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How humid air moves

mordors_eye | Posted in General Questions on

Home is 1,300 sq/ft, w/basement, in 4A MD.  Home was gutted, remodeled and (I think) well-sealed, except for the 4″ PVC pipe opening exposing the exterior areaway drain to the interior sump pit.  There are no doors in the stairwell connecting floors and I have no ERV/HRV.  Basement and 1st floor walls have ccspf.

Indoor humidity is typically in high 60’s.  I’d like to hardwire install an in/on wall dehumidifier (i.e. Santa Fe UltraMD33, Broan B33DHW, IW25-4 or similar?) but am limited on where I can install.  In a small house, there is limited wall space, basement ceiling height, or drain areas.  I don’t want to empty bins, and would like to avoid mechanical pumps and use a gravity drain directly to a sink/sump pump.  

The easiest spot is in the mechanical room by the sump pump (convenient). 

Question:  How readily does humid air move around a two-story home?  Can I expect the entire home to benefit from a dehumidifier placed in the corner of a basement?  I don’t want to assume the house will find “equilibrium”.  Thanks everyone for articles, products, or considerations.


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  1. Expert Member
    DCcontrarian | | #1

    I find that humidity tends to be more stable around the house than temperature.

    But I have to ask -- are you seeing humidity this high this time of year? Because I'm nearby and I'm seeing indoor humidity in the 40's. I would guess that either you have an extraordinarily tight house -- like you have to close doors slowly to avoid popping your eardrums -- or liquid water is leaking into your house somewhere. If the latter is the case the dehumidifier is a band aid, that water is going to cause issues over time. If it's the former some additional ventilation is called for.

  2. maine_tyler | | #2

    Yeah it depends on the moisture source(s).

    Do you have a good sense of what/where those are?

    If it's just ambient air infiltrating I think thr answer will be different than if it's caused by a wet slab, or lots of showering, etc.

    1. Expert Member
      DCcontrarian | | #3

      Or a hot tub/swimming pool.

  3. Expert Member
    BILL WICHERS | | #4

    You should probably try to find the source of all that humidity first. If you are, for example, getting a lot of moisture coming in through your basement floor, then you should consider sealing that first. It takes a VERY beefy humidifier to deal with a large, constant moisture source, and you have to pay for the electricity to run that dehumidifier -- and you have to run it *forever*. If you can cut down on the moisture that's getting in by finding and sealing the source, you can use a much smaller humidifier and pay a LOT less money over time.

    Regarding moisture moving around, I think you'll find that putting a humidifier in anywhere will likely help everywhere. Things are somewhat slow to stabilize, but you should find that if you place a humidifier near the source of moisture, you'll gradually bring down the humidity levels throughout your home.


  4. mordors_eye | | #5

    I average around 50% right now via a few (ThermoPro TP357) sensors I have around the house. It gets in the high 60’s Sept/Oct.

    I do have some smaller cracks on the exterior of the CMU basement I need to fill that undoubtedly let water into the hollow block when it rains.

    I installed a thick vapor barrier below the new concrete slab in the basement.

    I would have loved to excavate and paint a waterproofer on the CMU followed by a dimple board, but it wasn’t in the financial cards. I’m right on DC’s beltway, and my clay soil stays damp. I’m not sure if that’s migrating through the footer, but the basement walls have 2” of XPS taped at the seams, then 2” of spray foam, so not sure its coming though the walls. Plus, the new sump pump seems to be working fine.

    I’ve only been collecting data for less than a year (recently finished the gut job), but every day I wake up to moisture on the interior perimeter of my windows and think I need to do some mitigation. I appreciate all the feedback gents. This is such a great site with an even greater purpose.

    1. Expert Member
      Michael Maines | | #6

      At what temperature do you keep the interior? Do you have and use bath fans and a range hood? Do you vent your clothes dryer to the exterior? Do you have a large masonry chimney?

  5. kevinbmn | | #7

    If sump basin does not have a sealed cover, it may be (partial) humidity source. If sump is open, consider duct tape a sliced-to-fit piece of clear poly over it to the floor & ck daily for condensation on sump-side of poly. Regardless, open sump basin can also allow science project quality soil gas entry to home.

  6. Expert Member
    Akos | | #8

    "gutted, remodeled and (I think) well-sealed"

    A well sealed house will have pretty high moisture content, especially after a construction as any moisture from drywall work or new wood needs to be removed. Even once the moisture is out you will most likely need an HRV/ERV to keep winter time moisture down.

    Unlike temperature, dew point in houses tend to be pretty stable especially if you have forced air. The dehumidifier can be installed anywhere.

    Usually the best spot is in the basement where it can add a bit of sensible heat to keep it a bit warmer in the summer time. Around me, basement dehumidifiers are the norm, they are usually plumbed to drain either into the laundry sink or the basement floor drain.

    One thing to watch is some of the newer units with digital controls tend to cycle a lot which means they never get their nameplate efficiency or moisture removal rating. A better option is to crank them on max and use an external humidistat with a decent dead band to ensure long runs.

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