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Community and Q&A

Crawlspace Air Supply and Exhaust

Eric__S | Posted in Energy Efficiency and Durability on

Hi, first time forum poster.  I’m a homeowner in northern Virginia who is interested in energy efficiency upgrades to my 1977 home.  I’m a big fan of Musings.

Our house has a full basement in one section and a crawlspace (~1,000 sq ft) in another section. The crawl space is actually in pretty good shape because the site layout and bulk water solutions keep it dry except for our Virginia summer humidity, but it has the hallmarks of the failed vented crawlspace strategy: mold on joists and saggy under-floor insulation.  I did a lot of head nodding as I read Martin’s 2011 piece on crawlspaces and the merits of sealing them up.

So I decided to encapsulate and had a place come by for an estimate.  This place is a foundation repair company that also does encapsulations.  Their package solution contained many of the right elements including a super thick poly liner (20 mil) appropriately overlapped and taped and sealed to the perim wall, rigid foam along the wall (although they were using 2″ EPS w/ a film on both sides, not polyiso  ???), rim joist insulation, etc.  They also, interestingly, include seam-taped dimple mat under the liner.  They said something about it providing ground condensation drainage and also protects the liner from the copious amounts of gravel I have in there.

Anyways, then it got weird.  They recommended tearing out the saggy under-floor insulation and replacing w/ new R-19 under-floor insulation and putting in a dehumidifier.  This would address all the concerns of moisture, comfort, etc. I was perplexed and said, “I don’t think that’s code compliant.  You have to supply make up air from the living space via transfer grille and exhaust air out the crawl space rim joist (the depressurize strategy), or supply ducted air and allow it to return to the house via return or transfer grille.”  The guy said “well, we can poke a few holes in your trunk line if that makes you feel better.”

So I resolved to get an HVAC guy in to tell me how they’d do it.

First guy who came was actually a building performance supervisor and described doing the encapsulation and HVAC supply just as I had read about it in these forums.  Their standard poly is 12 mil and I asked him to quote me 20 mil, but meh, otherwise all good.

But then guy #3 came and he was a straight HVAC guy.  He said that the way the foundation repair encapsulators planned the work was correct (no air plan, just dehumidifying and putting in floor insulation).  He said they work closely with a local insulator and do this approach for all crawlspaces.  Blargh, that can’t be right….

I think I’m on the right ground to look seriously at dude #2 and pray his pricing is reasonable.  But I have one wildcard in play – our house has a radon issue which is mitigated through a fan-powered air draw under the full basement area’s slab.  I’m concerned with the crawlspace if it is conditioned and brought within the envelope, what will my radon risk look like?  Will the poly liner keep most of it out?  Do I need to pursue the depressurizing strategy to try to keep crawlspace air from being returned to the HVAC / livable area? And if it tests high after encapsulation is done, what system would they put into place to mitigate it…  Concerning to me is the sequencing of the work, if the mitigation system (if needed) would be best installed during the encapsulation.  Guess I need to put a call into the radon guys sooner than later.

Is there something I’m missing about why encapsulation guy #1 and HVAC guy #3 would recommend not conditioning the air in there fully?  Or are they simply misinformed?

Thanks for your thoughts!

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  1. Jon_R | | #1

    Some codes (eg R408.3 of the 2018 IRC) allow a dehumidifier. It has the advantage of working even when the AC isn't being used - no need to dehumidify the whole house just to produce some dry air for the crawlspace. If your air and vapor sealing are good, run-time will be very low.

  2. Expert Member
    Dana Dorsett | | #2

    The IRC calls out either mechanical dehumidification or a minimum of 1 cfm of ventilation for every 50 square feet of crawlspace floor area. That ventilation can be either exchange with conditioned space air, balanced ventilation, or exhaust-only.

    If it's easy to set it up to do 1 cfm per 50' exchanging it with basement air or conditioned air from the rooms above, the 10 cfm Panasonic FV-01WS2 would be adequate for up to 500 square feet of crawlspace, the 40 cfm / 20 cfm two-speed FV-04WS2 would be enough for a crawlspace up to 2000 square feet. As a DIY the component cost is less than a decent dehumidifier, and the operational cost is MUCH lower. The FV-01WS2 draws only a hint more than 2 watts.

