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Community and Q&A

Air Exchange for Unvented Insulated Crawlspace

paula_builds | Posted in General Questions on

My crawlspace is unvented and insulated.  As I am about to install the heating system, it is time to look at how I will circulate air within the house into the crawlspace.

-I do not yet have the 6 mil plastic down on the ground.  I had hoped to do that after the bulk of the underfloor work was complete in order to minimize the damage to the plastic from crawling on it.  There is round river rock on the crawlspace floor right now.  Maybe I should have put the plastic under the rock but it’s too late now.

-I have no holes cut in the floor of the house yet.

-My HVAC professional is installing the heat pump system.  It will supply air to both wings of the house from a closet, via ducts that are not very long, but it’s not a ductless system, in case that matters.

-I’m hoping to DIY the fans & return grills to exchange air with the crawlspace.  The site I’m exploring is ACInfinity.  Their tech support seems helpful in terms of thinking through options.

-I’ll use a fan like the T3 here, which is humidity and temperature controlled, to push air into the crawlspace.  I’m thinking the sensor should be located in the crawl.

-I could just use a passive return grille like this

-My crawlspace is a bit musty right now as it’s been unvented for about a year now.  Getting the ventilation set up, running fans, putting plastic down, and (if necessary) painting a limewash on the underside of the floor if needed, could all address any funkiness that has developed.

-I have a good drainage set up (rocks, pipe) outside the foundation, did all the recommended waterproofing, dampproofing etc so hopefully that will take care of me until I have gutters, downspounds on the building which of course feels pressing but may not happen for 6 months.

I’m on the rainy side of Oregon.  (Hence the worry about funkiness developing in the unvented crawlspace.)

I have no familiarity with what is standard practice with exchanging air into & out of the crawlspace.  I’m posting here to find out if what I’m proposing sounds fairly normal and acceptable.  What is the standard practice?

More specs:
-The underfloor ventilation requirements are 1 CFM per 50 sq feet”, which would mean 18 or so cfm would take care of my 868 sq. ft. house, according to a local building official.
-The T3 fan linked above is 120 cfm (and 30dba) so just having one fan seems adequate for the entire home.

-Would it be advisable to filter the returning air somehow?  Or is that not necessary since I’ll have the plastic down and space should be dry.

-If I do install filters, the ACInfinity person suggested that having fans for the return would assist in making that filtration more effective.

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  1. Jon_R | | #1

    Here is the latest code, which is helpful even if it isn't adopted in your area.

    Be careful that you don't pressurize the crawlspace any more than necessary - it would increase exfiltration. Be conservative and use 1+ sq inch/CFM of return grille.

    A dehumidifier in a sealed crawlspace will leave pressure the same, produce a more consistently dry crawlspace and reduce odor. It won't run much, so don't worry about operating cost.

    IMO, sensors/controls are a good idea and can improve performance and efficiency of the various crawlspace conditioning options.

    A humidity alarm is a good idea for when the system (fan or dehumidifier) fails.

    1. Expert Member
      MALCOLM TAYLOR | | #2


      Our code here in BC allows several approaches including just using sufficiently sized transfer grills between the crawlspace and house above.

      I'm not a big fan of sharing air between the crawlspace and living areas. My own preference is to include an exhaust fan in the crawlspace, which is part of the required house ventilation, and depressurizes the space which will draw air from the house above through passive vents.

      As Jon said, the trick is to neither pressurize or depressurize the space sufficiently to increase air leakage in either direction through the walls.

      1. paula_builds | | #3

        Malcolm, why do you not want to share air between house & crawl? Just the innate funkiness of the crawl air?

        Setting up a fan to exhaust out of the crawlspace would be a change from the current plan. I'm inclined to put down the plastic, condition the air, and have air in & out of the crawl into the house, unless I'm convinced that's a terrible idea.

        The big concern I see based on these comments already is that 50 cfm per square foot of crawlspace floor (868/50) gives 18cfm required, and the smallest fan I'm seeing on acinfinity website is the t3 at 120 cfm [Update: the fan can be set to run at 12 cfm up to 120 cfm (it has 10 speeds).], which would raise the concern of too much airflow, increasing exfiltration, I guess sucking air out of the house too hard would create the potential of cold air being pulled into the house itself? But if I have the air return (from crawl to house) balanced, then wouldn't it be ok? Or should I look for a smaller fan?

        As a side note I have been working on air sealing throughout the project, and will continue to complete that work.

        1. Expert Member
          MALCOLM TAYLOR | | #6


          I'm not keen on sharing air with spaces that don't get cleaned for decades at a time, may have radon, are usually damper than the rest of the house, and often get things stored in them that smell or off-gas. Those preferences aside, there is nothing wrong with simply connecting the two spaces.

          I'm not suggesting an exhaust only solution, but rather than you use the crawlspace fan as part of your balanced ventilation strategy. If you are installing an HRV/ERV then I would include a return air duct in the basement instead of a dedicated fan.

          1. paula_builds | | #8

            Radon is not much of a concern in this area, from what I've read. I don't plan to be cleaning down there regularly, so dust is probably going to accumulate, which supports the idea of filtering air before it returns.

            No HRV planned, but, what about having a return ducted to my heating unit from the crawl, with an inline air filter?

          2. Trevor_Lambert | | #15

            What is your ventilation plan?

    2. paula_builds | | #16

      Thanks! I think the trick is to figure out how to run the dehumidifier drain line out, perhaps combining (as code allows) with the condensate line from the heating unit(heat pump). But I like the idea of dehumidifying that space.

  2. brian_wiley | | #4

    I have a conditioned crawl (20mm liner up the walls with 4.5” eps insulation) that’s using an exhaust only strategy. It had been vented for 60 years, and despite the walls and floor being covered there’s still a slight mustiness to it that I can only attribute to the underside of the subfloor. It’s gotten better in the two years since I’ve lined it, but it’s still there.

