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Is this design OK for my vaulted ceiling?

Tooger Smith | Posted in GBA Pro Help on

I am new to this. I feel like I am back in college researching like crazy and finding too much info. This is the best site I have found. Keep up the good work. I have built several houses, but this one has a vaulted ceiling covering a 34 X 34 space. (This is my daughter-in-love’s request. How could I say “no”!) However, I am beginning to have second thoughts. This is what I have come up with due to a variety of reasons. Beginning from the outside: Metal roofing screwed down (sheets), radiant bubble wrap, one inch iso board felt covered, 7/16 osb, 2 x 12 rafters 16 in oc, either 7-8 inches of open cell foam or 10-11 inches of mineral wool, and finally car siding painted or stained. Leaving out the poly vapor barrier on the inside would allow drying to the inside.

We are in south central Indiana, last county in zone 4 next to zone 5. (70 miles sw of Indy) I have considered not doing the iso exterior, and go with 2 inches closed cell inside. However, the locals who do spray foam will spray closed cell directly to the roof sheathing only if I sign off, saying they are not responsible. They claim all I need is to use metal, bubble wrap, sheathing, 7 inches of open cell, car siding or drywall. They claim that with 7-8 inches of open cell, the water vapor never makes it to the sheathing. Help!!! 🙂

Obviously, there are some financial restrictions. But getting this vaulted ceiling thing right is probably more important than money in the long run. I do not want to do the vented thing.

Also, this same design would be used for the wall, using 2 x 6 walls and 4 inches open cell.

Again, “Thanks.” Mychief.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Tooger,
    You want to build an insulated cathedral ceiling, but you "do not want to do the vented thing." That limits your options.

    There are only two code-approved ways to build an unvented insulated roof assembly. You can either:

    1. Install an adequate thickness of rigid foam above the roof sheathing, or

    2. Install an adequate thickness of spray foam insulation on the underside of the roof sheathing.

    Either option can be used in combination with some air-permeable insulation (for example, mineral wool) -- but only if the foam insulation is thick enough. (Your plan to use 1 inch of polyisocyanurate above the roof sheathing will only work if you live in climate zones 1, 2, or 3. If you live anywhere colder, the polyiso layer isn't thick enough.)

    All of the information you need to design this type of roof assembly can be found in this article: How to Build an Insulated Cathedral Ceiling.

    By the way: bubble wrap is a worthless product and a waste of money. For more information, see Stay Away from Foil-Faced Bubble Wrap.

  2. Tooger Smith | | #2

    Thanks. I have read the "How to build..." So I could put 2 inches of iso above the sheathing. Then screw the metal roof directly through the iso to the deck and leave out the bubble wrap. Then proceed with the inside batts or open cell. Is this ok? Appreciate your thoughts. Tooger

  3. GBA Editor
    Martin Holladay | | #3

    Tooger,
    Q. "So I could put 2 inches of iso above the sheathing. Then screw the metal roof directly through the iso to the deck and leave out the bubble wrap. Then proceed with the inside batts or open cell. Is this OK?"

    A. You can't screw metal roofing through foam. If you plan to install metal roofing, you need either sheathing (plywood or OSB) or purlins (1x4s or 2x4s parallel to the ridge, installed 24" o.c.) above the rigid foam. Then the metal roofing is attached to the roof sheathing or purlins.

    Concerning your other question -- "Are 2 inches of polyiso enough?" -- there is no way to answer that question until you tell us where you live, or your climate zone.

    You can read about the minimum required R-values for above-sheathing foam in all of the North American climate zones in this article: How to Build an Insulated Cathedral Ceiling.

  4. Lucy Foxworth | | #4

    Tooger,

    There has been a lot of interest recently in insulating cathedral ceilings. Some of these Q & A posts give even more information on the problems associated with this process.

    This is a GBA prime only article, but it is very helpful. https://www.greenbuildingadvisor.com/articles/dept/musings/how-install-rigid-foam-top-roof-sheathing

    This is another Q & A question that is similar enough to yours to be helpful. https://www.greenbuildingadvisor.com/articles/dept/musings/how-install-rigid-foam-top-roof-sheathing

  5. Expert Member
    Dana Dorsett | | #5

    At any R-value the ratio of exterior foam-R to interior fiber R is what defines the average temperature of the roof deck, and thereby it's maximum & average moisture levels.

