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Roof assembly

user-7124595 | Posted in Green Building Techniques on

Looking for comment on the following zone 5/6 roof assembly on a 4/12 pitch.
Thank you in advance for your comments 

-2×12 SPF rafters dense packed with cellulose, no overhangs yet
-1/2 plywood sheathing
-fully adhered vapor open WRB on sheathing, walls and roof continuous
-2×4 horizontal purlins above with mineral wool batts, 
-2×4 (on the flat) vent channel vertically over purlins, sheathed with plywood
-purlins and vent 2×4 create overhangs
-bituthene and asphalt entire roof (its a two plane shed roof)

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  1. Expert Member
    Akos | | #1

    I guess it should work provided your roof sheathing is fairly permeable. That would be board or plywood sheathing, not OSB.

    It is a pretty complicated roof to build though.

    You could build a similar R value vented assembly by using 14" TJI, stapling a baffle to the underside of the top flange and filling the cavity with cellulose. Because of the low slope, you can even loose fill the cavity.

    I joists have much less thermal bridging than a 2x12 because of the thinner web, you would not need the exterior cross purlins and batts.

    The air barrier in this case would have to be the inside of the rafters. This could be taped OSB or one of the higher performance membrane products.

  2. user-7124595 | | #2

    Thank you Akos. The 12.5" of cellulose in your proposal would be approximately R-47.5 at R 3.8 per, likely less so if loose fill. I see my assembly at 11.25" at 3.8 (42.75) and the R-15 Rockwool Comfort Batt performing at R-57. The comfort batt in the horizontal purlins pretty much eliminates bridging. Several extra steps to be sure (doubling the plywood the most egregious)
    but a walkable 4/12 pitch roof with 2 x 4s will go quickly and safely. The 1.5" vent channel that cannot be compromised by any runaway insulation you cannot see is a big plus IMO. I should have included a sketch. I will let you know how the extra labor works out...Thanks again

  3. Expert Member
    Akos | | #3

    You are correct. Your assembly works out to around R53, my assembly is R45. In terms of whole house energy loss, the difference is noise. The ROI on the extra insulation is practically never in most climates.

    If you do want more insulation you can always bump it up to 16" TJI which would be an R52 roof.

    P.S. Much like exterior rigid insulation, the cross strapping can't eliminate thermal bridging, but it does reduce it quite a bit.

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