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Super-insulated steep pitched roof

| Posted in General Discussion on November 15, 2001 02:06am

*
My wife and I are designing a new house for ourselves. She very much wants a cape style — 10 in 12 to 12 in 12 pitch with the rafters coming down to the floor and shed dormers to provide more usable space and light upstairs. I’m adamant that our house be super-insulated so I would like to get about R-50 in the ceiling. I also want a vented cold roof. I’ve been racking my brain trying to figure out an elegant and efficient way to achieve this in a steep cathedral type roof with dormers. A couple of ideas I’ve come up with so far:

1) 16″ TJIs with something like 1/8″ masonite or even cardboard ripped to the right width and stapled to the underside of the top chords between rafters to provide an insulation stop. This would give me a 1 1/2″ vent channel above the insulation. Then I would use blown-in cellulose for a total of about R-50. But would the cellulose settle in such a steep application. This is my worry about this approach.

2) 2×12 rafters with R-30 fiberglass (for 2×10 rafters) stapled in place to provide a vented channel. Then I would nail up 2″ polyiso foam board to the underside of the rafters for a total of about R-45. 1x strapping screwed through the foam into the rafters would provide a base for attatching drywall.

Any thoughts out there on either of these two approaches or any others that might work?

Thanks for any help.

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Replies

  1. Steve_Zerby | Nov 14, 2001 10:20pm | #1

    *
    Jeff,

    If you've really gotta have the vent channel, I like #1, but use polyiso for the insulation stop, cut it a tad loose, and foam around the perimeter of it. Then you'll have a super air seal, and higher r-value than cells to boot. Then blow the cells as dense as you can get 'em in there. Settling shouldn't be a problem if you blow hard.

    Steve

    1. Mike_Gabriel | Nov 14, 2001 11:57pm | #2

      *There's a joke in there somewhere....

      1. Mike_Smith | Nov 15, 2001 01:20am | #3

        *do just as you said .. but extend your walls 12" above the 2d floor.... frame the roof (12/12) with conventional rafters (2x6 .. 2x8).. and make a false ceiling with a 10/12 pitch..do the same with your shed dormers... exterior pitch at say 6/12 and interior pitch at flat or 3/12...some of this can also be trussed.. as a matter of fact.. the whole thing can be scissor trussed.. with the shed dormers being half-trusses hanging from flat girder trusses.. supported by girder scissor trusses...use a 12" energy heel.... 12" at the eaves & 18" of cellulose at the ceiling line will give you an average R-50....the flat ceiling portion (if you have one ) can be a higher r-value than that ..

        1. David_Thomas | Nov 15, 2001 02:04am | #4

          *Jeff: In my own house (12:12 roof) I did something like what Steve suggests, but as described below. I have about R-60 in the ceiling and a VERY tight house (0.07 air changes per hour).Ripped 1-1/2" plywood strips to provide a spacer (not needed with TJIs). Ripped 2" blueboard (R-10) to 22.5" to wedge between 2x12 rafters. Had 7" of urethane foam (R-49) sprayed onto the blueboard for both it's high R-value and great sealing properties. A combination of 2" of spray-in-place foam and FG batting is a better return on your money, though.For my gable, I drilled holes at the bottom of all bays so that the "cold roof" was maintained everywhere. With the framing open, I was able to visually convective air flow under ALL the roofing. Or drilled more holes until I could.Easier than blueboard are the preformed foam baffles (sort of a cross between foam egg cartons and corrugated roofing). Can be had in sizes for 2x or TJI's and they go in easier than the blueboard - some of which need to be trimmed a bit different and secured in place. But the blueboard gives you better R per inch than FG. Compare locally for cost versus spray-in-place foam. For me, the first inch or urethane was $0.60 and each subsequent inch was $0.50. So the blueboard was definitely cheaper. But the tightness of the house comes from the urethane. (Or from very meticulous use of a small foam gun to seal all the edges.) -David

  2. Jeff_Chapman | Nov 15, 2001 02:06am | #5

    *
    My wife and I are designing a new house for ourselves. She very much wants a cape style -- 10 in 12 to 12 in 12 pitch with the rafters coming down to the floor and shed dormers to provide more usable space and light upstairs. I'm adamant that our house be super-insulated so I would like to get about R-50 in the ceiling. I also want a vented cold roof. I've been racking my brain trying to figure out an elegant and efficient way to achieve this in a steep cathedral type roof with dormers. A couple of ideas I've come up with so far:

    1) 16" TJIs with something like 1/8" masonite or even cardboard ripped to the right width and stapled to the underside of the top chords between rafters to provide an insulation stop. This would give me a 1 1/2" vent channel above the insulation. Then I would use blown-in cellulose for a total of about R-50. But would the cellulose settle in such a steep application. This is my worry about this approach.

    2) 2x12 rafters with R-30 fiberglass (for 2x10 rafters) stapled in place to provide a vented channel. Then I would nail up 2" polyiso foam board to the underside of the rafters for a total of about R-45. 1x strapping screwed through the foam into the rafters would provide a base for attatching drywall.

    Any thoughts out there on either of these two approaches or any others that might work?

    Thanks for any help.

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