Top-Vented Metal Roof Assembly
I’m in the planning process for a home I’ll be building in Quebec (Climate zone 7A. Wall assembly is pretty much sets (drywall as AB, VB, 2×6 with batt insulation, plywood, WRB, vapour permeable panel insulation, rain screen, cedar shingles), but I’m still debating over the roof assembly.
The roof is 12 12 pitch cathedral ceiling, framed with 2×12 and covered with corrugated metal roof. I want to avoid foam as much as possible for environmental reasons and because I already had an ants problem in the roof foam (XPS) of a chalet (they really love that stuff). I’m aiming for anything above R49, continuity would of course be a huge plus.
After reviewing pretty much every options there is, it seems that batt insulation between the the rafters with panel insulation on top would be my best bet (I ruled out dense packed insulation because I want to make most of the work myself).
Could anyone point to me a cold roof assembly that would work?
Until now, I have, from the interior :
-Drywall as AB
-VB (probably membrain)
-2×12 with batt insulation (Mineral wool or Hemp wool)
-Plywood as secondary AB
-wood fiber panels
-1 by 4 or 2 by 4 grid layed flat (vertical and horizontal) for air circulation under the metal roofing
-metal roofing with ridge vent
My questions are :
-Where should I put the WRB?
-Could I use waxed wood fiber panels with appropriate taping as my WRB like they do in most European wood fiber panel assembly?
-Is it possible to make this assembly a cold roof assembly? If so, how do I vent below the deck if the plywood layer is below the wood fiber panels? Can I just remove the ply from the assembly and have the grid under the metal roof as my venting?
-If this assembly can’t be made into a cold roof assembly, would moving the panel insulation on the inside and add vent baffles above the batt insulation achieve that in a smart way?
I attached a diagram of (approximately) what I had in mind (from the Steico website)
Thanks in advance!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part