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Crawlspace floor proposal

Monte_Main | Posted in Energy Efficiency and Durability on
I’m proposing a crawlspace floor assembly as part of a deep energy retrofit. The crawlspace has a 16’x16′ floor area (256 sq ft). This crawlspace is not used for any storage, there is a suspended dehumidifier in the space.
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. 
I’d ideally like to put down 3″ Comfortboard 80 directly on the ground and cover it with the vapor barrier per the video by Rockwool/Dan Edelman.
However, 3″ Comfortboard 80 is now around $5/sq ft and I can get recycled 3.5″ EPS at about $1/sq ft.
My proposed floor is as follows:
* Complete the radon mitigation/vapor barrier (not discussed here)
* Put down 3.5″ recycled EPS foam PSI 40 over the vapor barrier
* Lay 1/4″ Hardie board glued to the EPS foam with construction adhesive (making sure EPS joints are spanned)
* Fill the gaps between Hardie boards with fireproof caulk
Looking for comments/suggestions.

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  1. Expert Member


    Unfortunately, I think you would be lucky if the Hardi-backer or Hardi-panels made it through the installation intact. They are brittle and not designed for the point loads you get on floors. I'd use OSB.

    1. Monte_Main | | #2


      I wondered about that and checked the specifications. Hardee claims its much stronger than the average backer board. But R316.5.4 does say that 1/4 inch wood structural panels (OSB) are acceptable. And its only $0.57/sq ft, which is around 40% cheaper than the Hardie board.

      What do you think of using construction adhesive to glue it down to the rigid foam insulation? And using fireproof caulk at the seams? Or is there another way you'd recommend?

      1. Expert Member
        MALCOLM TAYLOR | | #3


        I'd favour using OSB, and gluing it should work fine. Are you sure you need to caulk the joints if it is fitted tight?

      2. brendanalbano | | #4

        I'm with Malcolm that the fireproof caulk may not be necessary. Always wise to check with your local building department if you are unsure.

        R316.5.4 Crawl spaces
        The thermal barrier specified in Section R316.4 is not required where all of the following apply: (long list of options, including the 1/4" wood structural panels you are considering).

        The list in R316.5.4 makes no mention of fireproof caulk.

        But again, you should always double check with your local building official.

        1. Expert Member
          Akos | | #5

          I would vote for 2x2 patio pavers.

          You can usually find the off fashion ones for free on-line if you can pick them up, even new they are pretty cheap. I've used these before, just flip them over so the smooth non-textured side is on top which looks much better.

          Easy to lay, takes a lot of abuse, no issues with water and can always be lifted up and moved if you have to get underneath them.

  2. Expert Member
    BILL WICHERS | | #6

    You don't need fire caulk. You don't need a fire barrier, you need a thermal or ignition barrier. Those three things are not the same. Fire barriers are intended to stop the spread of smoke fire, so they have to be sealed, this is how fire walls are build. Ignition barriers are supposed to stop things from catching on fire, and thermal barriers are supposed to keep things from getting hot enough to fail or catch fire, usually for some period of time (30 minutes, 1 hour, etc.).

    I agree with malcolm that hardibacker isn't going to survive. That stuff is very brittle, a little like heavy cardboard soaked with concrete. It especially tends to be prone to the corners crunching off. Your EPS is going to flex and deform just from you're crawling around on it, and that's going to crack and crunch the hardiboard.

    I would use OSB as Malcolm suggested, or plywood. I think plywood is a little safer in case of any water leaks, since it is less likely to degrade from moisture before you fix the problem compared to OSB, and pricing lately has been similar. I would probably use 1/2" CDX here. Code allows 1/4" hardboard too, which is like a very fine grained particle board, but that stuff isn't as cheap as it used to be, and it's also more prone to corner crunching problems than plywood or OSB.

    I kinda like Akos' idea of using pavers too -- I'd never thought of that before :-)


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