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Community and Q&A

How to vent a crawl space — “Musings” article of May 13, 2011

WEG | Posted in Building Code Questions on

I’m going to take the advise in the “Building an Unvented Crawl Space” by Martin Holiday that was posted on May 13, 2011.

It suggests putting a floor register in so air can move between conditioned space and the crawl space. It is then recommended that an exhaust fan to meet code requirements for conditioning the crawl space.

Is this fan exhausting to the outside? Or is it exhausting into the conditioned space. I think it is the latter but want to make sure. It is an 800 sf space 4′ high with volume of 3200 cubic feet, What cfm fan do I need? Where is best place to put floor registers?


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  1. GBA Editor
    Martin Holladay | | #1

    There are two possible approaches (according to the building code). Both options require the installation of a duct or transfer grille connecting the crawl space with the conditioned space upstairs.

    1. Option 1 requires “continuously operated mechanical exhaust ventilation [to the exterior] 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 to the exterior; 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.)

    2. 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.)

    -- Martin Holladay

  2. brendanalbano | | #2

    Since you asked, in case it's not totally clear, the exhaust fan (if you take that approach) exhausts to the outside.

  3. WEG | | #3

    Thank You User#6233497

  4. WEG | | #4

    And User USER-756436

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