I am looking at a project at a home that was built in the mid 1970s.  The living room / dining room area has a low slope (3/12 pitch) ceiling where a 1×8 knotty pine board is nailed directly onto the rafters.  There is batt insulation above, but the rafter bays are unvented at the soffits.  A ridgevent was added about 5 years ago when the cedar shake roof was replaced with asphalt.  Subsequently the air conditioning system was replaced and I fear oversized.  I am of the opinion that the house suffers from high humidity due to the lack of an air and/or moisture barrier at the ceiling.  I have suggested that the wood ceiling should be taken down completely, soffit vents cut in at each rafter bay, insulation baffles installed from soffit to ridgevent, then new batt insulation installed, then 1/2″ layer of drywall fire-taped only, and finally a wood ceiling can be re-applied for the aesthetic look.   I know ultimately a whole home dehumidifier might have to be added to compensate for the oversized AC units, but my first goal is to create an air barrier at the ceiling to prevent the condensation on the wood ceiling.  Is this an appropriate approach or is there a less invasive (and costly) option I am not thinking of?