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Community and Q&A

Mixed humid climate, conditioned crawl space – Part 2

BJHuffine | Posted in General Questions on

I hate to post a second question, but felt it would be best served as a separate one.  If anyone is interested in the first, the link is below. Basically, in the first, I was trying to get an idea of what would work best in a conditioned crawl space in my area.  I live in Tennessee, just above where the border intersects with GA and AL.  There seemed to be quite a few options and I wasn’t sure what I should be shooting for in new construction.  In the end, it seems that 1.) there’s the code where it relates to conditioning and 2.) different ways to pull air into the space to condition it and along with this 3.) no real “best-practice” just different approaches. But for this question, I wanted to approach a conversation I had this morning with a crawl space specialist in our area, which matches what another crawl space specialist recommends.  And neither regard conditioning the space.  Basically, the recommendation for new construction is to: 1.) Install insulation (one says on the wall with their borate impregnated foam board and the other with floor joist insulation). 2.) Install a single vent opening in the foundation (based on size of home and would require an installed exhaust fan). 3.) Seal the crawl space 4.) Do not install transfer grilles (no mixing of climates from the living to crawl space) 5.) Install greater than 6 mil polyurethane vapor barrier (one recommends non-chorded version to prevent smell in space). 6.) Install dehumidifier sized for the space. This is much different from the building science information out there about encapsulating crawl spaces as they go straight to dehumidification.  In other literature, the idea is to dry it first and if things work well, then no dehumidifier, but save that for the moment it is needed.  They state the reason is that we are in a humid environment already, so they feel it best to deal with the humidity this way.  It seems to me (though I didn’t think to ask them) that this would mean not placing HVAC supply/return ducts in the crawl space, but that leaves the attic and I know that to be a no-no.  Conditioning the space seems to suite having the duct work in the space.  So I guess I wanted to ask the community (especially those with experience in my region, or similar) if they have successfully conditioned the space or went the same route?  And for the experts, thoughts on this?

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  1. Expert Member
    Dana Dorsett | | #1

    Per IRC 2018 R408.3, exception 2.4 allows unvented conditioned crawlspaces as long as it is mechanically dehumidified with equipment capable of 70l/day or more for every 1000 square feet of floor area.

    Exhaust-only at a continuous 1 cfm per 50 of floor area would be allowed without the dehumidifier as long as there was a transfer grille to allow fully conditioned air, as would ventilating it with conditioned air at the same prescriptive ventilation rate, as would simply using the crawlspace as an HVAC plenum.

    Any of those work from a building science point of view.


    (Section R408.3 is near the bottom.)

    There is no reason to not use the conditioned sealed crawlspace for ducts & air handlers, given that it could also be used as the return plenum. If the ducts are sealed and the crawlspace is NOT being used as a plenum one of the other three solutions would be required.

    If there is a vent open to the exterior with exhaust fan WITHOUT a transfer grille to conditioned space it depressurizes the crawlspace, drawing in air from any available path, which could be soil gases depending on how perfect your vapor barrier seal is. Do either the exhaust, or the dehumidifier, but not both.

    1. BJHuffine | | #2


      Thanks for helping to clear up the code perspective. If you treat the crawl space as a plenum, does that mean it is best to have two transfer grilles so that there is circular movement? Or a supply and a transfer grill to allow supplied air to return back? Or either-or? I assume in any of these cases, placement of supply/return and/or transfer grilles would also be very strategic to ensure there's proper air flow across the space.

      Also, something I didn't think to include earlier is that they mentioned not using transfer grilles due to "musty" smell from crawl space. However, I was thinking that if conditioned and sealed well, that shouldn't be an issue, right? Though I understand there will be a 3 inch exposed foundation for termite inspection above the vapor barrier/insulation, I wasn't sure if that would be a problem or not. Any thoughts on that?

      Overall, I feel like the conditioned approach makes the most sense. Going straight to the dehumidifier first just seems like it is accepting that conditioning that space is impossible otherwise.

  2. Expert Member
    Peter Engle | | #3

    Even though it is allowed, you probably don't want to use the crawl space as a plenum. That just raises all sorts of other potential issues.

    If your house is not super tight, using an exhaust fan with a single transfer grille on the far end of the crawlspace from the fan will work just fine without a dehumidifier. The code allows this approach.

    If your house is super tight and you are going to make the crawl space super tight with well sealed seams on the vapor barrier, air sealing all perimeter penetrations, etc. then there will be very little moisture accumulation in the crawl space in the first place. The code allows you to condition this space with a dehumidifier. It will rarely run if you've done your air sealing properly. You could also condition this space with a supply and return from the HVAC system, so long as they meet the prescriptive airflow requirements Dana mentioned above. This approach has a slightly higher risk of odors from the crawl but again, if you've done your air sealing properly, that shouldn't be much of an issue.

