My plan for radiant barrier under metal roofing – GreenBuildingAdvisor

QUESTIONS:  How much will this radiant barrier setup help, and should I stop air movement in the air gap by putting plywood between the metal panels and the battens?

DETAILS:

1980 house in CZ-2.

Metal roof is in bad shape so I’m planning to replace with new metal panels.

While I’m doing that, I plan to install 1″ foil-faced polyiso, since the rafter bays are insulated with fiberglass batts, and batts-in-rafters requires some insulation on top of the sheathing.  There hasn’t been any rafter rot in 45 years, but since I’m replacing the metal roof anyway, I might as well do it right.

My stackup is:  sheathing, 1″ foil-faced polyiso (foil side up), 2×4 battens, metal roofing.  That gives me a 1.5″ air gap since radiant barriers don’t work without a gap.

There’s also insulation in the ceiling joist bays.  It’s not enough, but the attic is used for storage so I can’t easily add more insulation above the top of the joists.  The big issue is the knee walls separating the attic space from a conditioned attic living space.  I’m hoping the radiant barrier helps; I know it’s not a panacea, but right now heat is just streaming in.

I understand that the air gap is more effective when it doesn’t move, so it seems like I should be installing 3/8″ or 1/2″ ply on top of the battens.

Howzat?