GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Sealed cathedral ceiling in NC – foam contractors

Scott Schmeiser | Posted in General Questions on

Hello!  First post here!  Martin recommended I post my question here.

We are in the Charlotte, NC area, and have a pretty simple cathedral ceiling construction in our family room that hasn’t been closed in yet.  The room is approximately 22×16 and has a 12/12 pitch, 16″ eaves, 2×10 rafters 16 O.C., architectural shingle roofing over 15 lb (or 30 lb?) paper, and 3/4″ OSB roof decking.  The roof is part of an addition to the home, which is a historic home, so we elected to not use a ridge vent and opted for a sealed roof design so visually it looked more like the rest of the roof when it’s all done.  We plan on simple drywall for the finished side.

I have had a couple spray foam installers out to talk to us about their systems, and based on articles here (and other places) I haven’t heard anything that aligns scientifically to studies that I have read.  Unless I’m deciphering the articles wrong, it seems our best option for a sealed type cathedral ceiling is to have closed cell foam installed directly to the bottom side of the roof decking to the proper R30, or better, value.  One company that came out wanted to install open cell foam right to the decking.  Another wanted to install an air gap system with closed cell under that (even though we don’t have a ridge vent to allow the gap to work).  Neither of these seem to be the correct solution, and I’ve since scheduled yet another installer to come out and may ask a forth.

Am I missing something here?  Am I misunderstanding the science of how this all works?  Do these installers not understand how the science works for cathedral type construction?  Or are the concepts they advertise something that will work in a North Carolina climate?  If I DO understand this correctly, then maybe someone can recommend an installer in the Charlotte area that knows their stuff.  If I’m confused, set me straight!  I want to understand this before I go any further and make a big mistake.

Any help would be appreciated, even if it means you’ve gotta’ set me straight!  Thanks for your time!

Scott

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. John Clark | | #1

    Ideal solution is exterior rigid foam sandwiched between the roof deck and the asphalt shingles.

    Next best would be ~3" of closed cell spray foam (~R-6/in for total ~ R18) followed by something like fiber batts (R-19) = ~R37

    Option B: Rigid foam cut/adhered into place in lieu of ccSPF.

    Personally I would not use open cell SPF in a roof because in hot/humid climates moisture will eventually migrate to/consolidate at the roof peak. Closed cell at over 2" becomes vapor impermeable. Spray foam is tough because you need an installer who won't bugger up the mix which causes all sorts of post-install problems (fumes, improperly cured, etc).

  2. Scott Schmeiser | | #2

    Hi John. Thank you for the reply!

    So it sounds like I haven't lost my mind, at least. Based on what you're saying, it seems I am understanding the tech that's been provided on GBA. (that's a relief)

    I definitely like the idea of 3-4" of CCSF followed by batting. I'm actually doing Roxul/Rockwool batting in the walls (2x6 equiv) and maybe I continue this solution over top of the CCSF. I would have elected to spray foam in the walls as well, however with this room having the potential for changes technology/wiring wise, I'm electing to use batting. Price is part of it as well. Spray foam is definitely expensive, so I'm trying to keep it to where it makes sense.

    Thank you for the response and clarity check!

    1. John Clark | | #3

      Dana Dorsett can give you more reliable advice.

      Paging Dr. Dorsett !!!

  3. Expert Member
    Dana Dorsett | | #4

    >"...2×10 rafters 16 O.C..."
    --------
    >"have closed cell foam installed directly to the bottom side of the roof decking to the proper R30, or better, value."

    The IRC calls out R49 as code min for zones 4 & 5 (=all of NC). Charlotte is in zone 4, where it only takes R15 out of R49 (= 31%) to provide sufficient dew point control at the foam/fiber boundary, or at the roof deck with exterior insulation.

    With 2 x 10s you can install 2" of HFO blown closed cell foam (R14), and have EXACTLY the right remaining depth for R30 ROCK WOOL batts. That would provide signficantly more drying capacity for the roof deck than 3-4" of closed cell foam, and at R44 for the stackup still gets you pretty close to IRC's prescriptive minimum, at a lower cost than 3-4" of closed cell foam + cheap fiberglass batts. It's a heluva lot greener too, since rock wool has about half the CO2e footprint per R of HFO blown foam per R, and about 1/4 the foot print of HFC blown closed cell foam:

    https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png

    With 2" of closed cell foam the roof deck is fully protected from interior moisture drives in your area, but you'll still want a reasonably air tight ceiling painted with latex paint as the vapor retarder if going with the R14/R44 (= 32%) solution, or even a "smart" vapor retarder such as 2-mil nylon (eg Certainteed MemBrain) behind the gypsum board as some extra insurance to keep the rock wool dry over a colder than average winter.

