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Pier-and-Beam Construction: Insulating the Floor

jamesboris | Posted in Energy Efficiency and Durability on

Finalizing floor insulation details in Zone 2A, pier and beam, totally open crawlspace. Rim joists were insulated during framing. I have a lot of 3″ reclaimed felt-faced polyiso on hand and plan to cut&cobble it in the joist bays (w/thermal bridging, that’s a total assembly R-value of 14.1, code-compliant… and based on a Manual J that showed almost zero return beyond this thickness).

I need to air seal it, but no sealant or tape sticks reliably to this material. Many articles on GBA, like this one, recommend putting a continuous layer of rigid foam or plywood on the bottom of the joists. I don’t want to do that because it will prevent me from inspecting the joist bays for critters, leaks, mold, etc.

What if I ran a layer of *clear* 10-mil poly sheeting under the joists instead? Would the air gap between the foam and the sheeting be a problem? If so, I could instead run a strip of poly sheeting in each bay…

(Yes, it’s a vapor barrier, but so is the polyiso (to say nothing of the tile above the subfloor… which is a double-vapor-barrier problem I don’t know how to get around…).)

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  1. Expert Member
    Michael Maines | | #1

    James, have you tried Siga Fentrim? I bet it would stick to the polyiso felt facer.

    The plywood (or similar) layer is mainly to keep critters from getting in. Poly probably wouldn't cause any problems at that location but it also wouldn't provide any benefit that I can think of.

    1. jamesboris | | #2

      Hi Michael, thanks for commenting - I've learned a lot from your posts all over the place! I did try Wigluv and was underwhelmed... 2 benefits of the poly I thought of:
      1) Even if Fentrim works, it'll cost about 90c/ft... that's ~$900 of tape. Poly would run about $60, and maybe $150 for all the tape.
      2) Poly would air seal the joists themselves. My joists do have some surface mold already, though they've never seen a drop of rain. I was thinking I might hit it with 80-grit + bleach beforehand. I suppose I could use Membrain if vapor diffusion is a concern, though people just as often recommend exterior vapor barriers in Zone 2A.

      1. Expert Member
        Michael Maines | | #4

        You're welcome! An important part of the cut-and-cobble approach is to leave space around the perimeter of each cavity and seal them with canned foam. It's not as reliable an air barrier as some other options but you're right, Fentrim would get expensive. (As sticky as Wigluv is, Fentrim seems twice as sticky.)

        Your vapor drive in zone 2A is going to almost always be from exterior to interior, and weakly at that. Whatever vapor gets as far as the underside of the foam will be pretty well blocked from going further. My main concern would be whether enough moisture could accumulate to promote mold and rot at the exposed portion of the joists. A vapor-variable membrane would be safer. But I have no experience in your climate zone so I'm not sure how much of a concern it really is.

        1. jamesboris | | #5

          But wouldn't the polyethylene obviate the need for canned foam? Why seal around the edges if I can just friction fit the polyiso, and have my air barrier via the plastic? I hate working with canned foam overhead, and the movement of the framing will probably work the seal loose in time. This is really why I thought of the poly in the first place... poly taped on the perimeter is gonna stay taped.

          1. Expert Member
            Michael Maines | | #6

            It might matter more in cold climates, which have a greater temperature differential. It's nearly impossible to get a perfect friction fit, so you will have air leaks from the exterior to your floor sheathing. If the building interior is air-conditioned, that air will condense on the cool floor sheathing and lead to rot. The poly will slow the air and moisture but you really want the insulation to be continuous within the joist bay. I agree about using spray foam overhead, and would not recommend this approach in the first place.

        2. jlipkowitz | | #10

          Hi Michael,

          I have a related question. I am in midcoast Maine and have recently constructed a round cabin on piers. The cabin was from a kit, so the r-values were all baked in due to material choice other than the floor assembly. Walls are R 18 and Cieling is R 21.

          The floor assembly is supported on piers at the exterior and a steel ring in the center. The floor is framed with 2x6 PT joists, spaced 48" at the exterior of the house and about 6" at the central steel support ring. Essentially they create large pie shaped wedges between the joist bays. The subfloor is 3/4" ply.

          My conundrum is figuring out the best way to insulate this setup. I have about 2' between the bottom of the joists and grade.

          A few potential options:

          A. Wrap the underside of the joists with 1" EPS and tape the seams, protect it from the underside with plywood, and then blow in cellulose from the top.

          With the subfloor already being on, I would drill through the subfloor from the top to insert the hopper hose to blow the cellulose. My concern is that the pie wedge shapes would make the cellulose have a hard time filling the whole cavity and I would have to drill multiple holes per joist bay for that to work. It also involves the labor of attaching the foam/plywood under the structure in a tight space and the cutting difficulties of applying square sheets to a round structure.

          B. Hire a spray foamer to foam from underneath - probably 3" of foam and then apply 1" along the remainder of the side and bottom of the joist for thermal bridging.

          A few questions - GHG potential aside for the closed cell (I'd have the soy-based applied at least, but I realize it is a high-impact material choice)- Is there a reason from a building science standpoint to not apply the spray foam?

          My concern is that puts the vapor barrier on the outside of the assembly. Would this be problematic? If I went this route, would I want to make sure my finished floor wasn't acting as a vapor barrier so any moisture that did make its way into the floor assembly could then dry to the inside? Would the spray foam be a reasonable route and if so, what else do I need to keep in mind to make sure the performance and moisture issues are managed?

          Thanks and best wishes,


  2. Expert Member
    Akos | | #3

    My understanding is the issue in warm climate is the thermal birding of the floor joists. The joists are cooled by the interior so the exposed edges could be pretty close to dewpoint most of the time. This is probably why you are already seeing mold. Poly won't help much with this.

    Really the best solution is a continuous layer of rigid, doesn't have to be too thick either just enough for condensation control, under the joists to prevent this. If you go with something like GPS or unfaced low gwp XPS, it is easy to tape the seams with any standard tape.

    The fiber faced roofing polyiso is actually somewhat permeable so it should not create a moisture trap.

    This is a good read about this:

    1. jamesboris | | #7

      The space isn't yet conditioned, so I'm not seeing mold due to thermal bridging. But you're right that that's an issue... wouldn't the poly sheeting prevent it by shielding the cool joists from warm outdoor air (i.e. the source of the condensation)? Also, isn't the robust solution to this issue just... keep your AC setpoint above the outdoor dewpoint?

      I already have a boat load of fiber faced polyiso. My readings here have led me to believe that the permeance of 3" of fiber-faced polyiso is negligible. Martin: "Here's the short version: If the rigid foam is of useful thickness, don't expect much exterior drying."

  3. jamesboris | | #8

    Membrain would seem to be more dangerous than poly in Zone 2A -- the pores will open up when faced with our 90% humidity days, allowing warm air into the space. I'd think the solution to that though, is cooling above the dew point.

  4. jamesboris | | #9


    "It's nearly impossible to get a perfect friction fit, so you will have air leaks from the exterior to your floor sheathing... The poly will slow the air and moisture but you really want the insulation to be continuous within the joist bay"

    Right, because the friction fit is imperfect, the poly-sheeting is the air barrier, not the polyiso. 10-mil plastic taped with Zip should totally stop that air and moisture, shouldn't it? At least much more reliably than canned foam... right? Then factor in that the AC setpoint is above the outdoor dew point.

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