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How to build a cathedral ceiling

CgBRtH87DB | Posted in GBA Pro Help on

I just read how to build a cathedral ceiling and the voluminous responses.
I am a homeowner. We are about to have a shed dormer built with minimal roof slope and ceiling height.

I plan to push for the GBA detail for cathedral ceiling with emphasis on air sealing 6 sides of the f/g insulation.
Do I understand that the accuvent also must be caulked or sealed?

Since it’s a low slope roof the gc is planning on using ice and water shield over the whole roof. The polyiso will be installed on the rafters with joints foamed. Some of that will have nailers and drywall. From ceiling height up to the ridge the polyiso will be exposed (drywall will be on joists below)
This raises some questions:
1. Is the fume issue with polyiso a thing of the past? Is thermax the only product GBA recognizes in this application?
2. A local supplier said they had a black foil face.
3. I couldn’t find wall details with vinyl siding. I assume I have to use sheathing not shown in other wall details.
4. You have roof wall details at the vented section, but not side wall. How does that air sealing detail differ?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Dave,
    Q. "Do I understand that the AccuVent also must be caulked or sealed?"

    A. If you are using it with fiberglass batts -- absolutely.

    Q. "From ceiling height up to the ridge [I assume this means, "in the attic"] the polyiso will be exposed (drywall will be on joists below). This raises some questions: Is the fume issue with polyiso a thing of the past?"

    A. I'm nut sure what fume issue you are talking about. The concern here is flammability.

    Q. "Is Thermax the only product GBA recognizes in this application?"

    A. Talk to your local building inspector or fire marshal; local regulations differ on this point. Don't leave any rigid foam exposed in your attic without the OK of your building inspector.

    Q. "I couldn't find wall details with vinyl siding."

    A. What type of vinyl siding detail are you looking for? You may be interested in this installation manual for vinyl siding: http://www.vinylsiding.org/installation/manual/

    Q. "I assume I have to use sheathing not shown in other wall details."

    A. Your statement is unclear. Most walls include wall sheathing, that's for sure.

    Q. "You have roof wall details at the vented section, but not side wall. How does that air sealing detail differ?"

    A. Your question is unclear. If you are asking about the air barrier on your wall, there are many ways to provide one.

  2. CgBRtH87DB | | #2

    this link below is the detail i'd hoped to utilize. when the rafter intersects with the extension of the old collar ties i'd thought there was an option to continue the cathedral assembly with foil-faced polyiso without the drywall.
    the architect drawings show f/g in the ceiling joists which are extension of collar ties.
    that exposes f/g to the vented attic..
    what ways are there to get an r-40+ in a vented assembly without polyiso..and 10" rafters? i'm assuming poltiso is required.
    my question about the sidewall is limited to airsealing the wall roof juncture.(the wall in the above detail doesn't show sheathing)

    https://www.greenbuildingadvisor.com/cad/detail/vented-roof-mixed-climate-cathedral-ceiling-asphalt-roofing-vinyl-lap-siding-over-rigid-f

  3. GBA Editor
    Martin Holladay | | #3

    Dave,
    Q. "What ways are there to get an r-40+ in a vented assembly without polyiso..and 10" rafters? I'm assuming polyiso is required."

    A. There are lots of ways -- closed-cell spray polyurethane foam, for one. And you don't have to use polyiso if you don't want; if you need to use a layer of rigid foam, you could also use EPS or XPS. Here's more information on your options: How to Build an Insulated Cathedral Ceiling.

    Q. "My question about the sidewall is limited to airsealing the wall roof juncture."

    A. The detail you linked to showed that this juncture is air sealed with sealant and canned foam. So what is your question?

  4. CgBRtH87DB | | #4

    your article on "how to build a cathedral ceiling" is what prompted my questions.
    ice and water sheild is spec'd over the whole dormer because of low slope just as in your article. scary.

    you're concerned about unprotected foil-faced polyiso but suggest spray foam in its place. how are they different?

  5. GBA Editor
    Martin Holladay | | #5

    Dave,
    Q. "You're concerned about unprotected foil-faced polyiso but suggest spray foam in its place. how are they different?"

    A. Whether you install rigid foam or spray foam in your attic, you need to comply with all fire safety provisions. Most types of rigid foam and spray foam are flammable, and in some but not all applications, both types of foam have to be protected by a thermal barrier like gypsum drywall.

    Requirements in attics are handled differently in different jurisdictions. Some building officials allow exposed foam in attics; others do not. To determine the requirements in your area, talk to your local building department or fire marshal.

  6. CgBRtH87DB | | #6

    Took a while to get an answer. The foil faced polyiso can be left exposed in attic areas.