    See also:

    https://www.buildingscience.com/sites/default/files/migrate/pdf/BA-1001_Moisture_Safe_Unvented_Roofs.pdf

    Go to Table-3 if you want to cut to the chase. The 2" ccSPF + Spray Fiberglass column indicates just how well protected the roof deck is with only 2" of closed cell foam, even in climates colder than yours, at R-values greater than R30. With asphalt shingles it's good even for R49 in locations like Minneapolis or International Falls.

    >" I would have elected to spray foam in the walls as well, however with this room having the potential for changes technology/wiring wise, I'm electing to use batting."

    There's nothing "green" about closed cell foam. Going with cellulose would be the greenest, though that's usually an up-charge from batts. If it's 2x6 studs it's worth the upcharge to go with a high density R21 compared to crummy R19s (which only perform at R18 when compressed to 5.5" in a 2x6 stud bay) , but detailing the sheathing as an air barrier is a cheap but critical first step prior to any batt insulation.

    1. Scott Schmeiser | | #5

      Dana...
      WOOOW! Now THAT'S a detailed response! THANK YOU! That sounds fantastic. Now I know what I should be shopping for.

      I also really liked the suggestion about the "smart" vapor retarder such as 2-mil nylon. I will definitely consider that.

      Awesome! And the least toxins the better! I've been a little worried about any out-gassing that could occur. I don't expect an installer to be too straight with me on that, for sales reason. I have a 6-month old and a 2 year old that I wouldn't ever want to expose to unnecessary toxins.

      Thank you! That's some great stuff!

    2. John Clark | | #6

      Dana, I think Charlotte NC is on the northern edge of zone 3.

      1. Expert Member
        Dana Dorsett | | #9

        I think you're right Mecklenburg county is indeed the cool edge of zone 3 (mea culpa!)

        Hitting rewind...

        In zone 3 the IRC calls out R38 for code minimum, and as little as R5 out of R35 is sufficient for dew point control, and 1" of cell foam is adequate protection for the roof deck.

        https://up.codes/viewer/utah/irc-2015/chapter/8/roof-ceiling-construction#table_R806.5

        With 1" of HFO blown closed cell foam (R7) there would be 8.25" of space left for fiber. If blown cellulose that would come to R30-R31 which is "close enough". With the hygric buffering capacity of the cellulose it wouldn't need an interior side vapor retarder tighter than standard latex paint on gypsum board. The thermal mass / thermal diffusivity of the cellulose compared to fiberglass or rock wool also inserts a measurable delay in peak ceiling temps as well as a lower peak temp, lowering the overall peak cooling load in summer.

        This is by far the greenest solution, since the cellulose is "sequestered carbon", with a negative CO2e footprint.

        R30 denim batts would also fit reasonably in the 8.25" space and has similarly negative carbon footprint & thermal diffusivity to 3lbs density cellulose, and it's totally DIY-able (whereas dense-packed cellullose has a learning curve requires more equipment.) UltraTouch R30s can be ordered through the big orange box store if you can't find them elsewhere. A pallet load of R30 Ultra-Touch covers 300 square feet, costs about $550. I'm not sure if that's the minimum order- you may need just a little bit more than that. See:

        http://bondedlogic.com/pdf/denim-insulation/UltraTouch-Denim-Spec-Sheet.pdf

        Alternatively (and probably less expense if the batts are sourced somewhere other than a box store) , 1" of HFO blown closed cell + high density R30 "cathedral ceiling" fiberglass batts fit's the remaining 8.25" depth perfectly, delivering pretty much the same R-value, but without the somewhat subtle thermal diffusivity benefit, and no CO2e offset.

