GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Cathedral Ceiling in Cold Climate

Olimax84 | Posted in General Questions on

I’m building an accessory building in my yard in Montreal, Quebec. 15’x10′. 20degree sloped roof 2×6 16″OC. It’s framed and boarded now w/OSB. I want it to be well insulated but not too crazy. I’m really getting confused over which way to proceed with my insulation for the roof specifically. It will be unvented – cathedral. I want to do a corrugated metal roof but all the underlays for metal roofs here seem to be vapour impermeable, so I can’t do CC spray foam in the cavity right? Can’t dry to either side. My other idea is just doing rigid foam boards on top of the OSB roof sheathing and isulating less inside. Maybe half R value outside and half inside and keep everything air tight with drywall and sealant. Does this assembly work? Would the metal roof just cook the rigid foam boards in the summer? So much to consider.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member


    Your best bet is to stick to one of the un-vented roof assemblies in this link:

  2. Olimax84 | | #2

    Great article, what I described is similar to #3.

  3. Olimax84 | | #3

    Just in case someone stumbles upon this and is worried about putting foam boards under metal roofs like I was. All the EPS and XPS products I've found here in Canada, (soprema sopra, durofoam, foamular, etc.) all begin to deform/melt at around 70C-75C. Most underlays that were recommended to me are good for up to 115C so you can do what you want with that information.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |