I have a 12/12 pitch roof, no attic, tin roof. I need to insulate it. It is new construction. Do I leave an airspace between the insulation and the plywood shething so that I can get ventilation through the soffits up through the vented ridge cap or so I stuff it full?

His would have been a much easier task had you asked it during construction. Where do you live? What is your ceiling on the inside like and are there exposed beams?

There is a venting channel made for application between rafters. It is available at most lumber yards and big box stores. It is a light weight plastic that is stapled to sheathing. Then insulation is installed.

Chuck