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HRV / ERV for a conditioned crawlspace

Stu M | 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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  1. User avatar GBA Editor
    Martin Holladay | | #1

    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.

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