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Community and Q&A

Encapsulated Crawlspace Humidity and Temperature

1869farmhouse | Posted in General Questions on

I have a crawlspace/cellar that I encapsulated with closed cell foam and poly sheet – it also has an exhaust fan (only) moving around 70cfm outside.  

The house is ancient, and despite grading work and a perimeter french drain, it stays wetter and more humid than I would prefer.  At lease part of my problem is ground water as I sit in a low(ish) area.

I’ve been using a portable dehumidifier as a crutch (not sure how much more I can do otherwise), which is working well and maintaining ~40-45%rh.  My problem is that I also use this area for food storage, and the dehumidifier is keeping the temperature between 80-85 degrees.

I can’t help but realize that without the dehumidifier, the temperature is perfect.  But I repaired quite a bit of rot and I’m afraid of letting the rh creep up.  I’m unsure how I can maintain both temp. and rh?

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  1. Expert Member
    NICK KEENAN | | #1

    Air conditioner.

  2. Expert Member
    NICK KEENAN | | #2

    Longer answer:

    It takes about 1,000 BTU's to convert a pint of water from liquid to vapor. When the vapor is removed from the air that heat is returned. A dehumidifier dumps that heat into its exhaust air. A typical dehumidifier uses about 400 BTU's worth of electricity to remove a pint of water, and that heat is also dumped into the exhaust. So dehumidifiers are actually really good space heaters. If you don't want that heat, a dehumidifier may not be the right appliance. An air conditioner also removes humidity, but when it does it dumps the heat out of the building. A heat-pump water heater also removes heat, and with it humidity, and uses the heat to heat water.

    I find a dehumidifier and a heat-pump water heater are a good combo for a basement. I don't know if the crawl space has enough headroom. They do make ducted HP water heaters that can pull air from another room.

    1. 1869farmhouse | | #6

      You've peaked my interest with the combination of a heat pump hot water heater. I've considered getting a ducted unit like a Santa Fe 70 and putting it upstairs. I have a utility closet with an electric hot water heater I've been wanting to replace.

      Anyone have any idea if a commercial unit like Santa Fe 70 creates less heat than a portable Home Depot special?

      1. Expert Member
        NICK KEENAN | | #8

        The lion's share of the heat produced by a dehumidifier is the latent heat of the removed water. That is determined by physics and won't change. A more efficient dehumidifier will use less electricity and produce less heat that way but it's a small fraction of the total.

        And I'm not sure that ducted dehumidifiers are more efficient. They are quieter and more reliable.

        1. 1869farmhouse | | #10

          Damn it DC.... now you have me looking at 9k btu daikin quaternity units haha.

      2. CramerSilkworth | | #9

        If you're thinking that a ducted dehum upstairs will reduce the heat going into the crawlspace, it won't. The heat is in the outlet air, so as long as that's ducted back down to the crawlspace, the heat is basically the same.

        There are also split dehumidifiers (well, one that I know of) - the Ultra-Aire/Santa Fe SD12, but that's a pretty serious investment. If you can fit a window AC in there somehow that'd do the trick (but keep the dehum too).

        1. 1869farmhouse | | #11

          Thank you for the clarification - that is what I was thinking.

    2. Jon_R | | #13

      > A typical dehumidifier uses about 400 BTU's worth of electricity to remove a pint of water

      A typical Energy Star portable efficiency is 1.8 L/kWh = 3.8 pints/kWh = 900 BTUs per pint. About the same as the heat of condensation.

  3. Jon_R | | #3

    > an exhaust fan (only) moving around 70cfm outside ... I’ve been using a portable dehumidifier as a crutch

    Hopefully you aren't using both at the same time.

    Note that by warming up the air, the dehumidifier does less work to achieve the same %RH.

    1. 1869farmhouse | | #4

      I am using both. It's only 70 cfm, and is required by code for an encapsulated crawlspace in my area. It's pulling a small amount of reconditioned air from my living space through a transfer grill and the net result is actually lower humidity when compared to not running it.

      That is interesting, but I'm just concerned about my food being stored at suck a high temperature.

  4. charlie_sullivan | | #5

    Your 70 CFM exhaust fan is most likely pulling 70 CFM of outside air into the crawl. That air has high moisture content, and that's most likely what is creating the need to dehumidify. If you turn off the exhaust fan, seal that hole and any vents to the outside, your dehumidifier should be able to maintain low humidity without running nearly as much. Then it won't generate nearly as much heat either.

    I also recommend a target humidity of 60%, or at the lowest 50%, just because there shouldn't be any need to run it lower than that.

    1. 1869farmhouse | | #7

      The exhaust fan is pulling a small amount of air out of the crawlspace while simultaneously creating the negative pressure required to pull conditioned air from the living space through a transfer grill. I've measured with it both on and off, and the rh is lower while it is running. My crawlspace is TIGHT, it needs the air movement - in addition to being required by code.

      1. Jon_R | | #12

        Note that IRC R408.3 allows use of a crawlspace dehumidifier without any ventilation. With proper dehumidification, crawlspaces do not need air flowing through them. Agreed, local code could dictate otherwise.

        When it comes to humidity, 70 CFM is a large amount of air. Depending on the source air, it may require substantial amounts of dehumidification and heat to remove the moisture.

        > maintaining ~40-45%rh

        Your dehumidifier will generate much less heat if you set it to 55%-60%.

        For cool and dry, +1 on using AC. Maybe the crawlspace isn't an effective place to store food.

        1. 1869farmhouse | | #14

          I will try raising the humidistat to see what happens. In theory, the only air pulled into the cellar should come from the conditioned space through the transfer grill. Crawl space is probably a bad term for it - it’s a 800 sq ft cellar with 8 foot ceilings surrounded by crawl space on all sides. And when I say I encapsulated it well, I spent months giving attention to every detail haha

          1. Expert Member
            NICK KEENAN | | #15

            Which edges of it did you seal, and what did you seal them against? Walls, floors, ceiling? Against cold, air leakage, water vapor, liquid water?

          2. Expert Member
            NICK KEENAN | | #16

            With 8' ceilings you should definitely look into a heat pump water heater.

          3. 1869farmhouse | | #17

            I agree, I’m definitely looking into the water heater combo. I laid thick poly sheet (like the kind for outdoor ponds) and I ran it about 3/4 the way up the foundation walls. Then I sprayed closed cell foam on the walls, providing the insulation, air seal, and firmly affixing the poly to wall. I air sealed the floor joists non-asphalt based roof sealer, silicone, and mastic where needed. I live in the boonies, so I don’t have the luxury of a blower door test - but I put an industrial fog machine down there and pressurized the area with a 5ft fan. It’s TIGHT haha

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