Originally Posted by jtmann
Thanks for your input Bro931 and SPS-1. I want to did it right.
Bro's suggestion is not applicable for me - can't do it by code.
Do either of you see any problems in doing what I suggested - vapor barrier on both sides of the fibreglass batts in the basement ceiling joists?
I would take your concerns about the vapor barrier seriously. The previous owner of my house had put up frame walls with fiberglass insulation and the kraft paper was a not a sufficient vapor barrier. Water vapor from the inside of the warm basement filtered into the fiberglass-filled wall and condensed onto the cold block walls. This condensed water soaked the fiberglass, which soaked the frame wood and kraft paper. What a moldy, smelly mess! I had to tear out everything down to the bare block walls. If you decide to put fiberglass in the floor joists (ceiling joists for the basement), water vapor might filter through the floor into fiberglass. But you would have no cold surface (like a block wall) to condense water on. It might condense in the cold (lower) part of the floor joists or on the fiberglass insulation.
It is most important to have a vapor barrier on the warm side of the insulation. Such a barrier on the cold side would present a surface for condensation. But if everything is totally sealed, like fiberglass wool in a plastic bag, you have no worries. That's why XPS works so well.