Vented Cathedral Roof Assembly zone 5 – GreenBuildingAdvisor

We are trying to settle on a roof assembly for a 36×48’ barn / living space in climate zone 5 (Colorado, 8300’, 70psf snow load). I asked a similar question on here previously but some details have changed. Our walls are composite ICF (“The Perfect Block”), and our roof will be framed with open span Gambrel glu-lam rafters spaced 12’ on center. I will attach a drawing of the rafters for reference. All of the living space will be on the second level, within the cathedral ceiling. We are searching for the most cost effective and efficient way to insulate the space, and meet or exceed code. We’d also like to maintain some of the aesthetic features of the glu-lams by leaving a large portion of them exposed to the interior living space.

I have narrowed it down to 2 potential roof assemblies and am looking for some feedback / opinions. Maybe neither will work, I’m (obviously) new at this.

option #1

from interior to exterior:

Drywall or gypsum board

Horizontal 2×10/I-joist 24”OC between rafters 

Dense pack or batt insulation in between purlins

Smart vapor/air barrier, taped seems (flush with exterior wall surface for air sealing)

Vertical 2×4, on edge, 24”OC from eaves to ridge, to frame overhangs and additional insulation cavity

3.5” rockwool comfortbatt between 2x4s

Diagonal 1×4 vent strapping 

roof sheathing

synthetic underlayment

standing seam metal roof

My main concern is this assembly is unnecessarily complex. However it’s the best I can come up with to have exposed glu-lams, as well as adequate insulation, using “non-custom” glu-lams to keep costs down. It also seems the simplest method for a continuous air barrier at the top plate.  The top chord of the glu-lams can be either 2×8 or 2x10s sandwiched on both sides of the plywood webbing. We are hoping to use horizontal purlins (dimensional or I-joist) the same size as the top chord to make our insulation cavity/roof structure. This way much of the rafter is exposed and our purlins have a surface to attach to with joist hangers, as well helping the drywall sit flush with the plywood webbing. Standard sized rafters are only 16” deep at the wall top plate.  We’d like to avoid foam, so the 10” cavity is not enough depth to meet r49. Having the exterior insulation between 2x4s seems beneficial and should help with thermal bridging (Maybe?). Not having sheathing between purlins and vertical 2x4s seems odd but I’m assuming the 2x4s (toe nailed to purlins, and long screws used for overhangs) would add more rigidity to the purlins than sheathing would. Again, that’s an assumption from someone who’s never built a house.

option #2

from interior to exterior:

drywall or gypsum board

Horizontal 14” I joist purlin 24”OC between rafters

Dense pack cellulose in purlin cavity

smart vapor/air barrier, taped seams

Diagonal 1×4 strapping for vent space

roof sheathing

synthetic underlayment

standing seam metal roof

This option is slightly simpler. However the glu-lams would have to be a deeper, custom size if we still wanted them to be exposed. Would add cost and also make the top plate connection awkward, being that the ICF wall is only 10” wide. Again, no sheathing between purlins and strapping seems strange, but I don’t really see a need for it? Some blocking may be necessary to keep the purlins from bowing across the 12’ span before the strapping is installed. Also, thermal bridging would be a little more prevalent without the exterior mineral wool, although I can live with that. 

Is  1” vent space adequate? My thinking is the smaller the better in a high wildfire risk area. (Hence the Rockwool instead of foam).

Is the change in pitch of the gambrel roof an issue for ventilation? Does it need a vent to the exterior at that point, like some sort of one sided ridge vent?

Is a second, vapor open, air barrier needed between drywall and purlins?? Or is the drywall itself enough?

Is my vapor barrier in the correct spot?

Clearly, I don’t know what I’m doing. But I have been researching and reading obsessively, hoping to not screw this up. Thankfully we have about 18 months before construction starts. Looking forward to hearing some thoughts from the experts. If I come up with something that makes sense from a building science perspective, I can take it to our structural engineer to go over spacing, fasteners, etc.

Thanks for reading, and thanks to all the members who put out so much helpful information for people like me!