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What roof design is better, cheaper, and easier to build?

kenorakq | Posted in Green Building Techniques on

Currently planning a cathedral ceiling (VENTED) with R80 insulation. Since its vented all the insulation (not sure whether it will be Roxul batts or blown in Spider) will be inside the truss web which will have to be about 26″ deep (to allow for a vent over the 24″ insulation space.

This made me think about an UNVENTED roof plan with rigid insulation on the exterior. With the same goal of R80 in mind can someone please advise me the appropriate layup… and thicknesses required.

and given the two options which is more resilient, less expensive and easier to both build and and work with (I have a wood stove chimney that has to penetrate the roof).

Thanks from Kenora Ontario Canada P9N0E7 (zone 7).

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  1. GBA Editor
    Martin Holladay | | #1

    The easiest way to build an R-80 ceiling is to create a vented unconditioned attic with insulation on the attic floor. But evidently you aren't planning to go that route.

    Building an R-80 cathedral ceiling is challenging. Are you sure that you don't want an R-55 or R-60 cathedral ceiling?

    You probably want to read these two articles:

    How to Install Rigid Foam On Top of Roof Sheathing

    Combining Exterior Rigid Foam With Fluffy Insulation

  2. GBA Editor
    Martin Holladay | | #2

    If you want to combine above-sheathing rigid foam with fluffy insulation under the roof sheathing in Climate Zone 7 (as noted in my article, Combining Exterior Rigid Foam With Fluffy Insulation), at least 61% of the R-value of your roof assembly's insulation needs to be in the form of rigid foam above the roof sheathing.

    If you are aiming for R-80, that means that you would need R-49 of rigid foam above your roof sheathing, and R-31 of fluffy insulation under (and in direct contact with) your roof sheathing.

  3. kenorakq | | #3

    I just re-read both and note that the requirement for exterior foam is very high in my region...

    -In Climate Zone 7, you’ll need at least R-30 of rigid foam (about 7.5 to 9 inches of EPS, 6 inches of XPS, or 6 inches of polyiso);-

    This is a problem in itself as I am having a he77 of a time even sourcing the 6.5 inch screws for my walls (4" foam over 2x6 walls)... finding screws to secure 9 inches of EPS would be very spendy. I think that leads me back to the vented roof assembly....

    can you advise me why the recommendation for an R55 or R60 roof instead of R80? I'm likely missing the obvious :)

  4. STEPHEN SHEEHY | | #4

    We used raised heel scissor trusses. The roof pitch was 8/12 on the outside, but lower than that on the inside, so we ended up with pitched ceilings that were not as steep inside as outside The trusses were big enough to allow plenty of room for insulation.

    I attach a sketch that shows generally what they looked like. We had two different pitches on the south side of the house, with 8/12 on east and west and 3/12 in the middle. The attached sketch is for trusses in the middle section, but the same concept applied throughout.

    We filled the space above the ceiling with about 24" of cellulose. We did 2" of foam at the raised heel to get a bit more R value, since the heel was less than 24." It was a typically vented roof.

  5. kenorakq | | #5

    Stephen... thats an interesting looking roof... am I right that the span to the centre is about 37 ft so 64 ft wide room?

    My building is 30ft wide so 15 ft to center. and my wife wants 8/12 to allow a small loft...

  6. GBA Editor
    Martin Holladay | | #6

    Q. "Can you advise me why the recommendation for an R55 or R60 roof instead of R80?"

    A. Two reasons:

    1. The added expense required to go from R-60 to R-80 is probably not cost-effective. The last R-20 of insulation will save an insignificant amount of energy.

    2. It's physically challenging to build an R-80 cathedral-style roof assembly.

  7. STEPHEN SHEEHY | | #7

    Tim-getting to R-80 can be tough if you are filling the attic floor with insulation, as it takes up a lot of space. But if you do something like I did, so long as you have enough room above the cathedral ceiling, you can dump as much fluffy insulation as you want.

