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Community and Q&A

Window insulation and mold

carlone | Posted in Energy Efficiency and Durability on

We need to leave our house unoccupied this winter and I have a concern about heating and mold. Here are the details:
– 1275 sq ft super insulated house with HRV
– triple pane windows with rollshutters on the outside
– very cold, dry climate (northern Alberta). -40 temps not uncommon.
– we normally heat with a wood stove but need to use our back up electrical heat system while the house is vacant
– Plan to set thermostat at 5 or 7 degrees C (41 – 45 F)
– electricity is very, very expensive here
We plan to save on the potential catastophic cost of electric heat by making window inserts from rigid styrofoam insulation (R10). We have made four inserts so far and have tested them overnight while the building is occupied and fully heated. Outdoor temps at night are now around freezing. We noticeĀ  a LOT of condensation builds up between the glass and the insulation after just one night. My questions are:
– Are we risking mold growth around our windows with this plan?
– Is there anyway to prevent or mitigate mold growth?
– If we go ahead, should we keep the HRV running or turn it off for the winter?

Many thanks for your input.

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  1. GBA Editor
    Martin Holladay | | #1

    If you lower the moisture level indoors, and raise the temperature of your window glass, you'll reduce the chance of window condensation.

    To lower the indoor moisture level, you can:
    1. Get rid of the humans and all human activity like showering and cooking. You're planning to do that. So far, so good.

    2. Lower the thermostat setting. (Cool indoor air holds less moisture than warm indoor air.) You're planning to do that. So far, so good.

    To raise the temperature of the window glass, you can:
    1. Install storm windows or exterior insulation. You aren't planning to do that, but you might consider it. (In other words, the rigid foam belongs on the exterior side of the windows, not the interior side.)

    If you don't follow my advice, and you end up installing the rigid foam on the interior of your windo0ws, the key to limiting condensation is an airtight installation of your rigid foam to prevent warm, humid indoor air from reaching the glass. (That, and getting the humans out of the building and lowering the indoor thermostat.)

  2. carlone | | #2

    Thanks very much for the great info, Martin. A couple of further comments/questions:

    - We do have rollshutters on the outside of the windows which will provide security and some insulation from the outside. So, that leaves no room for insulation on the outside.
    - I have researched and never been able to determine the R value of the rollshutters. Do you have any idea?
    - Should we leave the HRV on or off while the house is vacant?

  3. GBA Editor
    Martin Holladay | | #3

    The exterior shutters will make a big difference. Considering there is an air space between the window and the shutters, they probably add R-1.5 or R-2 to the windows. If I were you, I would close the shutters and skip the interior rigid foam.

    Q. "Should we leave the HRV on or off while the house is vacant?"

    A. Leave the HRV turned off, in light of the fact that there won't be any human beings generating moisture while you're gone.

  4. Expert Member


    I'm a bit confused. I get that cold air can hold less moisture than warm air, but given that the amount of moisture in the air would be the same whether you heat it or not, wouldn't it be better to have warm air that didn't drop the moisture?

    That's the assumption I've always made here: Rooms in the house you leave at a lower temperature are more prone to mold and condensation.

  5. GBA Editor
    Martin Holladay | | #5

    There are a lot of factors here -- whether there is a moisture source in the house, for example, and how cold the window glass is.

    But if there is a source of moisture -- for example, a damp basement wall, which is typical -- you'll have faster evaporation of moisture from that wall to the interior of the home when the basement is warm than when the basement is cool. The air in a cool basement doesn't have much moisture (just as the air outdoors on a cool day doesn't have much moisture). But if the basement is warm or hot, the air in the basement holds more moisture. In fact, the warm air encourages evaporation from the basement walls.

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