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Insulating a Crawlspace Floor

amainiii | Posted in PassivHaus on

Just starting a deep energy retrofit on a 1956 ranch in Columbus, Ohio. I don’t know if we’ll achieve true Passive House but we want to do our best at each step. There is a crawl space under the master bedroom (12×16). Walls are CMU, floor is loose gravel. Clearance is about 36-39″. The current plan is 20mil vapor barrier across the gravel going up around 18″ on the side. 2″ of closed cell spray foam for air sealing and insulating the walls. Contractor is not planning on insulating the ground. I’ve seen crawl space floor recommendations for foam with 1″ of concrete over the top. I’d rather avoid the concrete. I’ve wondered if we could just lay unfaced mineral wool over the gravel and put the poly over the mineral wool.  I don’t know how strong the poly is. If plumbers had to climb over the top for some future maintenance, would that be doable without compromising the poly or the insulation? Any suggestions or feedback welcome.

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  1. user-6623302 | | #1

    CMU? Were are you, climate zone.?

  2. amainiii | | #2

    We're in Columbus, Ohio. Its Zone 5, a mostly heating climate. CMU = concrete masonry unit = concrete block.

  3. Expert Member
    BILL WICHERS | | #3

    I would recommend running the crawl space liner all the way up the walls. I would sandwich the liner against the wall using polyiso, then consider spray foam ONLY for the rim joist area, ideally using it to tie into the top of the polyiso on the walls so that you seal the entire assembly. I would not use spray foam against the walls since you should have pretty flat walls. I would only use spray foam on a crawlspace or basement wall that was an irregular surface (cut stone, etc.), where rigid foam would not lay flat against the surface of the wall.

    I would probably not insulate the floor here, I'd only insulate the walls. I would absolutely not use any kind of batt, mineral wool or otherwise, on the floor though. If you want to insulate the floor, use rigid foam. I would use XPS, but rigid mineral wool would also work (but is much more expensive). You probably don't really gain a whole lot insulating the entire floor of the crawlspace. You could put in insulated "skirts", which are sections of rigid foam laying on the floor coming in only 2-4 feet or so, which gains you a little without the expense of insulating everything. You gain most of the benefits of crawl space insulation by insulating the wall down to the frost line or a little below, and if your wall isn't going down that deep, the skirts can help.

    Note that with any kind of insulation on the floor, maintenance will be more of an issue unless you put in a subfloor of some type so keep that in mind.


  4. PBP1 | | #4

    I have a 12'x12' crawl that's about 36" deep with a floor of foam and concrete on top (about 3" or so). I don't regret the decision, water service line comes in and all my plumbing drains to sewer in that space (runs close to walls), it's also a good storage space.

    As the insulators forgot to hit an entire end bay against an exterior wall, I got a spray foam kit (and appropriate 3M mask), which was enough to do that bay and two of the walls. I agree with Bill though, it's easy to cut rigid foam and hit the edges with sealant/canned spray foam.

    The space has a small fan controlled by a humidistat, I installed a decent butterfly damper as the exterior vent flap alone was utter crap. I also have a vent to the laundry room above. The crawl is essentially a conditioned space - I'm in climate zone 6.

  5. amainiii | | #5

    Bill - Thanks for your detailed answer. I can find lots of resources describing crawl space insulation techniques, many of which echo the guidance you've provided. I can't find anything that is specific to passive house crawl spaces. If I understand correctly, Passivhaus walls are generally in the range of R-30 to R-50, ceilings are R-49 to R-80, and R-20 for slabs. I probably should have asked a basic question: What are the R-value goals in a crawl space under a passive house? Have you used this detail in passive houses? What thickness polyiso did you use? If you didn't insulate the ground, did you insulate the floor above? Since the crawl space is now conditioned space, it seems to me that all the usual passive house insulation requirements would apply. That would mean 4" rigidfoam on the walls, maybe 3-4" on the ground. Is your recommendation against closed cell foam due to cost or other reasons ? The contractor said he could use polyiso or EPS, but given this particular crawl space with a 29"x27" access panel. , he said he could do it cheaper with closed cell foam because of labor savings. I imagine his answer might change if it was 4" of closed cell foam.

    1. Expert Member
      BILL WICHERS | | #7

      You want to go with at least code minimum insulation on the walls regardless. In my climate zone, that means R15. I have a bit better than that, a bit over R17 on my own home, since I have (or will soon have in some areas :-), 3" of polyiso. I tend to pick my thickness of polyiso in this kind of project more based on what I can get a deal on from a reclaimer, so I go as much over code as current prices allow. A few years ago, one place had a great price on 1.8" polyiso, so two layers of that would give around R20. Unforuntately, they sold out of that material and current prices are not as a good (as is the case with pretty much everything these days!).

      For a passivehaus, you're probably going to want a lot more than code minimum. I haven't myself done a passivehaus, so I can't really give specifics, but below-grade walls might not require as much as above-grade walls. The reason for this is that below grade walls will never see as low of low temperatures as the above grade walls do, since the earth itself provides a little bit of warmth, and also insulates you a little too. This means you can typically get away with less R value on things like crawlspace walls to get to similar performance levels in terms of BTU loss per unit wall area compared to above grade walls. In your case, if you want passivehaus performance, I'd probably go for R20 or more on those crawlspace walls.

      In my own home, I did not insulate the floor, and I did not use skirts (my crawlspace walls come down to a little below the frost line, so I don't gain much with the skirts). I would never insulate the floor above. If you are wanting passivehaus performance levels, then you'll probably want to insulate the floor to minimize total heat loss. I would suggest R10 XPS at a minimum here, but you'll want to check with the Passivehaus people to see what they recommend.


      1. amainiii | | #9

        Bill - Thanks for the additional details. Why are you picking XPS over EPS? Are you suggesting reclaimed? If new, wouldn't EPS be better (environmentally)? I think even the new XPS is still 1.47 GWP (instead of 1.0 for EPS). Just wondering if there was a technical reason to avoid the EPS.

  6. amainiii | | #6

    PBP1 - Thanks. Was that new construction, or did you have that foam + concrete put in as part of a remodel? And 3" was that the concrete thickness? Or the foam thickness, or both? My crawl space already has a duct in (forced air system), so I think we are covered both practically and per code requirements.

    1. PBP1 | | #8

      Sorry I didn't give additional details, it was new construction, foam was 1.5" I believe and 3" concrete on top of the floor.

      1. amainiii | | #10


  7. amainiii | | #11

    An update for others who find this posting.

    In the 2015 and 2018 IRC (and Ohio 2019 Building Code), section R316, foam insulation is to be protected with a fire barrier that passes NFPA 275. Fire retardant vapor barrier is insufficient. R316.5.4 specifically lists what is required in crawl spaces.

    You can find lots of contractors around who will encapsulate your crawlspace with EPS/XPS and vapor barrier, who ignore R316. Suppliers I have talked to are unaware of the code requirement. Even the local building inspector I spoke with didn't seem to care. But, if only for myself, I want to attempt to follow the code.

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