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Community and Q&A

Using crawl space vents for soil gas/radon mitigation in an encapsulated crawl space

SamuelH | Posted in General Questions on

I’m planning on encapsulating my crawl space. For wall insulation, I’m considering doing the following:

1. putting a metal bug screen over the vents, leaving them open to vent
2. installing rigid foam (EPS with a vapor barrier on each side) over the vents, leaving a 1 inch air gap along the wall and ground

I’m thinking to do this for a few reasons:

1. the air gap will allow radon and soil gases to vent out
2. the air gap will allow the ground to dry to the outside
3. using rigid foam will prevent my walls, which seep water, from filling with water (versus spray foam), allowing water to run to my sump pit

My crawl space has a few inches of gravel throughout, which should help with the things above.

I’ve tested the radon levels in my home. They are low, 2 to 3 pCi/L. I’ve even tested in the crawl space; with the vents closed, vapor barrier on the ground, it’s only 4 to 5 pCi/L. So this venting would just be an extra safety measure. I’ll continue to monitor radon and odor levels and can add active soil gas mitigation later if needed.

I’m planning on air sealing all penetrations through the subfloor, so there shouldn’t be much stack effect air coming into the first floor.

Are there any reasons this would not work? I’m in the Nashville, TN area, climate zone 4A.

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    >"Are there any reasons this would not work?"

    Yes.

    To start with, it's almost impossible to adequately air seal between the crawlspace and first floor, and leaving the vents open allows stack effect to draw radon from the crawlspace air into conditioned space. Even if it were somehow possible to make it perfectly air tight between crawlspace and first floor, with the vents open and now cooler-in-summer insulated crawl, cool subfloor and joists will load up with moisture in humid summertime weather creating a mold hazard.

    Leaving the vents open will also result in condensation on top of the ground vapor retarder during the summer season, and do almost nothing for reducing the radon levels on the first floor. The 1" discontinuity where the vapor barrier meets the foundation wall just lets more radon & ground moisture in than if fully sealed.

    Instead, seal up the vents, seal the ground vapor barrier to the walls & wall foam to keep most of the radon from entering the crawlspace in the first place. Then, install exhaust only ventilation at the code-minimum 1 cfm per 50 square feet of crawlspace floor area rate, with a small floor vent on the far end of the house from the exhaust fan to purge potentially radon contaminated air from the crawlspace, replacing it with conditioned space air. That will have with a MUCH lower energy penalty, and effectively zero mold risk compared to leaving the vents open. Most houses can get there with less than 5 watts of continuous power draw from the exhaust fan (~45 kwh per year) many less than 3 watts (~25 kwh per year). The 3 watt Panasonic FV-01WS2 is good for 500' of crawlspace, the FV-04WS2 is good for 1000' of crawlspace at it's lowest speed, 2000' at high speed.

    1. SamuelH | | #2

      Dana, I wasn't clear. The crawl space would not be vented, you talked me out of that in my previous question. I'm suggesting that behind the rigid foam the vents are left open. Then the foam board would be sealed at the bottom with a taped vapor barrier, and spray foamed to the rim joist at the top. I want to allow water in and soil gas out. I would add ventilation later if necessary.

    2. cutrightrc | | #17

      Dana, first I want to thank you for all the knowledge and input you add to this site. I am looking at putting in the FV-01WS2 in my sealed crawl space. I am a little confused on the set up since the FV-01WS2 is a supply fan. Would I install it backwards on the crawl space wall to exhaust air?

  2. Expert Member
    Dana Dorsett | | #3

    >"Then the foam board would be sealed at the bottom with a taped vapor barrier, and spray foamed to the rim joist at the top. I want to allow water in and soil gas out. I would add ventilation later if necessary."

    You don't really want to let water in through the vents, even if it's air & water tight between the vent and crawlspace. Nor does leaving the vent open affect radon and soil gas issues when it's air tight between the crawlspace and vent.

