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Help with spray foam code – prescriptive vs. performance

eagleeyeshawk | Posted in General Questions on

Hi all,

Thanks for reading. We are building a home in climate zone 4; Nashville, TN

The spray foam quote on the exterior walls only calls for 4 inches of OC foam. Our exterior walls are 2×6 Tstuds. The insulation contractor said they are meeting code with 4 inches. I mentioned that for a 2×6 wall Rvalues are in the range of 19-21 and this would fall below code minimums. They noted they are not following prescriptive code, but rather performance code (chapter 11, 2015/2018 IRC, Simulated Performance Alternative, section 1105, R405). They said this bit of code allows them to only put in 4 inches of OC on our 2×6 Tstuds.

Also, they said specifically tha foam is very different insulation from fiberglass, mineral wool, and cellulose, in that it is effective against conductive, convective, and radiant heat transfer, using a strict R value comparison between the diff insulation materials is comparing apples to watermelons. Does this sound right?

After reading through the section, I didn’t find a specific statement regarding spray foam allowing them to only put in 4 inches in our climate zone.


1. My preference is to fill the entire cavity. Am I just throwing away money as the insulation contractors are saying?

2. We want to use these guys as they are only guys in town using a water blown CC foam.

3. Is Open cell foam typically water blown? If not, is it as damaging as closed cell in terms of global warming potential?

Thanks for taking a look. I really appreciate it.

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  1. Expert Member
    ARMANDO COBO | | #1

    It’s not quite that simple nor all true. Most insulators like to mention that under the performance code is ok to install less than minimum code required insulation under the prescriptive code, all by itself and just like snapping the finger. He does NOT get to make that decision.
    The reality is that you need to decide IF you want to apply for a Building Permit under the Performance or the Prescriptive code. Once that happens, you, your Builder or Designer can use REScheck to prepare the Report, since is one easy method to show compliance with the IECC code. Professionally, a HERS Rater does the calculations using a proprietary program call REM Rate, which does the same thing.
    The biggest problem I see with the Performance code is that many folks in the industry use it to cheat. That can put clients homes at risk since condensation control doesn’t care what code your permit is under, and installing less that required insulation can create bad issues, especially if you live in cold and humid climates, and your building envelope is flawed. Physics beats chemistry every time!
    Best case scenario, your insulator is trying to be price competitive with other insulators… wink, wink!
    FWIW, I usually specify dense packed cellulose. Better performance for less money and less impact on the environment.

    1. eagleeyeshawk | | #6


      Thank you for the suggestion on dense packed cellulose. We will take a look at that.

      Are there any problems to be aware of with cellulose? I recall a GBA article some time ago that mentioned that drywall needs to happen 24 hours afterwards or the moisture of the house can get absorbed by the drywall.

      I think mainly I am worried (as I was really worried with spray foam) about a possible smell issue.

      Thanks for your response on the use of performance code and how it differs from the normal prescriptive code. It’s good to know it’s kind of a thing that happens at the planning/designing phase rather than mid build.

    2. Expert Member
      ARMANDO COBO | | #7

      Dense-packed cellulose is installed dry through a stapled netting or mesh to the studs and plates. There’s no drying to wait for. Installation should have a density between 3.5 - 4 lbs/ft³ to avoid settling, should be rolled to "flatten" the bulging, and is best done by a professional.
      There is plenty of good info on the web, in articles and videos to learn about.

  2. Expert Member
    Michael Maines | | #2

    Spray foam manufacturers and salespeople are notorious for shady marketing. All insulation is tested for a combination of convective, conductive and radiative heat flow. Martin wrote about it here: Foam insulation is more airtight than other insulation types but it's not hard to make any assembly airtight, so it really shouldn't be a primary factor in decision making.

    In climate zone 4, the 2018 IRC/IECC calls for U-0.06 in framed walls, or about R-16.7 once the framing is factored in. Most open cell foam is rated at about R-3.5/in. There is little thermal bridging with T-studs; ignoring them completely, the math says you need at least 4.75" of R-3.5/in insulation. Modeling would probably show closer to 5" being required with the T-studs taken into account.

    If they are proposing a performance path, they need to show the total energy usage for the entire house, not just a single component such as a wall.

    Whether it's worth it to fill the cavity depends on your goals. There is a return on financial investment for the energy savings, potential to downsize mechanical equipment, comfort and indoor air quality considerations, and impact on health and environment. The ROI is different on every project.

    As far as I know, all open cell foam is water-blown. There is still a negative environmental impact due to the up-front carbon emissions of the plastic resin, and potential health risks if it doesn't cure properly, but it's generally safer and lower-impact than closed-cell foam.

    The only closed-cell foam I'm aware of that is water-blown has a relatively low density and R-value. It's somewhere between other closed-cell foams and open-cell foam.

  3. Expert Member
    PETER G ENGLE PE | | #3

    With OC foam in CZ4, you will also require an interior Class II vapor retarder or a "smart" membrane system.

    1. brendanalbano | | #5

      Unless I'm missing something, IRC 2018 R702.7 only requires an class I or II vapor retarder in marine zone 4. I don't believe one is required by the IRC in regular zone 4.

      And in marine zone 4, you can do a class III vapor retarder (e.g. latex paint) if you have vented cladding (a rain screen).

  4. Expert Member
    BILL WICHERS | | #4

    Spray foam R value is the same as the R value of other insulation materials (mineral wool, fiberglass, cellulose, etc.). That's the whole point of "R value" -- it gives a comparison of heat flow through a material using a standardized test methodology. Spray foam is no different than anything else in that regard. Where spray foam IS different is that it doubles as an air barrier, if it's installed properly in a thick enough layer. That air sealing does help overall building energy efficiency, but it doesn't change the R value of the material, and there are other ways to air seal besides spray foam.

    If you're putting in open cell spray foam, convention is to fill the entire cavity. That would be a full 5.5" layer, trimmed flush with the inside edges of the studs. Your contractor is probably trying to underfill to avoid the labor of the trimming step. What is normally done is to OVERfill the cavities, THEN trim flush, to ensure that the finished produce is a flat, even surface flush with the edges of the studs. That means extra material that gets trimmed off and trashed, along with the labor to do the trimming. Closed cell spray foam is usually underfilled, because it's very difficult to trim.

    If you do go with spray foam in your walls, and you use open cell, you do want it overfilled and then trimmed flush. This is the industry standard practice, and it's what your contractor SHOULD be doing. That said though, is there any particular reason you want spray foam in your walls? There are other ways to insulate walls that will perform just as well, and usually will be cheaper too. My own preference is to air seal the "old fashioned way", with caulk and canned foam (which is easy and relatively quick to do), and then use mineral wool batts in the walls. I only really recommend spray foam for niche applications such as unvented cathedral ceilings and irregular foundation walls (cut stone). In most other applications, there are cheaper options that are often at least as good as spray foam. Note that the mineral wool option I mentioned will perform at least as well as spray foam in terms of R value, for example.


  5. walta100 | | #8

    What is your goal for wall insulation in terms of R value and what is your local code requirement you are trying to meet?

    Consider filling the walls with damp sprayed cellulose and adding an inch of exterior ridged foam.

    Spray foam is the most expensive way to get an R of insulation. Yes, you are wasting money any time you buy the stuff. In most situations there is a less expensive way to get the insulation you want.

    Don’t be fooled by the DIY TV shows spray foam is not the mark of a quality job. My guess is every show feature spray foam because of a marketing program supplying it at little or no cost. I see spray foam in a set of construction plans as a red flag for laziness and total disregard for your budget.


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