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Make-up air in an older house

canada_deck | Posted in General Questions on

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  1. Expert Member


    Whether you open a window or install an air outlet, the make-up air is the same outside air. Fortunately, except for the summer when you have the windows open anyway, that outside air is much less humid than that inside once you warm it.

    I know that seems counter-intuitive in the damp PNW, but you can easily confirm it with a cheap humidity reader. My indoor RH is currently 52%, while outdoors it is 76%. If I open my front and back doors, it will drop several points in minutes.

    De-humidifyers will keep the whole house RH lower, but won't effectively deal with bathroom moisture. An ERV with boost function will do both, and improve your IAQ . With either of those, you will still have the same problem with make-up air unless you get rid of the bathroom fan.

    If your house is tight enough that it de-pressurizes when you run the fan, an ERV might be a good idea. Otherwise I would just open a window.

    1. canada_deck | | #2

      Thanks Malcolm. If I was to look at adding an ERV/HRV, which would you recommend (HRV or ERV)?

      1. Expert Member
        MALCOLM TAYLOR | | #3


        The PNW is the one region in North America where it doesn't make much difference. The choice may come down to that ERVs are much more widely available.

        I'm in much the same situation. My 30 year old, semi-well air-sealed house is tight enough that a fan or the dryer will make my wood stove back-vent, and like you I rely on opening windows year round to provide ventilation, and avoid de-pressurization. Right now I can't get too excited about mechanical ventilation for such a small house, using so little energy to heat.

  2. AD_in_AK | | #4

    How about one of the fresh air inlets that has a course filter and a small damper? I have one for the same purpose. I found it at a local electric supply house.

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