  4. Kevin Spellman | | #7

    I am in zone 4 NC as well(though in the mountains). I went with 2.5" of closed cell and the rest was open cell foam to R-49. It's very common here for installers and even energy raters to say using only open cell is ok with the classic "we do it all the time and don't have any problems." They also don't think R-49 is needed as code is years behind here. I elected to follow the research and go with the closed cell. Your plan looks sound.

    1. Scott Schmeiser | | #8

      Thank you for your response, Kevin! I appreciate it! And those are the same lines I've been getting from installers. "Yeah. This will totally work great for you." Also, they've all been telling me R-30, including the County inspector. So, that is a HUGE difference in R value that I get to correct for.

      So nice to hear from people that actually know. Now I know what I should be demanding of an installer. Hopefully I can find one, now.

      1. Expert Member
        Dana Dorsett | | #10

        R30 might be local code, since that's what it was for zone 3 under IRC 2009. (You're at the cool edge of zone 3, not 4- my mistake!) But it's not hard to hit pretty close to the current IRC R38 code minimum for much lower money- see the updated discussion in response # 9.

        1. Scott Schmeiser | | #11

          Yeah, it kind of depends on what chart you look at. We're actually in Concord, just North East of Charlotte, and some charts have us in Zone 3 and some Zone 4. Either way, I'm not much for aiming for the minimum requirements. If I can hit the higher R values for Zone 4, I'm going to go for it. I have the rafter volume to do it.

        2. Scott Schmeiser | | #12

          Dana,
          On a tangent topic, if you have anything you can add to my other insulation topic for the floor system in this same room, please let it fly:
          https://www.greenbuildingadvisor.com/question/insulated-sleeper-floor-joist-system
          Thanks!

  5. Scott Schmeiser | | #13

    Update.
    Had another installer come out that was MUCH more inline with the recipe suggested above. This guy really knew his stuff, including the actual product he is laying down and the values achieved from installing it. He's going to quote my ceiling with a full ccSPF and just the 2" (R14) worth with the idea of doing Rockwool batting over top. For fun, he's quoting me some green options for the walls, as well. He's sending me the product data sheets for everything. Fingers crossed the prices aren't out of this world.

  6. Scott Schmeiser | | #14

    The installer I mentioned that just came out uses a product by Johns Manville called "JM Corbond". Not sure I know how to decipher between a HFO and HFC blown ccSPF, but this says it has been given the "Greenguard Gold" and "Greenguard" certifications. Maybe that's just fluff on the spec sheet, but sounds promising as a low emissions product.

    Any Comments about JM Corbond?

    This installers prices were much more in line and his knowledge of the installation and the details of how he would approach the details of the job were on-point. If this is a safe product, I'm inclined to consider using this company. I also liked that he took personal interest and appreciation for the project as a whole was a plus. I find that people tend to do a better job if they appreciate and respect the envelope in which they are working.

  7. Kevin Spellman | | #15

    Prices around here are roughly $1/sf/in for ccSPF and $.30/sf/in for open cell.

    I think Corbond is just your conventional ccSPF with traditional blowing agent, not the newer HFO type.

  8. Scott Schmeiser | | #16

    Thanks Kevin!

    So is there a list of "safe" ccSPF brands I should know about so I can simply ask ahead of time before installers come out to quote?

    I'm starting to get concerned that I may not find someone that can install a safe product in my home.

    Also, in the event that the product might off-gas to an undesirable level, will installing a "smart" vapor retarding membrane before I put drywall up help to prevent potential introduction of toxins into my home?

  9. Scott Schmeiser | | #17

    Hey everyone! Just an update on this project.

    We're finally getting close to getting this portion of the house insulated. We had the company I mentioned above all quoted and ready to go and then Covid hit. Everything stopped for about hundred reasons. Now we're back in it and I called the company back out to re-up the quote and get something scheduled and it turns out they completely got out of spray foam. They no longer do it at all. And now I'm having troubles finding anyone that will blow HFO closed cell. Still more calls to make, but man things have really changed since Covid hit.

    So I'm re-upping this topic. I had the mentioned insulation company that I really liked come back today and tell me what else they do for a non-vented cathedral ceiling with 2x10 rafters. He explained they've been doing a high density cellulose. They install a strong and fine netting of some kind to the bottom of the rafters and just pack it in super dense in each rafter bay. Drywall (or preferred ceiling coverage, T&G, etc.) can go right over it after about an hour. He said they've been doing it that way for years and haven't had any issues. Obviously cellulose is a nice non-toxic insulation and has a great fire rating, I guess my only concern might be any settling of the cellulose and creating gaps over time. And also I don't feel like I can put much stock in the statement "we've been doing it for years" when problems often don't show up for a long time. But that's me as a homeowner being careful and not trusting anyone after getting a bad contractor in the past.

