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Retrofit: Partially replacing kraft-faced fiberglass with closed-cell spray foam

greendoug | Posted in Energy Efficiency and Durability on

I’m looking for some practical feedback on whether replacing kraft-faced fiberglass with closed-cell spray foam in some exterior partitions—but not all—potentially invites future frustration.

We’re working through a partial retrofit of several rooms of a 30-year-old 2×4 framed house and the architect has proposed to replace old kraft-faced fiberglass batts with closed-cell spray foam while our exterior walls are open. We’re concerned that having facades with both closed-cell spray foam (R-24, 0.23 perms) in some areas and fiberglass (R-13, 1 perm) in other areas will create moisture challenges when we eventually replace the exterior and want to add continuous insulation on the outside.

If we have exterior walls that are part fiberglass and part closed-cell foam (but not in the same stud cavity), are we inviting condensation issues if we later add continuous insulation around the outside? Or will we just need to install thicker exterior insulation around the whole house to both preserve the proper ratio and achieve uniform wall thickness? Alternately, are we going to invite problems if we never get around to the exterior insulation project?

We’re in Chicago, zone 5A, so we get an invigorating mix of hot and humid and deep freeze. Any suggestions or alternate approaches are appreciated.

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  1. Expert Member
    BILL WICHERS | | #1

    A few things to consider here. The big one is that you're never going to get R24 in a 2x4 wall. Spray foam isn't really R7 per inch over the long, or even medium, term, and you'll never get a full 3.5" fill with closed cell spray foam anyway, since closed cell is normally installed underfilled to avoid the need for trimming (which is very difficult with closed cell foam). A much more typical installation will get you an effective average depth of not more than about 3" or so, probably closer to 2.5-2.75" AVERAGE depth. A more realistic aged R value is about R6 per inch too, which gets you around R15-R16.5 actual R value in your wall using closed cell spray foam. Don't believe any of the industry BS about "spray foam R value is better than batt R value". R value is R value, period.

    Spray foam is a waste in walls, since you don't really get much, if any, R value advantage over high density fiberglass or mineral wool batts, and you DO pay a premium for spray foam! The only "advantage" would be air sealing, but that is easily done in other ways, such as canned foam in holes and big gaps, and caulk in small gaps. If you're already planning on putting up exterior rigid foam, then it's even easier: detail that exterior rigid foam as an air barrier and your air sealing is done (still put some canned holes in wire and pipe holes though).

    I would use mineral wool batts in the walls (R15 for 2x4 walls), and polyiso on the exterior. Put the cost savings from NOT using spray foam into thicker exterior rigid foam and you'll end up with a much better performing wall assembly for your money. The reason exterior rigid foam is a big winner here is that it is continuous insulation, so the studs don't reduce the effective R value of the foam due to thermal bridging. Thermal bridging of the studs robs performance from ANY insulation used in the stud bays, BETWEEN the studs.

    If you want to split this project into stages, I would recommend mineral wool in the stud bays, and a low-cost smart vapor retarder like MemBrain against the studs behind the drywall. Detail the vapor retarder as an air barrier, then install the drywall airtight too (which is pretty much just a bead of sealant around the perimeter, which is easy to do). This will give you a good performing, moisture safe, wall until you do the exterior phase of your project and put up rigid foam. I'd try for 2" of polyiso (or more, but more than 2" starts to make exterior details more complicated), which gives you a simple whole-wall R value of about R28, or a whole-wall R effective R value (allowing for thermal bridging of 2x4 studs on 16" centers) of about R26 or so. You'll also have good air sealing if you also detail the exterior rigid foam air tight (which is also easy to do, with tape and sealant), which is a big plus for overall energy efficiency of the home.

    Spray foam is not a magic material, and is really a waste in walls. I tend to recommend spray foam only for certain niche applications, which mainly means unvented cathedral ceilings, irregular foundation walls (cut stone, etc.), and some rim joist applications. I almost never even consider it for walls.


  2. greendoug | | #2

    Thank you, Bill—extremely helpful answer. Mineral wool + MemBrain and saving for exterior polyiso in the future is a much more straightforward approach. I appreciate the pragmatism.

    1. Expert Member
      MALCOLM TAYLOR | | #3


      At any given time the Q&A here has at least one discussion on how to remediate a spray foam installation which as gone bad. It's worth being aware of the risks before committing to what appears to be a pretty problematic material.

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