    The difference between R-60 and R-80 isn't all that much in terms of incremental benefit. We originally spec'd R-52, but our GC wasn't happy with the insulating sub's job on the walls, so to mollify the GC, the insulator dumped a lot more cellulose on the ceiling, so we ended up with about 2' of insulation.

  8. kenorakq | | #8

    Thanks..I was trying to build for the future... current code is R49 (might as well call it R50) so I thought the planning/building stage was the time to get ahead of the curve so to speak and plan for the 2050 code...

    R60 it'll be... still 20% better than R49

  9. STEPHEN SHEEHY | | #9

    Tim- the total span is about 38 feet wide. The house is about 60 feet long. I attach some photos from a recent article that show it better than I can explain it.

  10. JTyler | | #10

    You write, "The easiest way to build an R-80 ceiling is to create a vented unconditioned attic with insulation on the attic floor. But you don't want to do that." As someone who has been planning to build a vented unconditioned attic with ~20" insulation on the attic floor, I'd like to ask why you say this. Well...actually... I'd like to pretend I never saw this post and move on in blissful ignorance. But instead I'm asking: why do you say this?

  11. user-3258290 | | #11

    Martin said "you" to refer to the original poster specifically. The original poster has not chosen an unconditioned, vented attic, but that is the easiest option for anyone else that does choose it.

  12. GBA Editor
    Martin Holladay | | #12

    The main reasons that I think that it's easier to install R-80 insulation on an attic floor than along a sloping roof are:

    1. You can use one type of inexpensive insulation (cellulose) instead of two types of insulation or one type of expensive insulation (spray foam).

    2. You don't need deep framing members to hold the insulation.

    Of course, you do need raised-heel trusses or some other method of providing the required height at the perimeter. For some jobs, this is tricky.

  13. GBA Editor
    Martin Holladay | | #13

    Re-reading what I wrote, I now recognize the ambiguity. I have edited the comment to clarify my meaning. Thanks.

  14. JTyler | | #14

    Ok - I think I follow. My confusion was with the "you don't want to do that" portion of your answer. Like Keith poited out, it seems you were specifically addressing the original poster.

    Tim - for what it's worth, I am planning on a raised heel scissor truss with insulation on the floor of a vented unconditioned attic. This is perhaps not strictly a cathedral ceiling in that the ceiling pitch will not exactly match to roof pitch, so you would have to determine whether it meets your aesthetic design objectives. If your whole building has cathedral ceilings, I can see the logic behind exterior foam over the roof deck. You also mention a loft, which would lose square footage with a ceiling having a lower peak- so an unvented assembly may be beneficial in this respect. If portions of the building would have flat ceilings with unneeded attic space above, then it seems building a conditioned unvented assembly would mean a more expensive build and extra space to condition. I think either approach will yield a resilient roof if correctly detailed.

  15. JTyler | | #15

    Tim - I'm rethinking my comment that an unvented assembly may mean more space in your loft. Contact a truss company and explain your goals. They will let you know if they can give you the loft space you want under a vented assembly.

  16. Expert Member
    Michael Maines | | #16

    Tim, building an R-80 vented, cathedral ceiling is not difficult, just different and possibly not cost effective. The most straightforward approach is to use 24" parallel chord trusses or I-joists, run a continuous WRB over the trusses, run 1 1/2" furring over the WRB along the truss, and cap that with your roof sheathing.

    Loose-blowing a flat attic is much easier.

  17. user-5439398 | | #17

    Tim, here's an photo from my build. It's a parallel chord 30" truss, 2.5/12 pitch, 24' span, 55 psf snow load. The 7/16 osb holds up 24" blown fiberglass with a 6" vent space. The roof is 5/8 osb with high temp peel and stick and standing seam metal. We ran 3 rows of osb on the ceiling then blew insulation, working our way up to the high wall then along to a hatch that was completed with roxul scraps. More work than normal but not too bad, just a little harder to control the thickness of the fiberglass. I did not use cellulose due to weight concerns. The ceiling will be finished with ultra light drywall or plywood tiles.

  18. ethan_TFGStudio | | #18

    Chuck, how did you create your 6 inch vent space?

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