Herb, what you have is a insulation board clad with metal top and bottom and joined together with an "H" channel. I have in the past screwed down a 1/2" wood fiberboard and then applied Certainteed, "Flintlastic" base sheet and modified bitumen cap sheet. The key will be to properly fabricate wall flashing along the wall. We've done several like this and after many years, no problems.
What you are relying on is a caulk joint to make a water tight seal. The channel and the metal expand at different rates and will cause failure in the joints.