    Radon is another concern that I’ve heard others voice in sharing that air with the space.

    As far as the necessary CFM, you can always set a timer to cycle on and off to get to the necessary rate. Panasonic has some fans that go as low as 20 though, if memory serves correctly.

    1. paula_builds | | #9

      So with an "exhaust only" strategy, you push air out, but then air is passively made up by what..a grille? Just whatever happens, i.e. cracks or gaps?
      Is it a pretty low cfm?

      Because I've insulated on the outside of the stemwall, and then covered THAT insulation with a fancy layer of metal, I feel resistance to the idea of cutting a new hole for inserting an exhaust fan to the outdoors.

      (In my area Radon isn't supposed to be a concern.)

      1. brian_wiley | | #10

        For exhaust only, there’s a vent/grille connected to the living space for makeup air. I sized mine to 12”x8”, but that was really a guess based on CFM for a return grille. I have a 1285 sq ft house, so 25 CFM of exhaust. The code isn’t specific about what size or how many transfer grilles are needed, so it was my best guess combined with some monitoring for the first year.

        I used one of my old foundation vents from when it was a vented crawl as the area to pass the exhaust through, but I understand your hesitation to cut another hole in your envelope. You could go with just a dehumidifier.

        It seems like with all these strategies there’s pluses and minuses, and not everyone agrees. With the dehumidifier strategy, Martin Holladay isn’t a fan, but Dr. Allison Bailes is. Not that my opinion is worth much, but I think the most important thing is that you’re monitoring and watching the space at least for the first year or so. I bought a couple of inexpensive Govee Bluetooth temp/relative humidity monitors and have one in the living area and one in the crawl so that I can make sure that the humidity doesn’t get too wild down there. So far it hasn’t been an issue, but it’s nice to be able to check.

        I’d second Andrew’s suggestion of getting the vapor barrier down as soon as you can. I was amazed at how much better things were within just a few days of taping up the last sections of liner. Even a cheap temporary one seems like it’d be a big help. I will say that I was surprised how durable the 20mm liner is that I put down. I slid a 150 lb air handler across it last week, completely assuming that I’d have to replace the path it traveled, but there was barely a scratch. That said, if I did it again I wouldn’t go over 15mm, and would probably end on 12mm with just some key reinforcements where I often travel. My crawl has pier blocks and 4x4 posts every 42” on a grid, and that 20mm liner was a pain to detail around those obstacles.

        You’ve probably seen this before, but on the off-chance you haven’t I thought I’d pass it along.

        1. paula_builds | | #19

          Thanks for the link to the article, as well as your thoughts on plastic thickness, and monitoring the crawl space.

    2. Jon_R | | #12

      Code says "continuously" for the exhaust option, so no timer.

      My guess is that while it's off, radon or odors could enter the living space above.

      1. brian_wiley | | #13

        It does say continuously, but it seems like one of those common sense things where the necessary CFM is so low as to be difficult to find a properly sized fan. I only needed 25 CFM, and our building inspector was okay with a timed approach. It may be worth checking in with them first.

  3. Andrew_C | | #5

    Ignoring the air circulation for a moment, I think you may be overlooking the importance of the vapor barrier. It sounds like you've got a lot a work planned that may take a while. I would consider putting down a cheap 6mm vapor barrier now, even with imperfect installation, and then when the rest of the work is complete, add a more robust vapor barrier. The vapor barriers that I've had installed commercially are much thicker than 6mm.
    If funkiness is a concern, consider covering the floor while the rest of the work gets done.

    1. paula_builds | | #7

      Thanks, that sounds smart! If I'm going with a more robust vapor barrier, it should be able to withstand some occasional crawling around down there anyway.

    2. Trevor_Lambert | | #20

      6mm is almost 1/4". Did you mean 6 mil (0.006")?

      1. paula_builds | | #22

        That is what I assumed he meant.

    3. paula_builds | | #21

      Heres a 12 mil liner product with an antimicrobial element

      What do people think of having that antimicrobial element? Is there a downside to that? What chemicals are doing the job, and will they negatively affect my air quality??


  4. walta100 | | #11

    First before you put down your plastic would you have enough headroom put in radon vent pipe covered with 4 inches rock?

    If the old ductwork is in the crawlspace it may well have enough leakage to condition the space.

    The way I look at it the crawlspace is now part of your conditioned space it should be as clean and dry as every other part of your home.

    I say monitor it this winter if it is more than a few degrees cooler than the house them adjust your duct work. Monitor the humidity over the summer if it gets to be more than 10% above the living spaces you could add a dehumidifier or change the ductwork then.


    1. paula_builds | | #18

      my understanding is that radon is not a concern in this part of Oregon. Not sure if I should bother with that. I do have plenty of headroom, but there is a 2-4 inch layer of round rock already.

      This is new construction so there is no old ductwork down in the crawlspace.

      I like the idea of monitoring the situation. I would like to keep it nice down there, but I certainly won't be vacuuming regularly, for example. :)

  5. Jon_R | | #14

    Regarding any plan other than crawlspace dehumidification: in periods of moderate Oregon weather (humid outside but no AC or furnace use to dry the air), expect high humidity in the crawlspace. Some mildew growth may occur. So is there a benefit to applying mildewcides to the crawlspace surfaces and under the vapor barrier (ie, on the gravel)?

    1. paula_builds | | #17

      I have a limewash recipe that I think will act nicely as a mildewcide. It may be a tedious project but I'll brush it onto the exposed surfaces (walls, floor joists, etc) in the crawlspace. After I get the plastic down.

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