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Community and Q&A

Insulating Vented Crawlspace Ceiling

satinder | Posted in GBA Pro Help on

I am building a house in a flood zone over a crawl space that has flood vents. So my crawl space is vented. It has already been constructed with poured concrete foundation and a 3″ concrete floor over a vapor barrier. Drains were installed below the floor and it also has a sump pump and a radon vent installed below the concrete floor. My concern is how to insulate the ceiling of this crawl space to prevent a cold floor above as well as prevent air leakage into living area above. The insulation company is recommending a 3″ closed cell foam insulation (R-19.80) on ceiling and a 2″ closed cell foam insulation (R-13.20) over crawl space bands and ribbons. Is this going to be enough to separate the crawl space from my living space from temperature fluctuations? Is this going to create any issues with water condensation during summer since it gets quite humid where I am. I am in climate zone 4A. I would appreciate all the help! Thank you!

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  1. user-2310254 | | #1


    There are strategies for insulating cold floors that don't require closed cell spray foam. For example, see

    You don't tell us what type of ceiling you have in the home, but I would be concerned about settling for R-20 in that assembly. If you have cathedral ceiling, code would be R-49. Assuming you have this type of assembly, you can find guidance here:

    If you are building a tight home, it is important to have an effective ventilation strategy to provide fresh air while controlling humidity. See this article and its sidebar links for more information:

    1. satinder | | #3

      Thank you Steve. I am referring to the ceiling of the crawl space that would be the floor of my first floor living space. Wondering if 3" closed cell foam would be enough of an insulator . or should I also add a layer of batt insulation on top of the foam?

        1. satinder | | #13

          Thank you John, that was a very good article and good explanation regarding different types of insulation.

  2. JC72 | | #2

    IMO this particular application almost demands closed cell spray foam because it will help keep the wood dry during a flooding event. Make sure they spray all three sides of the floor joists.

    At 2 inches ccSPF typically is considered a vapor barrier but it ultimately depends on the formula. I've read it can be anywhere between 1.5 - 2.5 inches. Just ask the installer and contact the formulator of the foam.

    As for meeting the specific R-Value for the floor 2018 IECC indicates R19 for floor in non-marine CZ4.

    1. satinder | | #4

      I am near the water

      1. JC72 | | #5

        Well then it's R30 per the table in the link I provided.

        Q: What flood zone is the house in (A, AE, etc.)?

  3. Jon_R | | #7

    You would be better off with a crawlspace (with flood vents) that is sealed and dehumidified.

    1. Expert Member
      Dana Dorsett | | #8

      +1 !!!

      In 4A the summertime outdoor air humidity is high, along with a high likelihood of air conditioning. Even if the house is only air conditioned down to 75F the subfloor temp will be much cooler, and likely to take on significant moisture from the crawlspace ventilation air.

      Closed cell foam is expensive. A IRC code-min R19 on the bottom of the subfloor would run north of $3 per square foot. Insulating the crawlspace walls and band joist to the IRC code-min R10 can be done primarily with rigid foam tapes, and a bit of can foam at a much lower price per square foot, and in most cases it's usually fewer square feet. If a termite inspection strip is required/desirable in your area leave a 3" stripe of bare foundation to be filled with carefully cut strips of R15 rock wool batt, trimmed for a slight compression fit using a batt knife (or a 8-10" bread knife, which is nearly as good).

      For code-min references, see TABLE N1102.1.2:

      Even if one included the cost of the flood vents, an EPDM or 15-20mil polyethylene ground vapor barrier (already done in your case) it's usually substantially less money, more protective of moisture susceptible wood, and higher energy efficiency than insulating at the subfloor. It's far easier to make the foundation walls & band joists reliably air tight than the subfloor, and sealing the exterior walls of the crawlspaces reduces stack effect driven infiltration almost as much as air sealing the attic floor plane. (Sealing both is a good idea.)

      Once the foundation walls insulated there is no need to insulate under the floor for code purposes either the walls OR the floor insulation satisfies code. But it's fine to go ahead and insulate between with cheap fiber insulation, or a continuous layer of rigid foam board attached to the bottom edges of the joists, leaving the floor joists empty. Most crawlspaces don't have adequate access to bring in 4'x8' sheet goods, but cheap "contractor roll" R13s (or kraft faced R19) snugged up to the subfloor and side-stapled to the joists is a dead-easy DIY. But unless you have a radiant floor heating there isn't much point to that.

      With the walls sealed & insulated the crawl-space will not represent a very big additional heat loss, and the floor temps will stay pretty close to the conditioned space temperatures. With the subfloor insulated too the crawlspace won't drop much below the deep subsoil temp in your area as long the air sealing and insulation at the crawlspace walls are up to snuff. In most 4A locations the deep subsoil temps aren't much cooler than normal room temperatures, so the addition of floor insulation is against a much lower temperature delta than a fully vented crawlspace. See:

      1. JC72 | | #9

        IMO this approach doesn't make sense because you'll effectively turn the crawlspace into a bathtub after a flooding event. Buy insulating the floor with vapor impermeable insulation such a ccSPF any standing water remaining in the crawlspace essentially becomes a nonissue.

        Yes ccSPF is expensive but living on a floodplain is already expensive so the ultimate solution is to not live on a floodplain.

        Perhaps the best solution for the OP is just bring in a bunch of dirt and raise the grade of the house (happens quite often actually) then pour a slab. The OP can then get an elevation cert in order to avoid having to carry flood insurance. Save $$$ there because the cost of flood insurance will only go up. Unfortunately the rat slab and crawl are already built.

        1. Expert Member
          Dana Dorsett | | #11

          >"IMO this approach doesn't make sense because you'll effectively turn the crawlspace into a bathtub after a flooding event."

          No it doesn't. That's what the floodvents are designed to do. Most are designed open up when the tide comes in to relieve the pressure, but have to be manually closed. They stay open to let the water out. The amount of water in the "bathtub" at the end of the flooding event is the same as with open vents.

          >"Buy insulating the floor with vapor impermeable insulation such a ccSPF any standing water remaining in the crawlspace essentially becomes a nonissue."

          A continuous sheet of 6 mil polyethylene across the bottoms of the joists is a heluva lot cheaper than even an inch of ccSPF, and has a much lower vapor permeance too.

          It doesn't take a huge effort to bail out the standing water, and let the dehumidifier dry out the slab. Even with ccSPF encapsulating the joists & subfloor most people would still opt to bail out any standing water after a flood.

          1. Expert Member
            BILL WICHERS | | #12

            You could also easily pitch the floor towards a sump. This would let you put a small pump in the sump to pump out any standing water if needed. With a poly liner on the floor, the water would rapidly drain towards the sump and you'd have all the bulk water pumped out in short order. The dehumidifier could then deal with the small amount of remaining moisture to dry out the space.


  4. satinder | | #10

    Thank you all for your input!

  5. Expert Member
    Dana Dorsett | | #14

    >"You could also easily pitch the floor towards a sump."

    That's easily done BEFORE an (already existing) slab is poured, a bit of a PITA to do as a retrofit.

    And sumps in flood zones usually become a source of flood water during high water events rather than a drain.

    A smooth flat slab is easy enough to bail out after the tide has receded.

    1. Expert Member
      BILL WICHERS | | #15

      Ah, I missed the “already constructed” part in the original post :-) adding a slope with patching concrete would probably too much work to be worth it.

      A sump with no drain tile won’t be a source of any floor water. The idea would just be to have a lot spot where a temporary pump could be placed to pump out occasional flood water. This would be different from the usual sump pump arrangement tied into a perimeter drain that could bring in new water during a flooding event.


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