GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Air leakage or just interior humidity / stale air?

Aaron Dent | Posted in Energy Efficiency and Durability on

I have a 1930s home in Sudbury, 4 hours north of Toronto, Ontario. Temperatures get down to -40 degrees C in the winter here. The home is an old log home. 2 years ago, I installed 3″ polyiso on the exterior of the house, and replaced the windows with Fibertec, triple glazed windows, high SHGC on south face, mid-low on remaining orientations. I also chose fixed windows for better peformance on north and south walls, as prevailing winds are east-west.
For the window detail, I went with the large interior sills (9-10″) instead of moving the windows towards the inside of the wall and keeping them better protected for exterior winds… I think this is my problem.
Starting in September, if I’m not running my HRV 24 hours a day, I get some condensation on the bottom of the windows. Maybe I should be running my HRV 24 hours a day, but I’ve been using it mainly to keep my interior RH down – usually run it 30-40% of the time.
I have caulked the interior and exterior of the windows, so I can’t imagine how air leakage is a problem, but I’m wondering if there is a possiblilty that the windows have a problem…
Is there a homeowner test I can do to check for leaky frames?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. David Meiland | | #1

    On a calm day, if you turn on all of the bath fans, the dryer, and the range hood, can you feel any air leakage around the windows using the back of your hand?

    What is the temp and RH inside the house, at various spots?

  2. Hunter Dendy | | #2

    You can also pick up a smoke stick (or smoke pencil) for fairly cheap, which auditors use to find leaks. Turn on all exhaust fans test at each window. All else fails, find someone in your area with a blower door.

  3. GBA Editor
    Martin Holladay | | #3

    Thosten Chlupp addressed this issue in his JLC article, Installing Exterior Insulation in Cold Climates. In that article, Chlupp writes:

    "Windows can be installed either at the face of the sheathing — in a recess — or out at the face of the wall. From a performance standpoint, a recess is better, because the window is somewhat protected from wind-washing and the interior glass is more easily warmed by the heat in the room. By contrast, windows installed at the face of the wall are in an interior recess, separating them from the warm air inside (especially if a curtain is drawn) and exposing the outer layer of glass to cold wind. I’ve observed that in extremely cold weather — when it’s 25°F below zero, for example — frost tends to form inside windows installed at the face of the wall, whereas frost rarely occurs on inset windows. I’ve installed windows both ways, but because of the frost problem I now do only recessed installations."

    A further discussion of this phenomenon can be found in the posted comments to my blog, 'Innie' Windows or 'Outie' Windows?

  4. Riversong | | #4

    At -40° (which, for us 'Mericans is also -40°), there's little you can do to prevent condensation on the windows. But in September it probably wasn't that cold, so you clearly have another problem.

    What is the indoor RH when you run the HRV 30%-40% of the time on a cold day? If it's above 35%, it's probably too high for very cold weather. You might also check the RH on one of the deep window sills where it's likely to be considerably higher. Check the temperature there, too, as it's likely to be considerably lower than room temp.

    HRVs are generally sized to run 24/7 on low speed. You may not be getting enough air exchange, nor enough air movement to "wash" the window wells.

  5. Jin Kazama | | #5

    AARON have you news on your window problem with the fibertec units ??

    i've got the same windows and similar problems ( frosting and dereasonnable frame and glazing temperatures for this kind of window )

    please if possible update me on the status of your issue !

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |