Hi All,

I’m working on a climate zone 7 home renovation near Conifer, Colorado. The original house is a post and beam structural system with thick laminated T&G decking for both the floors and roof. The exposed interior face of the roof decking is cedar, beautiful, and needs to stay exposed. The slope of the cathedral ceiling / roof assembly is 3/12. The original structure was not built to accommodate the large snow load that we have so part of the new structural plan is to add a new ridge beam, eave beam, and 2×12 rafters above the existing decking. We are going to remove all of the existing roofing and all of the work on the roof needs to be completed from the exterior on top of the existing 2-1/4″ thick T&G decking.

Thanks to all of the incredible resources on this website, I thought I had come up with a nicely resolved detail that consisted of the following, starting with the interior:

Existing 2-1/4″ thick, 6″ wide T&G cedar faced laminated roof decking
New vapor permeable air barrier
New 2×12 roof rafters – cavity dense packed with cellulose
New 5/8″ sheathing
New 2″ rigid foam for thermal break
New 2×4’s on the flat at 24″ O.C. for continuous roof venting
New 5/8″ sheathing
New roofing felt
New standing seam metal roof

I revisited this detail while completing my construction drawings and realized that my air barrier layer wasn’t thought out at all and was reason for significant concern. The T&G decking does nothing to prevent air leakage and there is no way any sort of roll membrane product will survive the framers adding the new rafters on top of the deck.

I’ve been digging through these forums and several other resources and the only viable option that I keep coming back to is to spray a thin layer of closed cell foam in between the new rafters on the top side of the existing decking (at the bottom/interior side of the new rafter cavities). This of coarse would create an interior vapor barrier as well and so I would need to make sure my assembly can dry to the exterior. I would like to maintain the rigid insulation above the sheathing as a thermal break and so my thought is to use EPS since it is more vapor permeable and perhaps not tape the joints so that the assembly can breath to the ventilated cavity created by the 2×4’s laid flat. I would prefer to use open cell foam instead to provide the air barrier, but because we are in zone 7, I think I need a vapor retarder also which is why I keep coming back to closed cell foam as the easiest option. (An additional, different idea would be to sheath and tape the entire exposed roof deck with 1/4″ plywood? I would like to avoid closed cell foam entirely but an option like this just sounds like a ton of extra time and materials and border line ridiculous.)

My second concern / question is regarding the amount of rigid insulation I have above the sheathing. In climate zone 7, for a purely un-vented assembly, I would need R-30 above the sheathing to keep it warm. Is only using 2″ for a thermal break potentially doing more harm than good? This design was based on Building Science Inc’s vented/unvented hybrid for cold climates to prevent ice dams and I do not have the experience to confidently know that this will in fact work. Again, my thought is to keep the 2″ rigid above the sheathing but perhaps not tape the joints so that we don’t create a vapor impermeable foam sandwich.

I’ve attached a pdf of my eave detail which shows the roof assembly as it is currently drawn. I would greatly appreciate any thoughts or feedback. Thank you all for making this an invaluable website.

Dan