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Thermal bridging and closed cell spray foam

Trouble Shooter | Posted in General Questions on

Let’s say I put 2″ of closed cell in a 2×6 wall cavity. That leaves  a lot of wood exposed. How much does that exposed wood decrease my total wall R value?
as compared to filling the rest of the cavity with fiberglass?
or filling the entire cavity with closed cell foam?

Thanks

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Replies

  1. Expert Member
    Zephyr7 | | #1

    It's not a huge hit in terms of overall R value, but you can sometimes see the exposed wood acting as a condensing surface which can make any moisture problems worse. I would fill the rest of the wall with either batts or open cell spray foam so that there is no air space and no exposed stud surface.

    Usually spray foam isn't really necassary in a wall. The thermal bridging reduces your whole-wall performance, and you end up with only a little R value gain with spray foam compared to batts. I usually recommend using batts and exterior rigid foam, which will perform better than spray foam (the exterior rigid foam is continuos insulation, so it helps with thermal bridging), and it often costs about the same or even less than spray foam.

    Spray foam is best in the niche applications where it's the only good option, and walls aren't one of those places.

    Bill

    1. Trouble Shooter | | #5

      It's a barn in Southern Colorado that's already built. The walls are 2x6 covered with OSB and then steel siding. The roof is 2x4 trusses 16" OC with OSB and metal roofing.
      I could use fiberglass in the walls (do that myself) and then have 4" of closed cell sprayed on the bottom of the roof deck to encapsulate the 2x4" chord.
      Thoughts?

      Also thoughts on air leakage in a fiberglass wall vs foam?

      Thanks

      1. Expert Member
        Dana Dorsett | | #7

        >" ..4" of closed cell sprayed on the bottom of the roof deck to encapsulate the 2x4" chord."

        With the 2x4 top chords are FULLY encapsulated in 4" of foam (~R26-R28) on all sides (including the bottom edge/side facing the attic), even after adding in the air films and roofing/decking R values you'd still be WELL below the U0.026 IRC code-max performance for a zone 4+ location. To be clear, 1/U0.026 = R38.5 "whole-assembly", whereas with the full 4" encapsulation you'd be at best around R30 whole-assembly (= U0.033). See:

        https://up.codes/viewer/nevada/irc-2018/chapter/11/re-energy-efficiency#N1102.1.4

        >"Also thoughts on air leakage in a fiberglass wall vs foam?"

        An OSB sheathed wall can be detailed sufficiently tight to make fiberglass work, usually for a LOT less than 4" of closed cell foam. A 4" cavity fill would slightly underperform a 2x6/R20 code min cellulose or fiberglass wall for zone 5, despite the higher than code center-cavity R, all due to the shorter 4" (instead of 5.5") of the far more thermally conductive framing wood.

        Foam cavity insulation isn't some "auto-magic air sealer: either. While it air seals the cavities pretty well (there are installers that can screw that up too), it doesn't air seal the seams between doubled up framing (top plates, jack studs, etc) or the seam between the bottom plates & subfloor, all of which need to be caulked with a suitable sealant. (Polyurethane caulk is a pretty good general purpose sealant, though there are some purpose-made very low expansion can-foams that work too.) The effective cross sectional area of those seams in doubled up framing are much larger than they look. IIRC the folks at Building Science Corp model the framing seam leakage to be bigger than a perfectly smooth & straight 1/16" gap.

        1. Trouble Shooter | | #8

          Thanks Dana,

          Its my barn (shop) not house so I don't need to put code max in it. We plan to only stay there from April to October but when there want a warm place to work. Being a semi desert climate the nights do get cool there (Westcliffe Zone 6A)
          I think 4" of foam on the bottom of the roof deck and then R21 fiberglass in the walls should be sufficient. I will caulk every crack I can find. I have a nice Milwaukee cordless caulking gun that works great. I plan on keeping the shop at 50 degrees with either a radiant tube heater or forced air heater and then using wood heat while there.

  2. Jon R | | #2

    As I recall, filled or not, you lose about 1/3 of the R value of CC foam and about 1/6 of the R value of other fill insulation. You could use THERM for more exact answers.

    Use continuous insulation (interior or exterior) wherever possible.

    1. Expert Member
      Dana Dorsett | | #4

      Yup- the simple 2-D model is "...good 'nuff for th' kind o' girlz I go with." :-)

      While you'll get higher accuracy with a THERM model, the difference isn't particularly meaningful in practical terms. The fundamental problem is still that 25% of the face area of the wall is providing an ~R2.5-ish thermal bridge through the wall area that is ~R13-ish foam.

      A THERM model would just show that it's actually slightly worse than the 2-D model due to the excess amount of stud exposed to the indoor temperatures, increasing the surface area for soaking up heat. Since the exposed stud itself has at least some R-value the hit isn't as bad as with metal studs, where the conductivity is orders of magnitude higher.

      Filling the rest of the stud bay with R3.5/inch fiberglass puts more R between the excess stud and surfaces exposed to the indoor temperatures reducing the thermal bridging by a substantial amount.
      In the 2x6 case filling in the remaining 3.5" with R13 fiberglass or cellulose the R-value of the cavity area is doubled, cutting the insulated area losses by half, while R-value of the framing fraction goes from ~R2.5 to (~R1.25/inch x 5.5"=) ~R7, cutting the framing losses by MORE than half.

      So filling the remaining 3.5" of cavity depth with the cheap stuff more than doubles the performance of the wall compared to just 2" of closed cell foam.

      1. Trouble Shooter | | #6

        Thanks Dana, if you don't mind take a look at my reply #5 and comment there.

        Thanks again

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