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Want to reduce indoor humidity in a cool, humid environment. Outdoor humidity >70% most of the year. Would an ERV be useful?

willy55 | Posted in General Questions on

I live in a fairly new, well sealed house on the west coast of Canada. Ventilating indoor moisture most of the year is difficult as the outdoor humidity rarely goes below 50% and in spring and fall, usually in the 80-90% range, with temperatures averaging 40-60 F. I am researching ERVs, but unless they can reduce the humidity of the incoming air significantly, ventilation is only going to increase indoor humidity. Lots of condensation on the windows during fall, winter and spring. Air quality in the house is good.

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  1. charlie_sullivan | | #1

    What an ERV does is try to make the humidity of the incoming air match the humidity of the outgoing air. If you've run a dehumidifier inside, the help you keep the improvement the dehumidifier has made. On the other hand, if you don't have a dehumidifier, the best you can do is bring in outside air when the outdoor dew point is as low as possible. Using a HRV will work better than an ERV in that scenario.

    Looking climate data for Vancouver, on the theory that that might be similar to yours, I see that the highest humidity month are actually the lowest dew-point months. The dew point for 70 F and 50% humidity is 50 F. If you have 40 F dewpoint air outside, and 70 F and 50% humidity inside, running HRV will reduce your inside humidity ... and will reduce it more than ERV would have.

    The challenge might be the months when you have an outdoor dew point in the 50s. If you ventilate a lot with an HRV, you can probably keep the humidity inside below 65%, which might be OK, but if you want to do better at avoiding the potential for mold and dust mites, you might want to actively dehumidify, and then either use the HRV for just the amount of fresh air you need, or use an ERV.

  2. GBA Editor
    Martin Holladay | | #2

    In case Charlie's answer wasn't clear: if you are ventilating when the outdoor air is cool (in the 40s), the ventilation air will lower indoor RH -- because outdoor air can't hold much moisture when it is cool. Once that 50% RH air warms up to 70 degrees, it magically becomes dry air. That's psychrometrics at work.

    For more information on this topic, see HRV or ERV?

  3. Expert Member
    Dana Dorsett | | #3

    Use outdoor dew point as your guide, since it is a measure of the absolute (rather than relative) humidity.

    Ventilation air with an outdoor dew point anywhere between 35F and 55F is going to be just fine for human health & comfort.

    If you have dust mite issues make that 35F-50F outdoor dew points.

  4. Richard Beyer | | #4

    Here's a simplified Dew Point Calculator .....

  5. Expert Member
    Dana Dorsett | | #5

    FWIW, outdoor dew points in coastal B.C. only occasionally exceed 55F by more than a few degress and then only in summer. The mid-summer average right at sea level at the coast would be in the low 50s.

    While the dew point and outdoor temps can be narrowly separated during the foggy-dew drizzly mid-winter periods, those temperatures & dew points are typically in the 30s and low 40sF, which is makes for very healthy humidity levels at room temperature. Pull up a dew-point graph for say, Victoria, and you'll see:!dashboard;a=Canada/BC/Victoria

    To answer the question more directly, in that climate an ERV isn't of any particular use, since over 95% of the time you would be able to keep the interior RH below 60% by ventilation alone.

    If you absolutely need to get it down to 50% or less at all times, there will be a few weeks in summer where you might have to cut back the ventilation rate and run a dehumidifier to achieve that, but the ERV would only preserve that mechanical air condition, it can't dehumidify on it's own.

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