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Community and Q&A

I Aerobarrier and spray foam overkill?

88Clayton | Posted in Energy Efficiency and Durability on

I’m getting some competitive quotes on spray foaming my house walls, band joists, and roof deck, etc.

I also got an acceptable quote for Aerobarrier.  And would definitely go this route if I used traditional batts in walls.   Not only would I save money over using foam insulation in walls (open cell) but I’d save a significant amount on air sealing costs — especially if I had somebody other than myself do the sealing.  These savings would pay for the majority of Aerobarrier and allow me to go down to 1-ACH50.

The foam insulation contractors are saying Aerobarrier is a waste of money if they foam it according to their proposals.

The framers did a really rough job on my house so the effectiveness of Aerobarrier would help mitigate so many of these flaws that go with careless framing.  But foam would also tighten up a lot of this and seal.

Since they botched my Zip-R sheathing and compromised it’s shear strength, the idea of 2” closed cell in my walls seems like a great idea instead of the open cell.  This house is replete with shiners and overdriven nails.

So, in closing, what insulation combo might you guys recommend and is spray foam in the aforementioned locations PLUS Aerobarrier overkill?  Perhaps Aerobarrier is best suited for traditionally insulted houses.

I’d like my house to get down to 1-2 ACH50.

Aerobarrier will only take my house below 3-ACH50 if I have supplemental ventilation, but it’s the same cost even if they take it down to 1.0 on blower door.

Additional details:
2×6” studs
24” o.c. (except basement)
Zip-R6
All nails taped
Roof deck, band joists, and floor joists below 2nd floor knee walls to be foamed for sure.

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Replies

  1. Expert Member
    Peter Engle | | #1

    I agree that aerobarrier + SPF is probably overkill. Aerobarrier is so new that I don't completely trust it yet but i like what I've seen. So far, I know nothing but what they've shown in their advertising. Their adoption by some pretty serious big builders is an encouraging sign. Aerobarrier has a big advantage over SPF in a badly framed house, IMO. It is very hard to seal all of the wood-wood seams in standard construction, and the SPF people are typically not very good at this either. These seams can add up to a LOT of leakage. Aerobarrier finds all of those leaks and seals them. It would also let you substitute a greener insulation for the SPF. However, I still know nothing about the environmental costs of aerobarrier: manufacturing, installation, long-term, etc.

    1. Patrick OSullivan | | #6

      The CEO of AeroBarrier, Amit Gupta, recently appeared on The Building Science Podcast: https://positiveenergy.pro/building-science-podcast/2019/5/6/fix-a-flat-for-houses

      Arguably, you could call whatever he says “advertising” as well, but I found it to be very informative and it answered many questions I had about the product and process.

  2. Expert Member
    Dana Dorsett | | #2

    Aerobarrier can be used to take it down to any arbitrarily low ventilation, not just 3ACH/50. Give it enough time you can take that to 0.3ACH/50 (half the Passive House limit). If they guarantee 1ACH/50 for the same money as 3ACH/50, take it.

    If Aerobarrier + fiber insulation is cheaper than foam insulation (with no air tightness spec guarantee) it's sort of a no-brainer. Foam cavity fill only seals the the cavities where it's applied. Aerobarrier seals the seam under the bottom plate, penetrations of the band joist & foundation the ceiling/attic and every thing all on one go, every possible exfiltration path is treated.

    With R6 ZIP-R and R20 cavity fill there is sufficient dew point control for climate zone 5 or lower, but not zone 6 or higher.

    Adding 2" of closed cell foam in the cavities is the opposite of "...a great idea...". It buys almost nothing other than additional dew point margin. If shear strength is the goal, interior side steel T-bracing, steel flat bracing, or shear panels are all cheaper and far more effective (and greener) than 2" of 2lb foam.

    https://www.finehomebuilding.com/2011/05/19/4-options-for-shear-bracing-foam-sheathed-walls

    Steel flat strapping is easier to install, cheap, and doesn't increase thermal bridging, not that T-bracing adds a lot of thermal bridge. I has to be installed in big-X to get the full benefit, since it has almost no compressive strength. (T-bracing and L-bracing have some compressive strength.)