    These guys also provided the specification sheets on the products they use. This particular one is by "Advanced Fiber Technology". They even printed out everything I needed for code, etc. and offered to share any additional resources they had to answer questions about the products or methods.

    I would really like some input on this. I really like this company, and they have a great reputation. But if their dense-packed cellulose method is going to be a long term problem, obviously I've got to go back to plan A: 2" of HFO closed cell up against the roof decking and batting to fill out the rest.

    Thanks for the input as always!

  10. Scott Schmeiser | | #18

    Okay. I got the answer to my question regarding Cellulose right here on GBA. No go on the cellulose for an unvented roof structure. But now I'm running into contractors here in the Carolinas that say they successfully do unvented cathedral roof structures with closed cell foam all the time. I've had three of them out here with a great deal of experience that talk a great game and the prices are honestly decent.

    I feel like my head is going to explode. I'm trapped in the age old argument of open cell vs. closed cell and some batting on top. I'm exhausted and just want this part of the house done.

  11. Steve Knapp CZ 3A Georgia | | #19

    Scott,

    Nothing wrong with closed cell foam in an unvented cathedral ceiling. Sometimes there's an issue with having enough room to reach your insulation target.

    Also...

    It's generally cheaper to combine closed cell with air permeable (the flash and batt method).

    1. Scott Schmeiser | | #20

      Thanks for the comment Steve,
      Yeah, the suggested method above with 2-inches of closed cell and batting to fill out from there seems to really match what the science is saying. Assuming I understand the science here, I can’t figure out why all these contractors in my area prefer to just fill with open cell to about 8-inches (which seems to be the effective max R value) and call it good.

      Can someone explain to me why all these veteran applicators do this in my region? Is it that there’s no arguable difference in these two methods in the climate I live in? These guys almost get insulted when I lay out what everyone is suggesting to me. The response is almost always, “Well, we’ll do whatever you want, but this is the method we recommend”.

      Hmmph

      1. Expert Member
        Akos | | #21

        The reason you are seeing a lot of different opinions is that these options mostly work.

        Mostly means, that they generally hold up but occasionally cause issues.

        The question is do you want an assembly that might work or do you want one that is guaranteed to work?

        The 2" of cc SPF+batts is guaranteed to work in your climate.

        The rest (dense pack or open cell SPF) can usually be made to work with a diffusion vent. These are code approved in up to Zone 3, you might be just at the tip of where this is allowed.

        https://www.buildingscience.com/documents/guides-and-manuals/gm-2101-guide-building-conditioned-unvented-attics-and-unconditioned

        1. Scott Schmeiser | | #22

          Akos,
          Yeah, exactly how I'm feeling about this. I don't want a "this should work just fine for your application" response from these folks. None of these installers will tell me why they prefer not to do the closed cell or why they prefer the open cell. I'm wondering if they understand the science at all and why there is a potential for failure, or if they just keep doing things they way they are taught. Or maybe they simply just don't like working with CC for some reason.

          I have another call in to a company I'm familiar with. I'm going to get them out to see the structure. I've got to get this job done or we're not going to get insulation signed off of in time for my drywall guy to start later this fall/winter.

          I'm far from being an expert on this, but the science makes sense and everyone's responses and comments here have really paved the way for what seems like the RIGHT method. Either that or you guys are just great salesmen. HAHA!

          Thanks everyone!

  12. Steve Knapp CZ 3A Georgia | | #23

    "or if they just keep doing things they way they are taught."

    This, mostly. But, also this.

    "'I'm wondering if they understand the science at all and why there is a potential for failure"

  13. Jon R | | #24

    Nobody is more expert than Joe Lstiburek and his "Again, I repeat, it is ok to use open cell low density spray foam in conditioned attics….but the attics need to be conditioned" is found here. And similar "Low-density, open-cell foam is permissible..." here.

    Diffusion vents are only for air permeable (ie, fiber) insulation.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |