Providing vapor barriers after insulating attic
Q. We had 4 inches of thermal insulation blown into the attic of our home. Later, we added 2 3/4 inches of fiber glass, with the aluminum moisture barrier facing up. Then we added 6 inches of unfaced fiber glass. What can I now do to prevent moisture damage to the ceiling joists? Moisture shows in one spot on the ceiling below. Gordon T. Ersted Kettering, Ohio
A. The Mineral Insulation Manufacturers Association, Summit, N.J., suggests that homeowners contact their local building departments for recommendations on the issue of vapor barriers with insulation. Why? Because temperature and humidity vary so much from one locale to another.
Climatic extremes call for different kinds of insulation and vapor barriers.
A report entitled "Paint as a Vapor Barrier for Walls of Older Homes," by Gerald E. Sherwood, would be further enlightening, since painted ceilings and interior walls can inhibit the humidity from migrating to the insulation.
Why don't you write for the report?
My guess is that the vapor barrier in the middle insulation layer is the culprit.
Mechanical or gravity attic ventilation may also be in order, but follow the recommendations of your local buil ding officials.