Vapor Open Roof Assembly in Zone 6
We want to make a vapor-open, vented cathedral ceiling assembly (so we can add a loft when we have money sometime in the future) for a ~1100sf home in zone 6 (western NY on top of a hill, so plenty of wind and cold winters). We are trying to keep costs down so will opt for a shed roof. Our builder suggests i joists. We would particularly like to avoid spray foam. We need to be at least R49 to meet code, and would go higher if it means a better, more moisture-resilient assembly.
Our builder likes to use fiberglass in roof assemblies and wants to avoid blown cellulose in particular. We will have vents at top and bottom of the assembly, but I’m stuck on how to create site-built ventilation channels. I wonder if we could use purlins instead of sheathing and integrate the venting that way? Also, will we have issues with thermal bridging? I would welcome ideas and criticisms!
Potential Assembly:
Metal roof
Plywood sheathing
Venting – somehow!
Dupont Tyvek Commercial vapor-open membrane (OR Solitex Mento)
I Joists with approximately 14″-17″ Corning EcoBatt Fiberglass
Siga Myrex continuous air barrier (or Intello)
Service cavity/2x4s
Sheetrock
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Replies
Sarabeth,
There are a number of viable ways to do it. This is my suggestion from inside to out:
- Metal roofing.
- Impermeable roof underlayment.
- Plywood sheathing
- 2"x3" strapping parallel to (fastened to the top of) the I-joists to form a ventilation channel
- Tyvek Commercial permeable membrane over the I-Joists
- I-joists to interior as you have described.
This allows you to avoid ventilation baffles, has excellent drying into the vent cavity, and can use either batts or blown insulation.
For an article describing a similar roof see: https://www.greenbuildingadvisor.com/article/building-a-vaulted-high-performance-and-foam-free-roof-assembly
Interesting - thank you! Do you think thermal bridging with i-joists that deep would be a problem? I really liked the elegance of the roof assembly in that video, but they are in a warmer climate than we are.
Would the Tyvek Commercial be durable enough to withstand years and years of condensation/water from the underside of the roof dripping onto it with no sheathing to support it?
Sarabeth,
One of the benefits of I-joists - especially deeper ones - is that the thermal bridging of the webs isn't worth addressing.
I inadvertently omitted the roof sheathing from my description (I've corrected it above). There should never be condensation on the lower Tyvek Commercial membrane. It is there to limit wind-washing and to contain the insulation below - like any of the other materials used as baffles in vented assemblies.