Is it possible to add a vapor barrier to an existing attic, or would ensuring adequate ventilation (solar powered vent) be more effective. At this point I would have to pull up a layer o batts and the insulation between the joists to lay down a layer of poly. Since it would not be a continuous sheet and broken up by each joist not to mention that it will be held off of the ceiling by the strapping, how effective could it really be? It would have to stapled or taped to the joists to be viable?

Larry

Your best bet in an existing condition would be to apply a vapor retarder from the bottom. There are paints with a very low permiability rating designed just for that purpose. A regular paint store should be able to get you hooked up. The lower the permiability number, the less moisture that will pass through.