    Steel bracing can be engineered to be the ONLY shear strength support, but since this is only being done to make up for the hit in shear strength from the over-driven sheathing panel fasteners there's no real point do doing a full-on structural analysis. Even a half-assed strapping approach in conjunction with the poorly installed exterior sheathing will likely come in stronger than perfectly installed ZIP-R.

    If dew point margin is part of the issue, even adding an interior side smart vapor retarder would still bring come in cheaper.

    The structural capacity of closed cell foam is one of the aspects about closed cell foam than is often misunderstood. The shear strength of 2" of 2lb foam is less than that of wallboard. At 3" it can be useful as a glue holding the structural panels to the framing, enough to matter when considering uplift forces on roofs in hurricane zones, but that's not the issue here. And in the case of ZIP-R that's completely out the window, since it's only adhering to the facer of polyiso layer, not the structural OSB.

    https://www.retrofoamofmichigan.com/blog/spray-foam-insulation-misconceptions

    1. 88Clayton | | #3

      Makes sense. I’m CZ4. I like the strapping idea. Was already thinking about using it around windows and door framing. Good point about the foam ultimately being what the 3” of closed cell would “glue” to. One of the foam contractors is telling me 2” of closed cell will strengthen my house.

      Please see attached picture for what kind of strapping I had in mind.

      When I said closed cell seemed like a “great idea” I meant in comparison to the open cell foam every foam insulation contractor is proposing for my walls. It would offer me nothing. These contractors are quoting me 3.5” of open cell in my 2x6 wall. Not very impressive.

      I like the idea of superb sealing with Aerobarrier plus R-19 batts installed at Grade 1 standard. Although, this will probably be $1000 more expensive than the proposed open cell foam in walls plus standard air sealing. Unfortunately it’s not looking like an even swap on price.

      1. Expert Member
        Dana Dorsett | | #4

        >"One of the foam contractors is telling me 2” of closed cell will strengthen my house."

        Foam contractors come up with all sorts of dubious statements. Some will even tell you 2" is all you need, period, forget what the code says. (As it happens, 2" is the amount of HFC blown foam that can be safely sprayed in a single pass, requiring a cooling/curing period between lifts. Funny how that just happens to be the most you ever need. :-) )

        In fact 2" would of course add SOME strength to your house- more if it were ZIP rather than ZIP-R, but not enough to make it worth the premium, and not even the type of structure you're seeking. In hurricane zones 3"-3.5" became something of a standard since that's actually been third party tested in roofing assemblies multiple times, and it's what fits in 2x4 framed walls.

        This article (written by a spray polyurethane vendor) even cites how much racking strength as little 1.5" of closed cell foam adds to a (non structural) polyiso sheathed wall.

        http://rci-online.org/wp-content/uploads/2013-12-shidaker.pdf

        I'm sure 1.5-2" of ccSPF would add a meaningful amount of structure to a tent too, but so what?

        1. 88Clayton | | #5

          Haha. Great stuff.

  3. GBA Editor
    Peter Yost | | #7

    BSC did some research on hybrid wall assemblies some years ago, including the structural shear contribution of closed cell spray foam: https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/hybrid_wall_assemblies.pdf.

    One interesting note: if you are using fiberglass batt with AeroBarrier, you either do the AeroBarrier before cavity fill or after the drywall goes up...

    Peter

    1. Expert Member
      Dana Dorsett | | #9

      Note that in that BSC research that while the 1.5" of closed cell foam tripled the shear strength of the assembly compared to only polyiso for sheathing, but that's also a fairly weak starting point of reference. I suspect ZIP-R with somewhat overdriven fasteners is already more than 3x as strong as a polyiso-only sheathed wall with metal X-strapping due to the rigidity of the OSB relative to sheet polyiso + metal strapping, and the greater number of fastener points on each stud compared to structural metal strapping.

  4. Andrew C | | #8

    @Peter wrt Aerobarrier sequencing -
    I'm assuming the same is true for mineral wool batts as well as fiberglass batts...
    If your air barrier is external, e.g. taped plywood or Zip sheathing, I would think that you'd want to do the Aerobarrier prior to cavity fill. If you're not doing "air tight drywall", what's the advantage of waiting?

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