I recently bought a log cabin in KY. The ceilings are wood on top of the rafters. I think there is just a layer of tar paper on top of the wood, then a layer of plywood then the metal roof. The cabin looks good from the inside and outside, but it has no insulating properties at all in the ceiling. If I install styrafoam in between the rafters and fill that entire 6 inch area up, then install new a new wood ceiling, do I need to leave a space between the insulation and the existing wood ceiling?
yes if your going to insulate the roof rafters and then install drywall ceiling you need 1 1/2'' air space but that is only part of your problem.
You also need proper venting at the eaves and a ridge vent