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exterior condensation

user-7124595 | Posted in General Questions on

Looking for some opinions.

New triple pane windows in a zero energy home in southern NH. In this humid time of the year I am experiencing exterior condensation only on the south and west facing windows. The interior is conditioned at 72 degrees pretty consistently. It is a single story house. The fact that its only on the south and west is perplexing. I have a pretty significant stone drip edge outside with good overhangs and am wondering if the solar gain on those two sides creates a microclimate that perhaps contributes to the condensation. I’d appreciate any thoughts. Thank you

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Replies

  1. Expert Member
    Zephyr7 | | #1

    You'll get condensation anytime a surface drops below the dew point of the ambient air. This means you're most likely to get condensation on a window when it's humid, and when one side is cold compared to the other. In my area, I get summertime condensation on my windows sometimes in the mornings, since we get very humid (100% RH, basically) at night most nights, and when we run the air conditioning, the exterior of the double pane windows will drop below the dew point.

    In winter, it's the other way around, with the INSIDE surface typically getting condensation, more commonly at night when things are colder. The solution here would be to lower the indoor humidity levels.

    Triple pane windows REDUCE the chances of condensation, by better insulating one side from the other thermally. Triple pane windows do not PREVENT condensation -- if conditions are right, which means high enough humidity levels and low enough window temperatures that window surface drops below the dew point, then you WILL get condensation, even on a triple pane window.

    Bill

    1. user-7124595 | | #2

      Thank you Bill, I am in line with all that thinking. Any thoughts on why only on the south and west sides, that is what is bugging me. Seems like it has to be a micro climate, but even yesterday when it rained all day and the sun didn't shine, this morning the condensation was limited to those two sides of the house. Same in the garage which has inside and outside temperatures that approach being equal.

      1. Expert Member
        Zephyr7 | | #3

        Yeah, that's a tricky one. My guess would be those windows are either shaded at the right times, or there is something about the air currents (paths the wind takes) in those areas. Moving air will help to clear condensation more quickly than stagnant air, for example, when the conditions that made condensation possible eventually change. Shade will keep a window in the right conditions for condensation longer than a window that gets more sun.

        About the only way to know for sure is to watch for patterns -- when do you get condenstation? What are the outdoor conditions like at those times? Once you find the pattern, you can figure out why what you see is happening.

        Bill

  2. Tyler Keniston | | #4

    Since, as you say, condensation on only south and west windows is a bit counter intuitive, we need more clues as to differences between these and other windows.

    Is it center of glass condensation?
    What time of day?

    Are the windows the same make, size, type, etc.?
    Are they experiencing the exact same indoor air temps?
    Could they be radiatively cooling more than the others?
    Less wind, maybe yes.
    Different vegetation nearby..... Reservoirs of water causing hyper local evaporation...
    Are your dewpoints really near 72. That seems really high for nh. And is the exterior of your window really even that cold?

  3. GBA Editor
    Martin Holladay | | #5

    User 712,
    The cooling mechanism is night sky radiation. I'm willing to bet that your south and west windows see more sky than your north and east windows -- either because you have more trees to the north and west, or because your house is on a southwest slope.

  4. Expert Member
    Peter Engle | | #6

    Do the windows have different coatings? They might be lower solar gain coatings on south and west to prevent overheating. That could change their radiant cooling behavior.

  5. Expert Member
    Michael Maines | | #7

    I have two double-glazed casement windows facing NW here in Maine. Some mornings we have condensation on the exterior of one sash, sometimes the other, and sometimes both. I have not been able to figure out the cause but it's an interesting puzzle. I'm just glad the windows insulate well enough to allow condensation to form on the exterior, and not on the interior!

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