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Controlling Interior Humidity Levels

bmunsell | Posted in Energy Efficiency and Durability on

Wasilla, Alaska Zone 7 home. Built to state 5 Star + standard 10 years ago with ERV ventilation.
2100 sq. ft. in floor heat. 2 occupants. During the winter house is always 15% to 20% inside relative humidity while outside is rarely below 30%. Would like to figure out why and what is the best way to bring interior humidity up to more healthy levels.

NOTE: Outside humidity is always higher than inside.

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  1. DCContrarian | | #1

    Usually the answer is that the house is leaking air somewhere. In the winter outside air has a very low moisture content. Occupant activities -- bathing, cooking, breathing, sweating -- will introduce a fair bit of moisture.

  2. jwasilko | | #2

    If you end up needing a humidifier, we have the Aprilaire 800 steam humidifier and really like it (other than it's electrical consumption). It's super easy to maintain, and we get 2-3 years out of a canister.

  3. maine_tyler | | #3

    Outside air infiltrating/inside air exfiltrating.

    The outdoor air is mich drier in terms of absolute humidity. Do you have blower door numbers? Is your ventilation rate reasonable? Is that sort of inside RH atypical for Alaskan winters?

  4. DennisWood | | #4

    Bring outside air into your home measuring 30% relative [email protected] -10 C

    That air at 20C would be about 5% relative humidity.

    If you have a pretty high ventilation rate, humidity will be a challenge in zone 7. We use a water injection setup on our forced air furnace..Aprilaire. No moving parts or drums.

    Is your ERV using recirc, or exhaust during defrost cycles?

    1. bmunsell | | #5

      Thanks for reply. The ERV is using recirc during defrost cycles. For humidity I'm using an indoor/outdoor wireless weather station. Reading the discussion and wondering about the numbers I'm using from this weather station, the 70% outdoor humidity and 15% indoor humidity. Since, obviously the inside and outside temperatures are drastically different, 69 F inside and about 20 F outside tonight I must not be comparing apples to apples with these numbers. More research needed.

      1. DCContrarian | | #6

        At 69 F air can hold 107 grains per pound, so 15% relative humidity is 16 grains per pound.

        At 20F air can hold 15 grains per pound, so 70% relative humidity is 11 grains per pound.

        So the inside air actually has a higher moisture content than the outside air, which is what you'd expect. But it feels dry because it has so much more ability to absorb moisture.

        A rule of thumb is that each occupant generates one fifth of a pound of moisture (3.2 oz) per hour. If that moisture were staying in the house you'd be complaining about the dampness and the mushrooms growing on the walls. It's escaping somehow, probably being carried by air leaking out.

        If we knew how many occupants the house had and its volume we could estimate the leakage rate.

        1. bmunsell | | #7

          Thanks for the info. Given that the inside air actually has a higher moisture content than outside, I think we are over ventilating as mentioned above. Ventilation is through a Venmar AVS DUO 1.9 ERV and it is always run on auto. There are 2 occupants, 2100 sq. ft. of ventilated space, 9 ft ceilings with some cathedral ceilings. House is polyurethane spray foam and was rated 5 Star + here in Alaska, built in 2010. I wonder if Venmar ever updates its control software?

          1. DCContrarian | | #9

            Ok, here goes. If an occupant produces 3.2 oz per hour of water vapor, two occupants produce 6.4 oz per hour. If the interior humidity is constant then an equivalent amount is being lost. An ounce is equal to 437.5 grains, so 6.4 oz is 2800 grains.

            The indoor air has a moisture content of 5 grains per pound, so to vent 2800 grains of water you need to vent 560 pounds of air. Air at room temperature has a density of .0765 lbs per cubic foot, so 560 lbs of air is 7,320 cubic feet. That's per hour , it's 122 CFM.

          2. DCContrarian | | #10

            To get your indoor air to a more comfortable 40% humidity:

            At 40% RH the indoor air is 42 grains per pound so the delta is six times as big, so you need one sixth the infiltration or 20 cfm.

            Is that achievable? When infiltration is measured it's usually done at pressure. An oft-quoted (and oft-disputed) rule of thumb is that the natural infiltration will be 1/20 of what the blower door shows. So 20 cfm would mean 400 cfm in the blower door test, or 24,000 cfh. If we assume your 2500 sf house is 24,000 cubic feet it makes the math easy, that's one air change per hour (ACH). One ACH would be a very good but not extraordinary infiltration test result.

  5. walta100 | | #8

    If it were me, I would turn down the EVR until I could get the humidity above 40% in the winter.

    Yes, someone will tell you will suffer brain damage from the lack of ventilation. I say BS you will decide the room is stuffy and take corrective actions.

    One has to wonder just how tight your house is. Has it ever been blower door tested? What was the ACH50 number?

    Does the house have a fireplace or wood stove?


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