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Thick Open-Celled Spray Foam in Cathedral Ceiling

marc_m | Posted in Energy Efficiency and Durability on

Having over the years read and re-read the various articles about cathedral ceiling insulation (e.g. Five Cathedral Ceilings That Work”), along with comments sections, I’d like some guidance to this question: in Climate Zone 4A in the Blue Ridge mountains of Virginia, at elevations of 1800′ to 2200′, which doesn’t typically experience long stretches of high humidity, very high temps, or very low temps, and also can have fairly significant diurnal swings in temps and humidity, why wouldn’t really thick OC (i.e. 13″-14″ to get to R-49) foam work, using 14″ TJI joists? (Shingled roof.)

I haven’t been able to glean an answer to this specific question from these various articles/resources and also don’t have hours to spend wading through the mounds of articles trying to get this question answered. A builder tells me this is the most cost-effective path to building a cathedral roof and, moreover, has done remodels on homes with even thinner OC and has not seen any roof rot or moisture problems. He wants to go with this method on a current project for which I specified flash and batt (No. 5 Method in the above cited article).

So, can anyone provide some links to specific building science data and articles addressing this specific question? Thanks.

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  1. Expert Member
  2. marc_m | | #2

    Thanks, Malcolm. Those are among the other articles I've read, but again, they don't seem to answer my specific question. Your fist link is about attics primarily, not cathedral. The scond one as well, if I recall.

    I'm not savvy enough on the finer points of building science, like dewpoint calculations, but it seems to me that with this thick of foam I don't understand how the sheathing could get damp to begin with, given also the potential to dry to the interior, except for roof leaks (which I did see addressed—and which seems to me problematic for most cathedral ceilings in terms of being able to discover this, unlike with ventilated attics).

    1. Expert Member
      MALCOLM TAYLOR | | #3


      I'll take a stab at this, although it isn't something I've given much thought to as I try and design foam out of assemblies, and only use it where there are no good alternatives.

      The main problem with open cell foam is its permeability, which can allow moist, warm air to move though it, accumulate, and cause the roof sheathing to rot. I can't see any significant differences between conditioned attics and cathedral ceilings which would make the latter any safer, as neither have interior vapour-retarders.

      My understanding is that open-cell foam is used quite extensively in some regions to insulate roofs, and that the majority have no issues, but the ones that do would not have had problems had they used closed cell foam.

      One small caution. After some poor outcomes early in my career, I don't take any advice from trades solely based on their anecdotal experience - especially for things where the results are so important.

      Again, hopefully someone with more direct experience will weigh in.

  3. marc_m | | #4

    Your caution first: this is the reason I am posting here! :-) I was one of the unfortunate naifs who early on in the use of foam (and by "early on" I refer to the resurgence of its use; not the 70s-80s) bought the line about "effective R-value" and subsequently had to buy a roof rake to minimize ice dams. Fortunately my metal roof works well enough to keep eave problems at bay. I do believe that our particular climate lends itself to avoiding the roof rot problem. But again, and as you say, this is anecdotal, which again is why I'd love to see some sound building science which addresses this, one way or the other.

    Anyone? Anyone?

    I'll add here a comment (or question) which no doubt has been expressed countless times, and which I suppose undergirds the Pretty Good House concept. As much as I as a residential designer (that's what I do) would prefer to spec systems which work really well (note again that I'm being cautious about what the builder is proposing), most of these systems seem to be unreachable for the vast majority of my clientele, esp. now that building costs in our area are on average 50% more than they were pre-COVID. "Unreachable" refers of course to budget, but also is so because of the dynamics of the building industry in our area. I can and do educate my clients—the vast majority are not wealthy—but the budget choices presented by the builders trump the education 99 times out of 100. And in competitive bid situations a builder is not going to push the more expensive systems (as most of these are) at risk of losing the job. Most of the builders around here don't want to trouble themselves with better techniques. I try to baby-step them along with things they might be comfortable with, and even then there's resistance, mostly seen once I'm out of the picture and they're working with the homeowners. Most of my clients don't care to spend more on me for construction observation. An example of baby steps is crinkle building wrap for a nominal drainage plane. I spec it and now tell the clients to look for it, because many builders either miss or completely ignore the requirement and use whatever wrap is least expensive, or what they're used to building with. GBA readers and contributors are clearly a small minority, many seemingly with deep pockets who are willing to pay for the better techniques. The majority of my clients are not, and I would hazard to guess the vast majority of folks couldn't care less. So...

    With that dynamic in play, if you were to design the foam out of a relatively straightforward cathedral ceiling and keep the construction simple and PGH, what would you do to achieve R-49? Vented 16" TJIs with batts?

    1. Expert Member
      MALCOLM TAYLOR | | #6


      A vented assembly with TJIs or trusses.

      My practice is much like yours. Once the drawings are complete, the primary relationship is between the builder and client. With that in mind, I try and use assemblies and details that are familiar to them or similar to those they regularly use. The main driver for improvements in the envelope is our building code here in BC, which fortunately is very progressive. A code minimum build here yields a pretty good house.

    2. Expert Member
      Michael Maines | | #9

      A slightly different perspective, as a home designer: since I decided to focus on science-based building performance and comfort, I could not be any busier with work. I have learned that it's by far the easiest to work with like-minded builders; fortunately I have several of those and we refer work back and forth. They are also always busy. I still occasionally end up with a builder who wants to argue about details but I do my best to avoid them.

      Part of our goal with the PGH book and promotions is to make these things more mainstream, so they don't seem so "niche." Better building through peer pressure, you might say.

  4. Expert Member
    ARMANDO COBO | | #5

    "There is no worse blind man than the one who doesn't want to see. There is no worse deaf man than the want who doesn't want to hear. And there is no worse madman than the one who doesn't want to understand."

  5. marc_m | | #7

    Thanks, Malcolm. "I try and use assemblies and details that are familiar to them or similar to those they regularly use." That's my point. And thank you, Armando! That is an EASY baby step which comports with this approach, and is the kind of documentation which will aid in that effort. Of course I note Lstiburek saying, "Pretty sure about Climate Zone 4, but need more work to be sure." Do you happen to know off the top of your head whether there has been any follow-up? BSC, being a consulting firm, clearly states on their website it would be next to impossible to get that question quickly and easily answered.

  6. marc_m | | #8

    But then there's this: I note this is from long-time GBA contributor Allison Bailes. While written in 2014, with comments through 2015, there has been no effort on his part to retract, clarify, or edit this in light of any new data. So, does the roof rot issue come down to humidity control and/or poor installation practices?

  7. Expert Member
    Michael Maines | | #10

    The issue is all about vapor drive. If interior temperatures are higher than exterior temperatures for long periods, as in the northern part of the US, vapor drive can force enough moisture through the foam that it can accumulate at the roof sheathing. The greater the difference in temperature, and the greater the difference in the amount of moisture in the air, the stronger the force pushing moisture through the foam.

    Climate zone 4 is tricky because of the mix of heating and cooling. Perhaps using open cell foam with a vapor retarder on the interior would be adequate. Perhaps it's not even necessary to use the vapor retarder. It is too risky in zones 5 and 6 where I mostly work to even consider it.

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