    If the interior of the crawlspace is vented to neither the outdoors nor conditioned space, the IRC requires mechanical dehumidification. A tiny fan approach tends to use less power, but a dehumidifier is still fine.

  3. SamuelH | | #4

    >"You don't really want to let water in through the vents, even if it's air & water tight between the vent and crawlspace."

    I don't want to let water in through the vents. I want to let water in through the walls, and that's why I'm considering rigid foam instead of spray foam.

    >"Nor does leaving the vent open affect radon and soil gas issues when it's air tight between the crawlspace and vent."

    It wouldn't be air tight. I would leave a 1" air gap between the foundation wall and rigid foam. And a 1" air gap at the bottom of the rigid foam and the crawl space floor. The crawl space has gravel, so there will be an air path underneath the poly.

    >"A tiny fan approach tends to use less power, but a dehumidifier is still fine."

    Any exceptions for this? My crawl space is fairly big, about 2,500 sf and 5 ft tall, so 12,500 cubic feet. 12,500 / 50 puts me at 250 cfm. Actually the foundation wall is 5 ft and the the space from the bottom of the joists to the subfloor is another foot.

  4. Expert Member
    Dana Dorsett | | #5

    >"I don't want to let water in through the vents. I want to let water in through the walls, and that's why I'm considering rigid foam instead of spray foam."

    No, you really DON'T want water flowing through your foundation walls!

    >"It wouldn't be air tight. I would leave a 1" air gap between the foundation wall and rigid foam. And a 1" air gap at the bottom of the rigid foam and the crawl space floor. The crawl space has gravel, so there will be an air path underneath the poly."

    The air gap is a lot of complexity that buys you nothing if the foam itself isn't air tight between the foundation and crawlspace interior. It's easy to make the foam reliably air tight if it's affixed tight to the foundation, not so much if it's hanging on some spacers.

    >"Any exceptions for this? My crawl space is fairly big, about 2,500 sf and 5 ft tall, so 12,500 cubic feet. 12,500 / 50 puts me at 250 cfm. Actually the foundation wall is 5 ft and the the space from the bottom of the joists to the subfloor is another foot."

    The IRC specifies the ventilation rate by floor area, not volume. 2500' /50= 50 cfm.

    The FV-04WS2 is good for 40 cfm at high speed, and (code notwithstanding) should be enough if you do good enough job air sealing the vapor barrier & foam. If going for the full 50cfm, the FV-15NLFS1 at it's lowest speed runs 50 cfm but it's currently fairly pricey. The FV-0511VQ1 bath fan is less than half the price, and ~4 watts at it's 50cfm lowest speed, if installed close to where it exits the crawlspace, with minimal (and hard-piped) duct.

    1. SamuelH | | #6

      >"No, you really DON'T want water flowing through your foundation walls!"

      Wouldn't it better to let it flow through my foundation walls rather than let it sit inside the concrete blocks? Digging up the exterior for a perimeter drain would cost 20 to 25K I would imagine, and destroy my property (trees, patio, sidewalks, brick stairs, landscaping, etc.).

      1. Expert Member
        Dana Dorsett | | #7

        >"Wouldn't it better to let it flow through my foundation walls rather than let it sit inside the concrete blocks? "

        No- it wouldn't be better to let it flow. Concrete is highly tolerant of standing moisture, but water moving through the concrete slowly takes minerals with it. Do you have a lot of efflorescence on the interior of the foundation walls? (Efflorescence is minerals stripped from the concrete and deposited on the surface as the moisture dries.)

  5. SamuelH | | #8

    >"Concrete is highly tolerant of standing moisture, but water moving through the concrete slowly takes minerals with it."

    Understood, but it seems like leaving water standing in the bricks would cause odor and mosquito issues.

    >"Do you have a lot of efflorescence on the interior of the foundation walls?"

