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Unvented low-slope roof with exposed beams

Donna Riley | Posted in GBA Pro Help on

I am designing a 2nd-story addition to a home with a simple 2/12 pitch, shed roof to the east in Colorado, climate zone 5. The clients really want an exposed beam look so we have designed the roof with exposed glulam beams at 4′ centers. Above the beams we currently plan for 2x t&g decking, 4″ polyiso, and another 4″ nailbase insulation with polyiso, unvented. Roofing material will be Sarnafil thermoplastic vinyl membrane roof.

According to my research, we should include an air barrier between the decking and the rigid insulation as the t&g will not be air tight. The glulams are designed to protrude out beyond the wall to carry the 3′ overhang of the roof.

Questions?
1. What type of air barrier should we use between the t&g and the insulation?

2. It strikes me the the 7 glulams protruding out on both the west and east sides will make it difficult to air seal the joints created in the walls. I have been trying to think of a way around this. If we eliminate the glulam extensions, then we would need to add more framing above the decking to carry the over hang. This framing would seem to complicate the insulation of the built up roof. One way I thought of would be to eliminate the [email protected], and frame a structural over roof with 2×4’s to create the overhangs, spray foam in between, then add sheathing and 4″ nailbase insulation above. Then drywall between the beams (which would be acceptable). Any suggestions on this? or are the protruding glulams not a problem to seal.

Thanks for your input!
Donna

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Donna, air sealing the beams is quite easy. You provide a nailing base around the beam in the plane of the interior wall and use whatever sealant or gasket you'd prefer. Much harder is to air seal the t&g which will have both voids at the bevelled edges and between the boards. The best I have come up with is generous application of acoustical sealant as each board is installed.
    The larger problem with the glu-lams is that they act as thermal bridges. Not anywhere near as bad as steel or concrete, but still considerable.

  2. Expert Member
    Dana Dorsett | | #2

    Roofing foam manufacturers sometimes specify a slip-surface material to be used between the roof deck & foam. (Presumably this is all 2-lb fiber faced polyiso?) If none is specified, a layer housewrap would work for air-barrier purposes.

    I'd need a picture to figure out what you're proposing in question #2.

  3. GBA Editor
    Martin Holladay | | #3

    Donna,
    A synthetic roofing underlayment with taped seams or a European air barrier membrane with taped seams can be your air barrier. So can polyiso with taped seams, although a separate membrane is better.

    Tongue-and-groove ceiling boards that penetrate the air barrier are always a bad idea. It's best to terminate the tongue-and-groove boards at the plane of the exterior wall, and to use a different exterior soffit material, so that you avoid air leakage at the cracks between the boards.

  4. Donna Riley | | #4

    I was afraid of that Martin. The T&G was meant to cantilever out on the rakes and support an 18" overhang. So if I can't do it with T&G, then I have to add ladder framing above the t&G to create the overhang and infill that framing with insulation. Which means I don't get continuous insulation anymore.

  5. Donna Riley | | #5

    How about this detail?

  6. Donna Riley | | #6

    Maybe that extra layer of sheathing between the insulation isn't necessary if we can place the nailbase directly on the ladder framing and fit polyiso between the ladder framing. We had already ruled out SIP panels because they can be so difficult to seal, but it does seem like they would make dealing with overhangs alot easier.
    This detail is at the rakes. At the eves we have the glulams penetrating the air barrier to carry the 3' overhang. Do you think those penetrations can be properly air sealed?

  7. Expert Member
    Malcolm Taylor | | #7

    Donna, The other detail I've used to form soffits that gives the illusion of the ceiling material being continuous from ceiling to soffit, while including a break, is to stop the t&g 1 inch into the wall, then re-start it an inch from the exterior, filling the void with foam. To support the overhang you leave one board continuous every two feet .

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