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Crawlspace wall insulation

m854 | Posted in Energy Efficiency and Durability on

I think I should insulate my crawlspace walls. Originally there was unfaced fiberglass, but it was full of mouse and rat poop, so I removed it to clean everything up. I did the rim joists with cut and cobble foam covered with Rock wool, but the walls are bare. The walls are concrete, 3-4 feet high. On the floor is a 20 mil poly vapor barrier I put down, but I still need to find a way to seal it to the walls. Maybe I’ll tuck it in behind the insulation. The walls always seem to be dry, but moisture from the soil is a problem in the summer without the vapor barrier.

There are some signs that there has been standing water in the past, but probably not above the height of the footers. The soil is clay and there’s no drainage that I can see.

My main question is about what kind of foam I should use. I am concerned about GWP, so I don’t want to use XPS or spray foam. And I only need a relatively small quantity, and I’m having a hard time finding a supplier besides the hardware store. It is a townhouse, so I have only about 140 square feet of wall area. If I do two 2-inch sheets of foam the whole way down, that’s about 9 4×8 sheets of foam. That’s about R15 with 2-inch low density EPS.

Can I use hardware store low density EPS foam for this? Can I use polyiso? Or should I seek out higher density EPS or reclaimed form?

Almost forgot to mention:
I’m in Zone 5, and I’m making this a “conditioned” crawlspace, but since I have hydronic heat the conditioning is by means of continuous exhaust ventilation and maybe a dehumidifier if humidity continues to be a problem.

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  1. Expert Member
    Dana Dorsett | | #1

    Type-I EPS is fine here, if that's what's available.

    Used fiber-faced roofing polyiso will usually be quite a bit cheaper than box store Type-I EPS when it's available, and would be reliably R15 or better @ 3" total thickness (often labeled R17 or higher.) With polyiso it's best to keep the bottom edge above the high tide mark if there is a flooding history and should never rest on a slab or soil.

    A thin bead of polyurethane caulk is gluey-enough to make the vapor barrier stick to the foundation wall. It's fine if that bead of caulk behind the vapor barrier creates a small air gap between the foam board & foundation, as long as the seams and edges of the foam board are sealed. In my own crawlspace I used a combination of caulk as adhesive for the VB followed by duct mastic over the edges as a belt & suspenders approach to keeping soil gases out.

    1. Eric__S | | #2

      Did you take the VB all the way up the wall (minus 3 inches at the top), or did you only go up 12" from the dirt? If you went all the way up, how did you attach or adhere the polyiso to the wall?

      Martin had said in an earlier post/reply set that you definitely don't need to run the VB up the wall unless your inspector says you must, since the polyiso would act as an air barrier (and vapor barrier) if well taped.

      If you don't have to run the VB up the all more than a foot, I would prefer to adhere the polyiso to the wall with PL300 foamboard adhesive, but if I have to run he VB up the wall, the correct solution then becomes Hilti IDP fastners or Rodenhouse Plasti-Grip PMF fasteners through to the block wall of the crawlspace, right?

      1. Expert Member
        Dana Dorsett | | #3

        >"Did you take the VB all the way up the wall (minus 3 inches at the top), or did you only go up 12" from the dirt?"

        In my case it was about a foot above the dirt.

        >"If you went all the way up, how did you attach or adhere the polyiso to the wall?"

        This crawlspace has 4" of XPS (yes, I know- note that it was the dumpster-dived cut remainders from somebody else's project), attached with foam board construction adhesive.

        In the main basement I have 3" reclaimed polyiso, attched to the foundation with 1x4 strapping through screwed to the foundation with masonry screws, using the strapping to hang the gypsum board thermal barrier. That approach would work for a crawl space with a full-height vapor barrier too.

        Hilti IDP or Rodenhouse PMF fasteners make slick work of hanging the foam, but don't provide a nailer for installing gypsum board. You may be able to get away with using foam board construction adhesive for the gypsum board. (Some inspectors might balk without mechanical fasteners.)

        1. Eric__S | | #4

          Thank you Dana, that makes a lot of sense.

          I posted a question regarding my situation, about whether there's a penalty for or a condensation risk in omitting some of the rigid on basement / crawlspace walls ( Did you insulate the shared wall between your crawl and basement? If you did, on the basement side, did you insulate all the way up the wall, or only the bottom portion where there's crawlspace's earth behind the wall?


          1. Expert Member
            Dana Dorsett | | #5

            >"Did you insulate the shared wall between your crawl and basement? "

            There is a 1.5-2" air gap between the foundation and a beam for most of the common wall- it was never truly walled off. I had to hack out an access port in that section of foundation/retaining wall to gain access to the crawlspace. The foundation walls in the main basement have 3" of polyiso over it that stops about a foot beyond where the crawlspace begins. Yes, the penetrating foundation wall is a pretty big thermal bridge, but there are still bigger thermal fish to fry in this house before going after that one.

    2. m854 | | #8


      What kind of polyurethane caulk do you use? I can't find anything advertised as "polyurethane caulk" here. The vapor barrier I'm using is 20 mil plastic, so it's a little stiff. I think it's polyethylene which nothing special but notoriously hard to stick to. I set up an experiment with a few adhesives I had on hand or could buy in small quantities. Some of them work OK, but none of them I would consider to be a permanent bond. So far the polyurethane based products are the worst.

      Here are the results of an experiment with a few different adhesives gluing scraps of vapor barrier to a 2x4. The bond between the vapor barrier and the adhesive is always the weak link so far.
      Loctite PL Premium: Plastic peeled off by itself while drying
      Great Stuff foam: No adhesion
      E6000: Moderate adhesion. Needed to be clamped since it wrinkled the plastic
      Roof Pro "tripolymer sealant": Moderate adhesion, still not fully dry
      Loctite PL530 mirror adhesive: Still not dry
      Water based duct mastic: Still not dry
      DAP Weldwood contact cement: Moderate adhesion. No gap filling ability.

      I did one wall with QuietSeal Pro acoustic sealant (available from Lowes) a while back. I'll have to check on that and see how well it's holding up. I almost wonder if a tape would work better than an an adhesive, if there are any tapes that stick well to concrete. Flame treating the plastic sounds promising in theory but too much work in practice. And using heat to weld it to something else might work too, but again probably not worth the effort.

      I'll probably just end up using a pressure treated board fastened into the concrete to hold the vapor barrier in place.

  2. Eric__S | | #6

    Thanks, Dana, that makes sense. Cheers to bigger fish to fry! For me the bigger fish that I probably should be frying is air sealing the attic and adding insulation, but since I'm prepping to finish the basement and encapsulate the crawlspace soon, I'm trying to square away the final state of insulation now.

    Have a nice rest of the week.

    p.s. Matt V sorry if I hijacked your thread with my questions!

    1. m854 | | #7

      No problem. Its helpful discussion either way! I'm sure I'll have more questions as I get into the details.

      I just air sealed and insulated my attic, so the crawlspace is probably the "next largest fish".

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