Fraser writes from Ontario, Canada: "I have an old house with 6" insulation in the attic but no vapor barrier. The ceiling below the attic is plaster which will be covered with tongue and groove pine. Can I put a vapor barrier on the ceiling below the plaster before I put on the new ceiling or should I lift the old insulation and install the vapor barrier on the floor of the attic?"

Thanks, Fraser. I checked with my friends at Owens Corning but they do not have any test data on applying a vapor barrier directly to the plaster. I do not suggest pulling up all the insulation because that would be very labor intensive. My suggestion is to go to a local paint store and ask for a paint that acts as a vapor barrier. This is actually one of the best options for vapor barriers. Also, check with the local building code. Some codes do not require a vapor barrier between the flat attic area and living space as long as there is sufficient ventilation in the attic.