    In zone 5 you need a minimum of 40% of the total R to be on the exterior of the roof deck for dew point control in a roof.

    In zone 4 you need a minimum of 30% of the total be on the exterior.

    So, if the total R is going to be above the R49 code minimum, the prescriptive R values of the foam spelled out in IRC chapter 8 have to be increased proportionally, since those prescriptives for zones 3 & higher are based on a center-cavity total of R49.

    http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm

    And it gets even more complicated if the exterior foam is polyiso, which has performance issues at lower average temps through the foam. Polyiso needs to be derated to no more than R5.5/inch for zone 4A (that's you), no more than R5/inch in zone 5, due to it's loss of performance during the colder weather, which is when you are most concerned about dew point control at the roof deck. So since you're in zone 4, if you only install 2" (labeled R12 or R13, but only R11 after derating), that limits the total R to about R37, which is well below the IRC 2012 code min of R49 for zone 4.

    At 7" most half-pound foam is still about 5 perms, about as vapor open than a single coat of latex paint. But it's also only about R26, barely half code-min. At 13" you'd make code-min, but you would still be over 2.5 perms, not nearly vapor tight enough to be protective of the roof deck for the long haul. It might be OK at 13" if the roofing color is dark so that the roof runs warmer than a more reflective surface, but this is getting into the hair-splitting realm, not exactly good practice, and definitely not meeting code.

  6. Tooger Smith | | #6

    All of this because my daughter-in-love wanted a loft to look out over the lake. Mercy!! But I already said "yes", so......(many thanks for the info. Even at 70 years and many new and remodel houses, I still need help. Just ask my Chief if you don't believe!!)

    How about this. Metal roof, 2 inches ISO, 7/16 decking, 3 inches ISO carefully "cut and cobbled", mineral wool batts or open cell, drywall. I can get the iso very inexpensively and I have the time to cut and cobble. Thanks again.

  7. Jeff Carroll | | #7

    Have you considered metal SIPs? Their spans are much greater than OSB SIPs since they're structural, and if you want unvented cathedral and are going with a metal roof, they might be a alternative. We're building a Cap "Odd" near Hannibal, MO using them.

    The only drawback: the companies with the structural panels are mostly based in the SE. We're having them trucked in from Georgia, for example. http://www.permatherm.net.

  8. GBA Editor
    Martin Holladay | | #8

    Tooger,
    We've dropped a couple of hints, but you still haven't told us your location or climate zone... so it's really hard to give you advice.

    I think that cut-and-cobble is a risky way to insulate an unvented cathedral ceiling. I also think that it's a bad idea to sandwich roof sheathing between two layers of foam -- exterior foam and interior foam. For more information, see Cut-and-Cobble Insulation.

  9. Charlie Sullivan | | #9

    Martin,
    The second paragraph of the original question says:
    "We are in south central Indiana, last county in zone 4 next to zone 5. (70 miles sw of Indy)"

  10. Tooger Smith | | #10

    Charlie, thanks for coming to my "rescue" about my location. :) So, one option is metal, membrane, osb, 3 inches iso, osb, 8-10 inches mineral wool or open cell, drywall, paint or car siding. Another option is metal, membrane, osb, 3 inches closed cell, 8 inches mineral wool, drywall, paint or car siding. The second option leaves thermal bridging tho with 2 x 12 rafters perhaps it Is not a big deal.

    An irony of this, is that I live in a pole barn house about 150 yards away from the build site that I built in 2000. The walls are metal screw directly to the 2 x 4 girts with 5 inches of open cell then rough wood planks for the inside wall (obviously not air tight). The ceiling is a vaulted ceiling with metal, bubble wrap, purlins, 5 inches of open cell, and covered also in rough planks (plenty of air). All these years, I have never had any condensation problems. (I did have a leak because I had partially missed a purlin when I was screwing down the metal. So, go figure. Are the local open cell people really wrong when they tell me to just put metal, membrane, osb, 7 inches of open cell car siding or drywall?