    An insulater thinks the dead space between drywall (on ceiling joists which are extension of collar ties) and the cathedral assembly will be prone to condensation since it's above the bath. (24" tapering to zero). It's adjacent to the vented attic, but sealed from it. Walls and ceiling will be green board.

    He offered f/g in the joists covered by polyiso. This would be part of the vented attic. I objected because my understandin is f/g should be encased on all six sides.

    Do u agree with his moisture comment?

    If f/g is exposed to vented roof environment, am I right to be concerned in not encasing it?

    Dave

  7. GBA Editor
    Martin Holladay | | #7

    Dave,
    I'm sorry, but your questions are again unclear.

    If a drywall ceiling has warm, conditioned air on both sides of the ceiling, there is no reason to insulate that ceiling because the ceiling is entirely within conditioned space. There is no condensation risk if the drywall is exposed to interior conditions on both sides.

    Concerning your second question -- is it OK to omit an top-side air barrier from a horizontal layer of fiberglass batts installed on the floor of an attic -- the answer is yes. (Even the Energy Star program allows the top-side air barrier to be omitted.) The main reason this is allowed is to respect the long-term habits of installation contractors. If the slight loss of thermal performance due to wind-washing bothers you, the usual solution is to install deeper insulation.

  8. CgBRtH87DB | | #8

    Thx Martin. Path of least resistance is thicker f/g with polyiso and dry wall.
    On a personal note, thx for your amazingly fast responses. Sorry I wasn't clearer with questions. To quote ric masten "....words - when it comes to words, it's a wonder we communicate at all."

  9. CgBRtH87DB | | #9

    Referring to my question # 6 and your # 7.
    The dead air I referred to was that created by the drywall nailers when using polyiso over f/g insulation in a vented attic.
    The contractor feels the dead air in a bath is a potential moist area. Further, if he screws drywall through the polyiso he expects nail pops (head of screw not as big as nails).
    He normally glues and screws drywall. If he glues the polyiso is there any value in gluing drywall to polyiso.
    Height limitations don't allow more than 8" f/g, on average. We'll probably compromise with a half inch polyiso rather than the inch I prefer.

  10. wjrobinson | | #10

    Dave, there are glues that work on foams. Read labels, call manufacturers if needed. Most nail pop issues are a thing of the past. To avoid such, screw instead of nail, if nails used at all they must be ringshank, and for best results the frame of the building needs to have lost moisture to the point of being less likely to shrink away from the drywall which causes most modern day issues with finding raised locations at fastener locations. And this happens more the more the walls are banged into with humans and furniture. Stay clear of your walls and there will be less "pops."

    A good Panasonic bath fan of the appropriate size run on a timed switch twenty minutes after a shower and properly ducted to the exterior is your icing on the cake for moisture control.

  11. CgBRtH87DB | | #11

    Thx Aj. My question concerned the gba ceiling detail in a vented attic with polyiso on rafters which are filled with f/g.
    The detail shows a 1X nailer that the drywall is attached to. The contractor, architect and insulater all take issue with the dead air space created by the nailer. (it's a bathroom)
    The contractor routinely glues and screws drywall and would glue polyiso. But, without the nailer he says they lose the "feel" of the screws in the drywall.
    What he terms "nail pops" are actually drywall paper not being strong enough for small screw heads and sagging.
    While he's willing to do it, any problems would be mine. I don't have the real world experience to make a good decision. I'd like the extra R's, but don't want problems.

  12. GBA Editor
    Martin Holladay | | #12

    Dave,
    The air between the furring strips used to strap the ceiling will be no more or less humid than the air in the bathroom itself. There will be no condensation in this area, because there will be no cold surfaces.

    If the entire bathroom is so humid that it is leading to drywall problems, this has nothing to do with the furring strips or the air space. This means that you have very high humidity in your bathroom. The way to handle such a problem is to operate your bathroom exhaust fan.

  13. CgBRtH87DB | | #13

    Thx Martin. Are the nailers required to fasten the drywall securely using screws?
    Dave

  14. wjrobinson | | #14

    Dave, ask your contractor to do whatever he is comfortable doing. That is your sticking point. Either way would work. I double screw when foam gives me pop thru trouble. Set first screws, then second, then back to first and tighten. Or just strap it.

  15. GBA Editor
    Martin Holladay | | #15

    Dave,
    When I install drywall overhead under a continuous layer of rigid foam, I definitely prefer strapping (furring strips).

    The layer of rigid foam complicates fastening. Without strapping, the drywallers have to use longer screws, and it's harder to find the rafters or ceiling joists. Moreover, the fact that the foam is somewhat yielding can lead to more screw pops.

    So -- go with the strapping.

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