In the June 2021 issue 299 of Fine Homebuilding, there’s an article called “A new take on insulating a roof” that uses a new approach to venting a vaulted ceiling. I am wondering if this assembly can also be used with metal roofing (instead of shingles)? Or, is there a simpler assembly possible if using metal roofing?
I’m building in a humid zone 5 mixed climate. I have a vaulted ceiling and want to use blown cellulose insulation in the roof. I understand I’ll need a vented roof system.
According to the article, the assembly (from rafter outwards) is:
Rafters
WRB
strapping nailed parallel along each rafter
OSB sheathing
roofing underlayment
shingles (in our case, metal roofing).
I’m more familiar with an assembly that looks like:
Rafters
OSB sheathing
WRB
strapping nailed perpendicular to rafters
metal roofing
Would both of these options work? Do either pose condensation issues? In the later option, does this leave sufficient air channel to allow for the roof to dry to the outside?
Thanks!
Replies
https://www.finehomebuilding.com/2021/04/28/a-new-take-on-insulating-a-roof
Your modification would not allow the flow of venting air from soffit to peak.
If you plan to mount the metal roofing to horizontal strapping, (and this is ok with the material manufacturer), you would be better off treating this as a substitute for the sheathing. (or laying the strapping over a solid layer of sheathing)
I get the impression many metal roofing makers want a solid deck below the roofing material.
The vertical 2x4s mounted flat over the barrier along each rafter are there to allow space for venting air to transit low to high, removing unwanted moisture along the way. The other details in the article are important, including calculation of the net free venting area based on square footage of the covered space.
https://www.energycodes.gov/sites/default/files/2019-09/2018_IECC_residential.pdf
Seems like you would need R49 insulation for the ceiling in your zone.
So your rafters would have to be 15 inches high or so. (could be less depending on the insulation installation methods)
The point here is to make sure you understand how much R value you can get between your rafters.