Vented Cathedral Roof Assembly zone 5
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!
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Replies
For option 1 forget about the 1x4 strapping. Use 2x6 and fill with 3.5" MW batts. This give you a code min space without extra material.
For option 2, you need to bump up the strapping to 2x4 and it needs to run from soffit to ridge, not diagonal.
The smart VR should be on the inside in either case but you probably want an air barrier membrane over the purlins to air seal. This membrane can be well detailed housewrap.
That makes sense. Is there any concern with the 2x6 buckling due to only being toe nailed? Does size of vent space matter in a wildfire area?
I was under the impression that strapping for a standing seam roof needed to be horizontal or diagonal due to the panels not having horizontal rigidity. Vertical would be way easier in either case.
Typical rafters get toe nailed to the top plate unless you are in a windy area where you need hold downs. So not much holding them down. With these rafters running over purlins and toe nailed to each, it will be a lot sturdier.
Vent gap has nothing to do with fire risk. Fire resistance comes from having the correct fire rated intake screens, you never want any ambers to get inside your roof.
You need a solid roof deck for the metal, that is your roof sheathing. The metal sit directly on the deck so it is fully supported. The vent space is under the roof deck.
So your option 1 stack up should be
-drywall
-vapor retarder
-purlins with dense pack
-sarking membrane (heavy duty house wrap is fine)
-2x6 rafters with 2x4 batts
-roof deck
-synthetic underlayment (class A in fire areas)
-metal roof
That is all super helpful. Thank you so much!
Hi Jackson!
Just curious - why have you decided on a vented roof vs an unvented roof assembly?
A few reasons. We get lots of snow, and snow has an R value. If enough of it sits on a warm roof it can create ice dams. The way I understand it, a small vent space prevents that by keeping the roof surface colder. It also lets the assembly dry, should any moisture (vapor) find its way into the insulation or rafters. Also, metal roofs are prone to condensation. The underlayment protects from that but the vent space also helps the sheathing dry should any moisture make it through.
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Why no options of having insulation above the sheathing?
What issues would that solve?
Rigid insulation is pretty expensive, and I’d rather not use foam. The metal roof we’re hoping to use requires sheathing rather than firring strips, so if we did a roof deck, then insulation, we’d still need another layer of sheathing. And the ice dam issue requires a vent space either way. Two layers of sheathing seems redundant. This just seems to be the most cost effective, reduces thermal bridging (albeit not as effectively as rigid), and the vent space minimizes any moisture risks. I’m open to suggestions though!
Jackson-
If you use a product like RayCore (a "type" of SIP product & yes, foam) you may be able to skip the roof sheathing per RayCore's literature (see screenshot & attachment), but obviously you should consult & get approval by a licensed engineer & then get it OK by the building inspector (as it says in the fine details of their literature at the top).
How?
You would take one of their roof panels that has a structural element going all the way thru it (2X4 or 2X6) & mechanically fasten that to the Glulam rafters that are 12"oc.- I'm thinking these in essence would become your roof purlins tying the roof structure together.
Then cover the Raycore panel with an approved membrane, strap them, add sheathing & metal roof (per manufacturers guide)...
Just a thought - hope it may help!
I have explored the Rayco panels and even spoke to their tech department who was very helpful. These panels use a closed cell foam. They will not allow vapor to escape should it accumulate at the ridge. These also do nothing for thermal bridging. In the early stages of design we thought we wanted to use SIPs or these Raycore panels. Mainly cost, as well as moisture issues steered us away.