    Obviously, an R-50 ceiling does sound better than my little house's R-20.

    Again, I really do appreciate the feed back. Obviously, I am still looking for the best combination of No drips, high R, low Dollars. Some poet could make a limerick with that!

    Tooger

  11. GBA Editor
    Martin Holladay | | #11

    Tooger and Charlie,
    I'm embarrassed that I missed the reference to Tooger's location. Sorry for the sloppy reading -- and thanks.

  12. Tooger Smith | | #12

    Martin...no problem. At least you can read!! And if I got through the day with only that mistake, Chief would not have to use her Styrofoam bat on me so much! (I keep telling people that I am married to a mean women. However, like all men, I married up!!)

    About half hour ago, this dawned on me. I don't know if it were a dream or if I were in la la land. And maybe after reading my offering, you might think I had a nightmare.

    Here goes: We have 2 x 12 rafters, 16 in oc, 7/12 pitch. They are notched and strapped to 2 x 6 wall and notched and strapped to the LVL beam. They are resting on top of the lvl beam. If we pole barn nail the purlins vertically @ 24 in oc ( like we usually do for pole barn construction with rafters 4 ft oc), then we could put 3 in of ISO board ripped at 22.5 inches, then jam the next purlin, jam the next iso, etc on up the roof. The metal could be screwed directly to the purlins. Inside, spray 1 in of closed cell. Then "fill" the cavity with rock wool or open cell, then drywall.

    Among other advantages, soffit support is good, rain is not a problem like with osb. (Plus, it is a slab floor, we would cover the iso each day as we work. I pondered about any membrane use. Tweak this for me, please. Was it a good dream or a nightmare? :)

    As always, thanks for the help.

    Martin, are you old? I thought only old people get up at 5:30 am!!!!!

  13. GBA Editor
    Martin Holladay | | #13

    Tooger,
    Yes, I'm old.

    You are describing a variation of the "cut-and-cobble" method -- a method that requires cutting rigid foam into thin strips and inserting it between framing members. There are several problems with this approach, most of which are related to air sealing. It's hard to keep the seams sealed, especially because framing members move due to settling, wind, and changes in temperature and humidity. Moreover, this method isn't code-approved for hybrid unvented roof assemblies that combine rigid foam (or spray foam) and air-permeable insulation.

    There have been reports of failures -- mold and rot -- in unvented cut-and-cobble roof assemblies.

    For more information, see Cut-and-Cobble Insulation.

  14. Tooger Smith | | #14

    Discussing this with Chief this morning... she said I was in a hypnagogic state! (She may be a mean Yankee, but she is smart! I grew up a Cajun just below I-10. So everyone above I-10 is a Yankee. Just thought this might increase your vocab one more word.

  15. Tooger Smith | | #15

    I put post 14 before I read your post 13. In our county, there are no building codes. We only have to have the septic system permitted. After that, we are free to build as we want to build. No inspections. However, I do want this to work.

    Wouldn't the inch of closed cell stop the vapor migration? If there ever is any moisture, it can dry to the outside as well as the inside. If It can not dry to the outside, then it is sealed. If it is not sealed well enough, then it can dry.

    The air space between the iso and the metal and the ridge channels of the metal themselves would not give any venting if a ridge vent cap was put in place?

    So you are saying, this is more of a nightmare rather than an epiphany! :)

    As always, Thanks. Chief says I am hard headed. But I am loyal!!

  16. GBA Editor
    Martin Holladay | | #16

    Tooger,
    Once again, I must have been reading too quickly. Yes, the one inch of closed-cell spray foam will provide an air barrier. I imagine this would work, as long as you don't have to explain it to a code official.

  17. Tooger Smith | | #17

    Thank you, Sir, for your time and input. I believe this solves the problem and gives me the best R, no drip, and dollar value. I will try and send a pic or so when this happens, Lord willing and I'm still alive.

    Chief says "Thanks" also. Maybe now I can leave the computer!!!

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