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Community and Q&A

Closed crawlspace ventilation choices

user-433254 | Posted in Energy Efficiency and Durability on

I’m in the process of closing my 900 sqft crawlspace and need to find a way to ventilate the space. One thought I had was to use the old oil furnace chimney/flue as a way to send the air up and out with the help of a small(20cfm) bath fan. The flue is centrally located and close to an existing outlet. Or is it better to seal the flue (or remove it all together) and duct the air through the foundation wall.
zone 4 asheville nc
house built in 1969
neighbor had his crawl space tested for radon and was negative.
13mil barrier sealed to block wall
r10 insulation wall r15 on the bands
floor above 5/8 ply with 3/4 t & g wood

Any help with pointing me in the right direction for finding an appropriate 20cfm fan would also be appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    Concerning your question about a 20 cfm fan: you already asked the question on another thread (it's here), and you will notice that I already posted an answer to that question.

  2. GBA Editor
    Martin Holladay | | #2

    I advise you to seal up your old chimney flue and to use a separate outlet if you want to install an exhaust fan. I assume that you are aware of code requirements concerning sealed crawl spaces; if not, they are discussed in this article: Building an Unvented Crawl Space.

  3. Expert Member
    Dana Dorsett | | #3

    What Martin said- depressurizing the crawlspace (and whole house) with a flue may even pull more soil gases into the crawlspace than it otherwise wood. Best case, ventilating it exhaust-only into the flue would increase the whole house infiltration rate, which would be potentially over-drying in winter and raises both heating loads in the winter, and cooling loads in summer.

    In an Asheville NC location ventilating the crawlspace with outdoor air would raise the average humidity in the crawlspace, and increases the mold potential in the crawlspace. If instead you use the fan to ventilate the crawlspace with conditioned space air from the rooms above, it guarantees that the humidity (& radon) inside the crawl space tracks that of the fully conditioned space, to be managed by air conditioning & ventilation of the the conditioned space.

    To do that, a small floor grille on one end of the house and another on the far end to vent the crawlspace exhaust works. Setting it such that the fan pressurizes the crawlspace relative to the house, and letting the exhaust come up in a bathroom can be a good approach, since the crawlspace would then be slightly positively pressurized relative to the surrounding soil, and the average bathroom humidity would be lower than it otherwise be, limiting the mildew risk in the bathroom. (When the bathroom fan is running it would still depressurize the bathroom enough to exhaust the extreme humidity and odor periods.)

    A continuous 20cfm is probably overkill unless the crawlspace is unusually large in volume- running it on some duty-cycle is probably worth it.

  4. user-433254 | | #4

    Dana, so what you are saying is push air from the living space to the crawl space via a small fan. Then the slightly pressurized air would come up through another vent in the bathroom and return to the rest of the house or be exhausted throught the bath fan.
    I should also add that the house has resistance heat now but will be adding a minisplit this year. It has no central air/heat now. Then at some point may add a stand alone heat pump to my water heater that will be located in the crawl space. Will these affect this system in a negative way that you can think of?
    I'll be going back to read code requirements to make sure I understand the system.
    Thanks very much for your time and advice.

  5. Dana1 | | #5

    Your HVAC & hot water upgrade plans are sound, and don't affect the crawlspace ventilation recommendations.

    Is the crawlspace even tall enough to accomodate a retrofit heat pump on the water heater? I suppose the heat pump could be plumbed along side a low-boy HW heater rather than on top, if need be.

  6. user-433254 | | #6

    Dana, I have about 30" of headroom(below the floor joists) in the crawl space. I was thinking of a Geyser R and putting it directly below a water heater that will be in a closet above. Unless I can find an affordable heat pump water heater that fits in the crawl space but so far I haven't seen anything.

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