    From a radon abatement point of view ventilation with basement air is preferable to exhaust ventilation or ventilation with fully conditioned air from upstairs, since it doesn't depressurize the crawlspace relative to the outdoors, which could draw radon into the crawlspace, and doesn't inject crawlspace air directly into the primary conditioned space above.

  3. Eric__S | | #3

    Thank you Dana and Jon. I should have been looking at the 2018 IRC, vice the 2015? version. That extra line permitting a dehumidifier only made my statements to the other guys incorrect =( I will have to apologize.

    I'll get with the radon guys next and see what they have to say.

    Thank you for pointing out the new code.

  4. Expert Member
    Peter Yost | | #4

    Hi E S -

    how connected is your crawlspace foundation to your full basement?

    Why would you re-insulate the floor assembly when you move to a conditioned/unvented crawlspace?

    In terms of radon mitigation: we all wish that there was a predictable impact on radon levels in a home as you make it more airtight/reconfigure below-grade spaces; unfortunately, that is not the case. Any plan you have for changing the way the crawlspace works should take into account that the new crawlspace has to have a radon mitigation component.


    1. Eric__S | | #5

      Hi Peter,

      Thanks for asking. To be honest, I have no idea why the encapsulation company (guy #1 above) proposed to re-insulate the floor. Doesn't make any sense to me at all, especially because the rest of the encapsulation process included insulating at the perimeter and band joist. Even if we decided to just do the humidifier in there vice any deliberate supply air or exhaust, I would imagine you'd be better off without insulating under floor.

      As far as the radon goes, yeah, I can imagine it would be really tough to accurately predict what was going to happen exactly when you seal it all up and handle the air in whatever way you choose to handle it. I guess the only thing to do is try to encapsulate to manage moisture and improve thermal efficiency and then test for radon concerns and mitigate post encapsulation if needed.

      Thanks for the reply.


      The layout is like this:

      crawl = 30' x 33' ~1,000 sqft
      full basement = 48' x 26' ~1,300 sqft unfinished but dry and clean
      access to crawlspace is from basement 2.5' x 3' sliding plywood door which is positioned to be at the top of the foundation wall (see "O" on diagram)
      radon mitigation pipe from subslab to joist level and run out thru rim joist to fan and piping up to roofline (see "R" and "F" on diagram)
      main hvac trunk line runs from basement thru crawlspace on center line

      ```````````` _____________
      ```````````` | ........................|
      ```````````` | .....crawl.........|
      ```````````` |.........................|
      | ......................................................R...|
      |................unfin basement................|
      |............................................................| - Walk out

  5. Mogana | | #6

    Hi, i'm new to home renovation service. I started a course few months back which has been a DIY course mainly. And now i'm noticing my crawl space leaking out moisture due to lack of encapsulation. So, i am thinking of insulating it myself, though i read a couple of tips share by as a clue to basement insulation but still skeptical about it DIY. But if i have to hire a professional i would spend more and that's another big consideration for me. My question, should i do it myself or hiring a professional best?
    Pls, i need assist on the way forward......

  6. Eric__S | | #7


    Can you provide some more specifics about your crawl space?
    How do you access it?
    What's the height from floor to bottom of the joists that you have to work in?
    What is the floor of it like?
    Does any of the HVAC ducting for the rooms above it run through it?

    I have almost completed my crawl space DIY encapsulation and it has turned out really well, but it took a long time and I started with a pretty good jumping off point (fairly dry, gravel floor, easy access to all parts of it, etc).

    Happy to help give some thoughts if you provide some more specifics.

    Eric S

  7. cchaser | | #8

    Good morning. I am in a similar situation. I have 2 40x20 Crawlspaces that I am considering insulating the walls and redoing the vapor barrier on the floors. I have installed dehumidifiers in each and have the RH down to 50% however the Dehumidifiers have run continuously. I have access to TX-1200 for the vapor barrier and RMAX TSX1500 for insulating the exterior walls of the Crawlspace. My plan is to run the vapor barrier to 1-5 in up on the insulated walls and all the way up on the interior/in insulated walls. When reading on this site, it gives me pause putting a vapor barrier on the wall. Am I misreading? Will this cause mold? Is a soil mitigation system needed to prevent smells in the home? Are the materials I listed good for the job? TIA. C. Chase

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