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Where does the water vapor go?

tedinnh | Posted in General Questions on

This is a question more of curiosity and needing an education rather than a specific request for advice or guidance on a building issue.

I wonder about this every heating season. So here in southern New Hampshire this past weekend we had a reminder that winter is just the turn of a calendar page away. Fired off my pellet stove for the day Sunday. RH in the house dropped from 60% to about 50% as the temp went from 64F to 72F. During the core of the heating season when that pellet stove runs 24×7 RH will drop over time to 25-30%. This happens in every house during the heating season. Everybody knows that. But did you know that I have no idea where that moisture goes? Heated air holds less water vapor I am told by a10 year old who can’t believe I am even asking the question. OK, I will go with that.

But where does the water vapor not being held by that warmer air go? Is it pushed into the walls and out of the building envelope? Or is the drop in RH just a factor of how RH is measured by my trusty $5 hygrometer?

Ted

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Ted,
    Water is moving into and out of your house all the time. If we are talking about water vapor in the interior air, the main way that water vapor leaves your home is via exfiltration. The exfiltrating air carries water vapor with it.

    The volume of exfiltrating air is always replaced by an equal volume of infiltrating air. During the winter, infiltrating air is quite dry. That's the main reason that indoor relative humidity levels are lower during the winter than during the summer.

    If you make your house more airtight, you'll have lower rates of exfiltration and infiltration, and your wintertime indoor relative humidity will rise.

    If you wonder where the water vapor goes once it is outside, the answer is it joins the water vapor in the atmosphere. There, it participates in the wonderful, well-known cycle of cloud formation, precipitation, and runoff to lakes and oceans.

  2. Expert Member
    Dana Dorsett | | #2

    "RH in the house dropped from 60% to about 50% as the temp went from 64F to 72F."

    The dew point (a measure of the absolute moisture level) of 64 F/ 60% RH air is about 50F.

    The dew point of 72F / 50% RH air is about 52F.

    You weren't LOSING water vapor in the air during that warm up- you likely GAINED some!

    "Heated air holds less water vapor I am told by a10 year old who can't believe I am even asking the question. OK, I will go with that. "

    The 10 year old has it upside down- send him/her back to class! :-)

    Warmer air has a higher capacity for absolute moisture content not less. The "relative" in "relative humidity" is relative to the temperature.

    If you cool a closed volume of air to it's dew point, it reaches 100% RH. Removing more heat from that air at that point results in a phase-change of water vapor to liquid water, without reducing the temperature (much). The removing heat from the air at a constant rate results in a fairly constant tempearture drop per BTU until it reaches the the dew point. The saturated 100% RH a changes the temperature much more slowly, since the bulk of the heat being removed is the "heat of vaporization" of the phase change from gas (water vapor) to liquid water.

  3. tedinnh | | #3

    Thanks Martin & Dana. I see the cause/effect relationships clearly now. Also, I know my house is far from airtight and I am trying to tighten it up as much as possible. But with the bad construction practice of clapboard over OSB with no house wrap what-so-ever (pitiful for 1988 building) all I have between me and the environment is 6mil poly. So that contributes significantly to the RH dropping during the heating season it seems. Good news anyway, as a long time lurker of GBA I long ago dumped my humidifier that was in constant use during the winter months. Probably saved some of that OSB from turning to oatmeal with that move alone. This is a good time of year to restate your views of wintertime inside RH for those who may have missed it.

    Thanks Again,
    Ted

    P.S. Ha! I finally get one up on that smarty-pants 10 year old niece of mine. But I'll give her big credit for trying to find an answer when the "adults" around the table at the time couldn't agree on how what RH really indicated, never mind why it really moved during the heating season.

  4. kevin_in_denver | | #4

    To me, there is still one unanswered question:
    Is there a way to be more comfortable in the winter while sleeping and not have your mucous membranes dry out? (I'm assuming that's why you had a humidifier in the past)

    Because of the the reasons above, you might try reducing the temperature of the bedroom. Cold air doesn't have as much drying potential. Don't open a window, however, and see if you can air seal the bedroom.

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