    All of these approaches probably use about the same amount of energy in the end. Small changes in details could swing the energy balance either way.

    1. BJHuffine | | #6


      Thanks for clarifying options and their constraints and comparisons like the tightness of the air sealing, source of moisture for the option in question, likelihood of odor, etc. This is exactly the information I would like to see on all the options presented by the great folks at GBA or Building Science.

      After thinking through your comments, it does seem that either an exhaust with transfer grille or dehumidifier-only (no vents, no transfer grilles, no supply/return) seems to be the way to go with less impact. The first at least offers air movement, versus stale air in a dehumidifier-only approach, which seems to me to offer more drying opportunities. Then again, to some degree I assume the dehumidifier would be circulating air as it extract moisture, but probably not as much.

  3. Jon_R | | #4

    > Going straight to the dehumidifier first just seems like it is accepting that conditioning that space is impossible otherwise.

    It's certainly possible to condition the air in the house such that it will be sufficient to dehumidify the crawlspace. The question is if that's most efficient. Ie, do you want to condition the entire house ($) during periods where you would normally open windows - just to reduce crawlspace humidity?

    While probably too much trouble to implement, it would be easy to show that the most energy efficient reliable solution is the ability to switch between either (dehumidifier or air from conditioned space).

    I'd only accept the energy hit of exhaust ventilation if there was an odor/radon issue or the air flow was part of the interior ventilation plan.

    1. BJHuffine | | #5


      You're right. There still seems to be a lot of advocates for conditioning the space directly and supposedly if you get it right you're okay. But then there are also many who have mentioned that on the off-seasons when the HVAC is not working as often, it becomes a problem. That definitely makes sense and I'm not necessarily questioning that as I am variations or approaches that maybe others are taking to avoid that issue. At the moment, I'm beginning to wonder if those with more success during off-seasons may be in a different climate than I am to begin with and maybe mute. Then again, I may also talk with some HVAC specialists and see what's available. Maybe there's more opportunities to ensure continuous air flow with today's technologies. Then again, it still has to be weighed as Peter and yourself have brought up. I think I'm almost talking myself into an exhaust with transfer grille or dehumidifier only (no vents, no transfer grilles). Off the top of my head, the first seems to operate more as an independent system, though using conditioned air from above, which may be best.

      As for figuring out how to do both? That smells like a patent for someone. It's amazing the technology used to control commercial environments, I bet a controlled system approach for residential application would do well.

  4. Expert Member
    Peter Engle | | #7

    You could do both inexpensively without automatic controls.

    Just use a spring-loaded damper or a fan with an automatic damper to control outdoor air exchange when the fan is not running. Leave the transfer grille open - it won't hurt anything. Put the fan and the dehumidifier on switches you can control from upstairs.

    Run the dehumidifier in hot/cold weather when indoor air is expensive, especially winter when the heat gain is a bonus, and the fan in the shoulder seasons when indoor air is free. Install a remote-read humidity monitor in the crawl space to keep an eye on things.

  5. Stockwell | | #8

    Hi Jason--As I mentioned on your other thread, I have been through this process of choosing what to do with the crawl and I settled on no conditioned air, dehumidify and spray foam the underside of the floor. I had the duct discussion with my HVAC guy. I told him as long as he followed the principles here that I would be ok with it. In my case, they will seal the ductwork, wrap in R-13, and then it will be buried in R-30 of foam. I opted to not go the conditioned route because we do like to have the windows open as much as possible and not rely on HVAC so much. That's why I chose to live in the mountains!

    1. BJHuffine | | #9

      That's an interesting and innovative approach. Personally, I'm not sure I'm ready to dive head first into the spray foam world. But I totally agree with opening windows and living in the mountains. I live on one myself!

      Curious, the Building Science article addresses it from the perspective of the attic. Does the temperature and moisture source differences between an attic and crawl space affect it?

      1. Stockwell | | #10

        I think the principles are the same whether attic or crawl--ductwork sealed tightly, ample impermeable insulation. Crawl should actually be a little more forgiving since there is not(or shouldn't be) warm moist air to contend with if you have done things correctly.

        1. Jon_R | | #11

          Most of the crawlspace ventilation options involve pulling air from the interior - which will sometimes be warm and moist.

          1. Stockwell | | #12

            Jon--I was referring to my own setup which has no ventilation

    2. Jon_R | | #13

      Kevin, as I understand it, you have a sealed and dehumidified but thermally unconditioned crawlspace. Ie, different barriers for humidity and heat. While interesting, is this code compliant? What happens when it's < 40F in the crawlspace and the dehumidifier can't function? Perhaps too cold to grow enough mold to be an issue? Or perhaps stack effect and small amounts of heat from the ducts prevent a humidity problem in cool weather?

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