Replies

1. GBA Editor
   Martin Holladay | Jul 12, 2019 03:46pm | #1

   William,
   Q. "I would like to add 12 inches of ComfortBoard outside the existing roof and applying a new metal roof."

   A. Good luck. Installing mineral wool insulation on the exterior side of sloped roof sheathing is rarely attempted in North America. You will be a pioneer, and may face pushback from roofers.

   Q. "What kind of material should go between the new metal roof and the sheathing?"

   A. Roofing underlayment. (Note that roofing underlayment isn't optional -- it's code-mandated.)

   Q. "What kind of provisions (if any) do I need to make to prevent the ComfortBoard from sliding on the roof (7/12 pitch)?"

   A. Presumably, you will first install a frame of 2-by lumber, as high as the thickness of your mineral wool, around the perimeter of your roof. That will help. Friction and the compression from the screwed plywood, in addition, should take care of the issue. But you may want to consult an engineer.

2. 
   Aedi | Jul 12, 2019 04:12pm | #2

   I second the call for an engineer. 12" of comfort board is quite a lot. What climate zone are you in?

   If I understand your plan right, your layers (from bottom to top) are:

   -Old roof

   -Peel and stick

   -Comfortboard

   -Plywood

   -New roof

   If this is correct, then it is pretty strange. As Martin points out, you'll need underlayment on top of the plywood, so I'm not sure what benefit the peel and stick adds. If you use 2x4 strapping instead of plywood (permitted by some roof manufacturers), then I think you can rely on just the peel and stick? Again, its an unusual setup, so it is hard for me to be sure off the top of my head. And again, an engineer is a good idea.

   If you can, use foam instead. If you are worried about the environmental impact, you can use reclaimed foam.