I have a late 70’s pre-fab home with 2×6 raftered cathedral ceilings.  The ceiling assembly is made up of 8ft wide panels that run from the ridge to the exterior walls, about 12-1/2ft long.  They appear to be full length 2×6 rafters with plywood on top and a fiberboard type drywall panel on the interior.  The roof has a ridge vent as well as aluminum soffit with vent holes.  I haven’t had a chance to remove the soffit yet to look for a vent channel from the walls to the peak, but am thinking there must be a second deck above the layer of plywood attached to the top of the 2×6’s if a vent space exists.  Does anyone know if this was a common building technique for these style of houses?  I’m located in central Wisconsin and we had a couple prominent prefab builders in the area at the time, but I can’t find any details about what techniques they used.  The reason for asking is that I’m trying to figure out the best way to add insulation without replacing the roof.  Right now adding a layer of rigid foam to the interior seems to be the most cost effective approach.  However I don’t want to run into issues if for some reason the roof turns out to be unvented where moisture might get trapped between the shingles and foam board.  I also want to be sure that adding interior foam would be acceptable if the roof is vented.  Any insight would be appreciated.  Thanks