with condensation resistance.

The Insulating Glass Unit (IGU)

IGUs are divided into edge of glass and center of glass for calculating both U-factor and condensation resistance performance.

For U-factor computations, edge of glass is the outer 2-1/2 in. of the glass surface measured from the glass/sash interface along the entire perimeter of the glass. Center of glass is the glass surface that is not within the outer 2-1/2 in. of the edge. For condensation resistance calculations the edge of glass is measured to 1 in. from the edge, rather than 2-1/2 in., but center of glass remains at 2-1/2 in. from the edge.

Both U-factor and condensation resistance computations for windows consider edge of glass and middle of glass as…

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  1. Expert Member
    Michael Maines | | #1

    Greg, thanks for this information, and your contributions in GBA comments over the years. I have been curious about windows I installed in my own house, Mathews Brothers double-glazed casements, made near me in Maine. I don't recall the performance values but I don't think I asked for anything special, so probably around U-0.30. They face northwest and on many mornings the outside is covered in condensation to the point that we can't see through them. We rarely use air conditioning and rarely open these two windows. Sometimes it's only one window. I'm guessing it's night sky radiation super-cooling the outer surface, and it goes away as the outside air warms up, but I'm curious if you have other advice on what might be causing it and whether there's a way to address it.

  2. [email protected] | | #2

    Thanks Michael, I truly enjoy the participating in Q&A so submitting an article for consideration seemed like a natural fit.

    I would agree that it sounds like night sky radiation. Just an odd thought, but have you ever tried the lighter or phone light trick on those two windows just to see what surface your LowE coatings were on?

    So far as limiting the formation, some folks swear by rainx or similar products to avoid exterior condensation. I have never used it, but might be worth a try to see what happens.

    1. Expert Member
      MALCOLM TAYLOR | | #3


      I didn't realize you were Oberon.

      I've learned a lot from your contributions in the Q&A. and thanks for the article. It went straight to my technical folder.

      1. [email protected] | | #7

        I have learned a lot from your contributions as well, so really glad that you found the article useful and thanks for all you do to help keep GBA as a valuable resource.

    2. Expert Member
      Michael Maines | | #4

      I have not tried the lighter trick but I will do that! I believe RainX contains PFAS or other forever chemicals, which we already have a problem with, so I'm not inclined to do that. It's a minor annoyance at most. Thanks!

      1. Chris_in_NC | | #6

        I think RainX is siloxane based.

        1. Expert Member
          Michael Maines | | #10

          I should know better than to post comments without research! Original Rain-X and Rain-X shower door treatment are composed of two types of alcohol plus acetone:

          There is also Rain-X Mud-X, for vehicle surfaces NOT including glass, which includes a siloxane compound:

          1. [email protected] | | #13

            Thanks MIke, interesting links!

      2. [email protected] | | #8

        I think most of the regulars who frequent GBA would be familiar with NEAT, Cardinal's surface 1 "easy clean" coating. Unlike Rain-x and other similar aftermarket products, which are hydrophobic, NEAT is primarily titanium dioxide and hydrophilic.

        While NEAT works great at sheeting water (no leftover water beads like you get with Rain-x) caused by rain or a garden hose, I don't recall ever seeing any information on how well it sheds surface 1 condensation. NEAT is not available as an aftermarket solution in any case since it's applied as a sputter coat in the coating factory.

        Enter X89 which, as I understand, is sort of a hybrid combination of i89 (indium tin oxide) and NEAT (titanium dioxide) that was specifically developed to offer both the easy clean performance of NEAT and to eliminate surface 1 condensation.

        Although X89 is still on Cardinal's list of coating options, I don't recall ever hearing of anyone actually offering the product. I do recall hearing at one time that there were some development issues, but that was a long time ago, and again it is listed as an available coating. If it is actually available, like NEAT, it's not an aftermarket solution anyway.

        1. Expert Member
          Michael Maines | | #11

          Thanks Greg/Oberon, I was not aware of NEAT or X89.

          1. [email protected] | | #12

            You're welcome Mike.

  3. russellchapman | | #5

    Awesome article. Thank you Greg!

    1. [email protected] | | #9

      Thanks Russell, glad that you enjoyed it and hope you found it useful

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