My construction is as follows:
Outside skin
1" ribs 16" on-center.
Plastic shims edge of each rib against the skin for minor air space and rain screen.
1" Formular150 R-5.0 cut tight.
Bead of Great Stuff at corner of rib and skin.
Press fit insulation into cavity.
Two layers of sill insulation over outer face of rib for thermal break and spacing.
Small bead of GS on outside edge.
Tape.