I haven't posted many pics of our work but I thought this was an interesting venting solution that hasn't been discussed here, at least of late.
Our customer had major condensation issues with his old metal roof. The existing 2X6 rafters had little to no insulation and zero ventilation. We removed the existing metal roof and prepped the existing 1X decking for the installation of new materials.
The Hunter Panel does not call for a vapor barrier/underlayment but does recommend it. We opted to install Titanium UDL 30 as you would under a normal roof. We then installed a Cool Deck panel manufactured by Hunter. This had a 1" Rigid ISO spaced with 1X strips, laminated to a 7/16" OSB coverboard. This was a fairly easy installation. With the exception of the screws required for fastening it cut and installed just like OSB.
We installed ice guard in the valleys and eve's as usual. We then Installed another layer of Titanium over the new roof deck. A 16" SL Panel manufactured by Bridger Steel in Casper, WY was selected as the prefered covering. We had a bit of trouble keeping things square as the home is OLD and has been added onto a few times. For an old saggy roof I think it turned out pretty well.
As a bonus, it has snowed several time since with no sign of condensation!