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Last Word on Opening Windows to Reduce Indoor Humidity?

jamesboris | Posted in General Questions on

Short version: If the dew point is lower outdoors than indoors, will opening windows *always* make it less humid inside?

Longer: I know that dew point is a better metric than others here (as Alison Bailes writes here). Just trying to zero in more. “Shoulder season” humidity control is a big hassle here in Zone 2A, where a 60-70° F day’s dew point will often swing from 30° to 60°. Swamp mornings become desert afternoons become coastal evenings.

I’ve built a tiny house with no ductwork, so I’m afraid I’m SOL as far as a “pleasant” dehumidifier goes… basically I have to deal with (a) a loud, hot standalone unit, (b) a somewhat quieter, but way weaker through-wall unit, or (c) my pricy Mitsubishi GL12NA’s “dry” function, which cools the room, period. So, proper “window management” seems very important for me. Would appreciate any thoughts. Thanks!

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Replies

  1. maine_tyler | | #1

    Basically yes.
    Replacing indoor air that has a higher dewpoint (absolute moisture content) with outdoor air that has a lower dewpoint will ultimately reduce the absolute moisture content of the air within the structure.

    The caveat is that whether that's the most efficient or comfortable thing to do is less simple and requires more specifics.

    If the RH of the outdoor air is higher (even with a lower dewpoint) then bringing lots of that air in could potentially increase the RH of the indoor air, which may make it feel 'more humid.' This scenario implies that the outdoor air is cooler, so maybe that's all around a good option in your climate. And if the air is too cool, you can heat it, further reducing the RH.

    On the other end, if the outdoor air is warmer but with a lower dewpoint, bringing it in may require cooling, but if it's too warm it may not be the most efficient option. Another concept to investigate in this regard is enthalpy.
    https://www.computrols.com/enthalpy-hvac-free-cooling/

    1. jamesboris | | #3

      Thanks for chiming in Tyler, I've learned a lot from you here on GBA. Some followups:

      "If the RH of the outdoor air is higher (even with a lower dewpoint) then bringing lots of that air in could potentially increase the RH of the indoor air, which may make it feel 'more humid.'

      What is the difference between "feeling more humid" and "moisture as it becomes a problem for building materials"? Thinking of condensation on drywall, *hardwood floor expansion* (so fussy!), etc.

      "This scenario implies that the outdoor air is cooler, so maybe that's all around a good option in your climate. And if the air is too cool, you can heat it, further reducing the RH."

      To clarify... which scenario? Still trying to grasp muh psychrometrics here!

      "On the other end, if the outdoor air is warmer but with a lower dewpoint, bringing it in may require cooling, but if it's too warm it may not be the most efficient option."

      Would the more efficient option in this case be to run a dehumidifier? My guess is that if the air outside is *that* hot/dry, I'd be running the AC, no?

      1. maine_tyler | | #5

        First a disclaimer,
        HVAC is 100% NOT my expertise. I have a decent understanding of the science behind psychometrics and that is all. The practical's of installing and operating HVAC equipment is beyond me. I am also very unfamiliar with your climate zone!

        >>"What is the difference between "feeling more humid" and "moisture as it becomes a problem for building materials"? Thinking of condensation on drywall, *hardwood floor expansion* (so fussy!), etc."

        The sensation of humidity is complicated, but RH is one element since it affects our mucous membranes and affects evaporation rates. What that means for the building moisture question depends. If your concern in your climate is outdoor air condensing on cold drywall, then its probably most simple to just think in terms of the drywall temp and the dewpoint of the air. When you bring air in at a different temp and RH, technically you change multiple parameters so the sum effect is hard to say, but thinking in terms of temp and dewpoint here makes sense to me.
        In regards to wood-- wood is hygroscopic so it 'tunes' to RH. So a higher RH means wetter wood (put simply). There are charts that will correlate ambient RH with wood moisture content for a given temperature (temperature affects it a bit but not tremendously).

        >>"To clarify... which scenario?"
        I was just saying that for outdoor air to have a lower dewpoint but higher RH, it must therefore be cooler (than interior air). Because for a given dewpoint: RH goes up as temperature drops--and RH goes down as temperature rises. So it would be impossible to have outdoor air that has a lower dewpoint AND higher RH AND higher temp.

        >>Would the more efficient option in this case be to run a dehumidifier? My guess is that if the air outside is *that* hot/dry, I'd be running the AC, no?

        Yes, most likely this scenario would not be a humidity issue at all, and it would probably have to be pretty darn dry for that situation to exist. But I guess my point was that you might just want to run the AC and not bring that air-- if it is very hot-- into the house just because it has a lower dewpoint.

  2. Expert Member
    Michael Maines | | #2

    Yes, I once asked building scientist Bill Rose a similar question and the response was unequivocal--moisture always moves from high vapor pressure to low vapor pressure, with no exceptions. Dewpoint is very closely related to vapor pressure.

    I question your observation, though. Dewpoint usually stays within a fairly narrow range. I think you might be confusing relative humidity for dewpoint. As the temperature changes over the course of a day, the amount of moisture in the air remains fairly constant, but the relative humidity changes. When weather fronts move through is when you'll get fast changes in dewpoint, and resulting storms.

    1. jamesboris | | #4

      The typical morning here is dewy enough to run the gutters (right now it's 64 F outside, dew point is 63). My observation may be a little overstated as applied to 24 hour chunks, but it's not unusual to have very dry and very wet days in the same week. Also, we do get a lot of random fronts where we are, on the intersection between coastal plain, central TX swampiness, and much more arid country.

  3. Expert Member
    DCcontrarian | | #6

    Take a look at the Cerv2 by Build Equinox -- https://www.buildequinox.com/cerv2/ -- not because it would be appropriate for a tiny house, but because its operation is instructive. It has six possible modes: the fan can either ventilate or recirculate, and the air can be either heated, cooled or dehumidified. (There's also a seventh mode where it does nothing, which it calls "energy saver.") It has logic which looks at interior temperature and humidity, and exterior temperature and humidity, as well as the user set points, and decides which mode is most appropriate.

    To the original question: "If the dew point is lower outdoors than indoors, will opening windows *always* make it less humid inside?"

    The answer is yes, but the longer answer is that ventilation isn't always the most effective way to get comfort. Recirculation with either dehumidification or cooling may be more appropriate depending on the indoor and outdoor conditions.

    1. jamesboris | | #7

      Thanks, that makes sense. I feel like in my zone it breaks down to:
      1) Hot days (wet or dry): AC should take care of it
      2) Cool-ish, wet days: standalone dehumidifier, though the noise is super annoying and I see no way around it
      3) Warm-ish, wet days: standalone dehumidifier heats the house too much, AC cools it too much, "Dry" mode doesn't seem to do much... so I end up running dehu+Dry, hoping for the best.
      4) Warm-ish/cool-ish dry days: hooray, I can open a window~~~

      1. Expert Member
        MALCOLM TAYLOR | | #8

        Jamesboris,

        It's raining heavily here (when isn't it?), 4C and 85% RH. Inside it's 21.5C, and 50% RH. Opening all the doors for 10 minutes will bring the indoor RH down into the mid-40s.

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