I am renovating an old addition to our house, dividing it in to 2 rooms with a partition. The smaller room will not have much heat and will be kept at around 40 degrees during the winter. The ceiling in this part has old drywall that I do not want to remove. Is it necessary to use a vapor barrier on this portion of the ceiling? The attic above this will be insulated. The other room will have a vapor barrier over the drywall and a wooden ceiling added. Is it necessary to insulate the whole depth of the wall cavity or would 6 inches of insulation be sufficient? Will the bats stay in place if the cavity is not completely filled?
Answer:
Wherever there's insulation, there should be vapour barrier. I don't want gaps between insulation and vapour barrier so you might fill the whole cavity. R-28 bats are available for that. Even at that, eventually, they may slump in the cavities so you should be prepared for it. I'd take off the drywall, insulate with R-20, apply vapour barrier block (as opposed to strap) with 2X2s horizontally 16" O.C. and then apply new drywall.