    I have a fair amount of efflorescence. It's spotty throughout the walls. Some places are worse than others.

  6. Expert Member
    Dana Dorsett | | #9

    >"Understood, but it seems like leaving water standing in the bricks would cause odor and mosquito issues."

    With air tight foam &/or vapor barrier between the conditioned crawl space and the CMU there isn't a pathway for odors or mosquitoes to get in there.

    If the water table or periodic flooding is high enough to accumulate in the CMU cores you need more than just a ground vapor barrier (which would be floating atop the water, if it's liquid-tight.)

    The efflorescence is an indication of moisture passing through the CMU, a process you really want to stop. Cleaning it off and applying a masonry sealer before adding the foam may be worthwhile.

  7. SamuelH | | #10

    >"With air tight foam &/or vapor barrier between the conditioned crawl space and the CMU there isn't a pathway for odors or mosquitoes to get in there."

    Mosquitoes and odors wouldn't be an issue on the inside of the building, but I meant on the outside.

    >"If the water table or periodic flooding is high enough to accumulate in the CMU cores you need more than just a ground vapor barrier (which would be floating atop the water, if it's liquid-tight.)"

    I installed a sump pump and did a lot of trenching around the low lying areas of the crawl space. There's gravel throughout too.

    >"The efflorescence is an indication of moisture passing through the CMU, a process you really want to stop. Cleaning it off and applying a masonry sealer before adding the foam may be worthwhile."

    It's worth it even on the interior foundation wall?

  8. SamuelH | | #11

    >"It's easy to make the foam reliably air tight if it's affixed tight to the foundation, not so much if it's hanging on some spacers."

    You raise a good point. Foam board probably would be hard to make air tight and isn't worth the extra effort, spray foam would be a better option. If I have issues with radon or odors afterwards I can work on vapor barrier air leaks with mastic or install a radon mitigation system.

    >"A tiny fan approach tends to use less power, but a dehumidifier is still fine."

    The fan would use less power, but what about overall energy consumption with continuously exhausting conditioned air?

    1. Expert Member
      Dana Dorsett | | #12

      Taped foil faced foam board can be made air tight if firmly attached to something that isn't flexing, and has few penetrations to seal (such as the foundation wall).

      The Panasonic FV-04WS2 burns 8.2 watts at 40cfm. Running continuously that would only burn through 72 kwh per YEAR.

      A typical room 70 pint dehumidifier burns more than o.5 kwh per hour of active operation. Even running a 5% duty cycle it would use 220 kwh/year.

      1. SamuelH | | #15

        Should I be concerned about insects and mice entering the crawl space fan's exterior exhaust port? Should I install a duct filter box inline with the exhaust duct to prevent pest's from entering? Or something similar?

    2. Jon_R | | #13

      > what about overall energy consumption with continuously exhausting conditioned air?

      As Dana demonstrates, the cost of exhausting conditioned interior air is often missed - fan operating cost alone is not the accurate cost.

      Also not mentioned is that exhaust doesn't improve crawlspace humidity when you aren't conditioning the interior (eg, mild days/weeks). You may get mold.

      Do add a monitor so you get an alert if either system fails.

      1. SamuelH | | #14

        I will probably do both a fan and dehumidifier, and hopefully the dehumidifier will run at a very low duty cycle, much less than the 5% example Dana gave. If I have time I'll figure out how to make an estimate of the energy penalty for exhausting the air. Not sure it would influence my decision though, air quality is a big concern of mine, especially if I'm going to have someone spray foam the walls and rim joist.

        I do already have a wireless temperature and humidity sensor. I may get at least one more so that I can monitor the humidity distribution across the crawl space.

        1. Jon_R | | #16

          Energy wise, you don't want to exhaust the air you just paid to dehumidify (this can cause the dehumidifier to run a lot). Install a dehumidifier, seal the crawlspace well (no vent to the interior) and use as little exhaust as possible to control pollutants.

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