Guys I need to pick your brain on this theory: say you have an old house, no vents to work with. roof is metal and brand new, no insulation. I am stuck with a whole bunch of XPS boards I want to use. Here's the plan: install well-sealed xps board with a gap away from the rafters, say 1". Note there is nothing between rafters. Then drill holes in walls and at gable end and install vent covers. possibly use an electric fan to get some air moving. climate zone 4 ridge vents and soffit vents would cost too much as a retrofit so don't want to do that. Would this work?

If I understand this correctly, you want cross-ventilation from rafter-bay to rafter-bay to be able to vent the roof deck, terminating the vent space at the gables to the outdoors?

Theoretically it might work, but you'll need at least 1.5", and maybe even 2" of gap for the cross-ventilation, not 1". Power venting the space isn't recommended, since that would actively drive air leakage through the XPS (which would be quite difficult to air seal reliably.) With R15 of XPS you'd be able add more R below the XPS (up to ~ R50 total center-cavity R) with minimal danger of condensation though.

Unless you really need the space for storage or there are conditioned attic rooms with kneewalls, etc. you're probably better off insulating and air sealing at the attic floor.