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Community and Q&A

Should I Insulate The Floor of a Crawl Space

GreenBuildingAdvisee | Posted in GBA Pro Help on

I have a ~200sq foot crawlspace under a section of my home connected by an approximately 4ft by 3 foot opening to the full basement (32’x25′) which has decent ventilation and is unfinished. Currently the crawlspace is a dirt floor with poured concrete walls. The home is from 1916 and is in New York( Zone 5). I’ve been looking to decrease air leakage and increase home comfort by encapsulating the crawlspace and adding spray foam along the walls and rim joist. In the larger basement only the rim joist will be spray foamed in all areas where spray foamed I’ve requested a thermal barrier paint.

 One contractor I had bid on the job is suggesting I use thick rubber (60mil EPDM) instead of plastic on the dirt floor and then apply 2 inches of spray foam on the entire floor in addition to the walls and rim joist. I was wondering would it make sense to tell the contractor to not apply the spray foam to the floor and leave it just as the vapor barrier? Also is EPDM OK for this use? Is it better to have the foam starting about 6-12 inches up the wall overlapping over the floor vapor barrier?  The quoted price is not really different from the other quotes for plastic over the dirt with no spray foam on top. Is there any disadvantages to spray foaming over the floor if it costs the same amount?



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  1. Expert Member
    Dana Dorsett | | #1

    With poured concrete walls it's cheaper and greener to do the wall insulation with rigid polyiso or EPS. Unless the floor of the crawlspace is going to have some mold-susceptible material stored on it, there's not much point to insulating the floor. There can be a VERY long energy use rationale for as much as 2" of EPS (not polyiso) under the EPDM vapor barrier floor, but 2" of closed cell spray polyurethane foam will never (ever) "pay back" it's carbon footprint unless you're heating the place with resistance electricity supplied only by a coal fired grid.

    1lbs density foil faced polyiso has about half the CO2e footprint per R as HFO blown closed cell polyurethane, and about a quarter that of HFC blown closed cell foam.

    Using RECLAIMED rigid foam board is greener still, since it has effectively zero CO2e emissions- it only extends the benefit side of the cost/benefit equation for an environmental hit already taken (often decades ago).

    There are multiple vendors of reclaimed 2lbs roofing polyiso & EPS in NY, costing 1/4-1/3 that of virgin stock goods. There is probably a vendor near you- what's your ZIP code?

    Polyiso is fine to install on the walls, but the cut bottom edge may NOT rest on soil or it can slowly wick ground moisture. With EPS under the EPDM and the EPDM sealed to the wall, it's fine to have the polyiso in contact with the EPDM.

    EPDM is commonly used for membrane roofs and ground vapor barriers, and is one of the preferred materials for this application.

    1. kdodson6 | | #5

      I got some reclaimed polyiso but I’m looking for code is 29020.

    2. kdodson6 | | #7

      Also Dana, how would you go about sealing off a porch that is included in a crawlspace? The porch has stairs that lead to main entry door. The floorIng is staggered wood planks. Should I build a wall of sorts and tie it into the piers that line the deck/porch?

      Ultimate goal is to encapsulate the crawl fully (South Carolina) and this situation makes it impossible.

    3. GreenBuildingAdvisee | | #8

      Thank you for the reply. I have quite a bit of 2 inch fiberglass faced recycled polyiso from an attic insulation I got in bulk from ReUse Actin in Buffalo NY. I don't plan on using the crawlspace for storage so it seems best to just not insulate the dirt side.

  2. user-457184 | | #2

    The thermal barrier should be along the floor of a vented crawl space and on the foundation walls in a closed crawl space. You always want the insulation between the regions with the highest temperature differential, since heat flow is proportional to the difference in temperature. There is only slight temperature differential between the ground and the crawl space atmosphere. You would be wasting money installing insulation there.

    You don't need an EPDM vapor barrier or one with that thickness. Again, you are trying to slow moisture flux. Save the EPDM for your koi pond.

    1. Expert Member
      Dana Dorsett | | #3

      >"You don't need an EPDM vapor barrier or one with that thickness. "

      I was assuming the 60 mm (2.36" ) was a typo, and that it's actual 60 mils (0.060") which is pretty rugged and walkable compared to a sheet polyethylene solution.

      It's the "deluxe" version, to be sure.

      Insulating a crawlspace floor in a zone 5A climate is more about summertime mold mitigation than energy use. It might be "worth it" on an energy use basis for 2" of used Type VIII EPS at 15-25 cents per square foot, or even virgin-stock EPS at 75-90 cents per square foot (over the very long term), but definitely NOT for 2" of sprayed closed cell polyurethane at $2-2.50 per square foot.

      1. Eric__S | | #4

        What's the detailing need to be if you were going to put the EPS under the EPDM (or 20 mil polyethylene)? Do the EPS sheets need to be tight/taped? If there's a place where the contour of the crawlspace floor makes laying them flat a headache, what do you do?

        Not asking for a friend. Encapsulating my crawlspace this year too in Virginia.

        Thank you, Eric

      2. GreenBuildingAdvisee | | #10

        This was a typo; it is in fact 60 mils. I've edited the original post to correct this. Despite it not being necessary I like the idea of the EPDM since it allows one to be less careful of ripping the vapor barrier flooring when doing work in the crawl space. Thank you for the cost analysis that helps a lot.

    2. Expert Member
      NICK KEENAN | | #6

      This raises a really good point -- is the crawlspace vented or closed?

      In a lot of older buildings there aren't sharp divisions between conditioned and unconditioned space, attics and cellars and crawlspaces are sort of in-between. You gotta pick one or the other.

    3. GreenBuildingAdvisee | | #9

      The crawlspace as it currently exists is what I'd call 'semi-vented', opening to the leaky larger basement but not to the outside directly. I figured that by sealing the rim joist in both the larger basement and the crawlspace I'd be moving the crawlspace towards a closed environment. I've got a dehumidifier to deal with new moisture problems from reducing air flow in the basement. Your point about the temperature differential is spot on, the basement sits at around 50-55 degrees. By sealing it I'm looking to reduce stack effect; I have no intention of creating a finished basement. However, if I want the floor above the crawlspace to be warmer is there anything additional that can be done?

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