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HRV / ERV for a Conditioned Crawlspace

stusrenos | Posted in General Questions on

I’m in climate zone 5-6. 3′ conditioned crawlspace, with 12mil. plastic sealed floor & closed cell spray foam insulation installed on crawlspace walls & rim joists. Forced air gas furnace- ducts into crawlspace with one small flapper vent allowing some air into crawlspace when furnace/ ac is running. I currently run a dehumidifier about 12-24 hours a week in summer months to maintain a humidity level consistent with main floor. I’m about to seal the main level with new windows and 3″ of polyso around exterior, and thus also want to install an HRV/ERV.

My question is- how should I handle the crawlspace regarding HRV?…..should I include an intake and or exhaust into the CS? Currently it is decent down there, but due to exposed fibre cement fireproofing over spray foam and exposed floor joists etc….certainly not as nice an atmosphere as rest of house.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Stu,
    Building codes permit crawl spaces to be sealed, as long as you comply with one of two options for conditioning the crawl space air. These two options are explained in a GBA article, Building an Unvented Crawl Space.

    The code lists two options for conditioning unvented crawl spaces; both options require the installation of a duct or transfer grille connecting the crawl space with the conditioned space upstairs. Option 1 requires “continuously operated mechanical exhaust ventilation at a rate equal to 1 cfm for each 50 square feet of crawl space floor area.” In other words, install an exhaust fan in the crawl space that blows through a hole in the rim joist or an exterior wall; make sure that the fan isn't too powerful. (The makeup air entering the crawl space is conditioned air from the house upstairs; since this conditioned air is drier than outdoor air, it doesn't lead to condensation problems.)

    Option 2 requires that the crawl space have a forced-air register delivering 1 cfm of supply air from the furnace or air handler for each 50 square feet of crawl space area. (Assuming the house has air conditioning, this introduction of cool, dry air into the crawl space during the summer keeps the crawl space dry.)

    If you are worried about mixing the crawl space air with the air upstairs (because the quality of the air in the crawl space is suspect), then I would choose option 1.

    Note that neither option requires an HRV or an ERV. I don't recommend that you include any HRV or ERV supply registers or exhaust grilles in the crawl space.

  2. newbiemike | | #2

    Martin, thank you for your helpful comments to Stu.

    We are building in Eastern WA in Climate Zone 5B (almost 6) and considering including running a supply and return duct from the HRV to and from the unvented crawlspace. Can you please elaborate on why you do not recommend connecting an HRV to an unvented crawl for ventilation?

  3. kevin_in_denver | | #3

    Martin has retired.
    If the air in a crawlspace is suspect for any reason, it shouldn't be connected to the house air in any way.
    A sealed, insulated crawlspace doesn't need to be conditioned. Emergency heat of some sort may be needed to prevent freezing.

    1. Expert Member
      MALCOLM TAYLOR | | #4

      Kevin,

      As Martin points out in his reply above, conditioning the crawlspace is a code requirement.

      1. Deleted | | #8

        Deleted

  4. Expert Member
    Akos | | #5

    Since one of the options is to have exhaust only ventilation for the crawlspace, there is really nothing wrong with putting a stale air pickup for your ERV there. There is no point for a fresh air feed, that should go to bedrooms and living room.

    This would provide the dual benefit of drawing conditioned air from the house into the crawlspace plus exhausting any smells without the energy penalty of exhaust only ventilation.

    To me this is better use of a stale air pickup than trying to vent a bathroom.

    1. newbiemike | | #6

      Akos, thank you, this is helpful. I was trying to avoid the code-required 50CFM ( 1CFM/50 sq ft) exhaust only fan from the crawl. That seems like such a waste.

      Now I will have to figure out a good way to connect the unconditioned crawl with the main floor. Not sure if the registers should just be in the floor or up the wall in a cold climate. Any tips??

      1. Jon_R | | #7

        Limit the registers+leaks to about 1 square inch per 5 CFM. This will maintain a little bit of negative crawlspace pressure (a good thing).

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