I have a 1960s split-level with an HVAC supply trunk to the upstairs bedrooms housed inside a soffit on the garage ceiling. There is a (poorly plugged) outlet into the garage in the duct floor and near it, the start of some mold growth on the gypsum. I'm guessing the root problem is a combination of air leakage condensing inside the drywall and simple condensation from having a very cold AC system in the humid garage and organic gypsum.
I plan on pulling down the soffit, sealing the outlet with a suitable galvanized sheet, etc. but I want to insulate/vapor seal the duct properly so the condensation and mold won't occur again. The garage is pretty poorly sealed at the door, so the cavity temperature and humidity variation is pretty large.
I'm thinking of using either polystyrene or polyisocyanurate (foil-faced foam) board to insulate and vapor-seal around the duct, then frame out the soffit with 2x2 and 2x4 and fill the remaining problem areas (joints and around seams) with spray foam.
Is this a reasonable plan or should I use something else to seal/insulate the duct? Would duct-board be better or worse than the XPS/RMAX sheets?
Dan
I plan on pulling down the soffit, sealing the outlet with a suitable galvanized sheet, etc. but I want to insulate/vapor seal the duct properly so the condensation and mold won't occur again. The garage is pretty poorly sealed at the door, so the cavity temperature and humidity variation is pretty large.
I'm thinking of using either polystyrene or polyisocyanurate (foil-faced foam) board to insulate and vapor-seal around the duct, then frame out the soffit with 2x2 and 2x4 and fill the remaining problem areas (joints and around seams) with spray foam.
Is this a reasonable plan or should I use something else to seal/insulate the duct? Would duct-board be better or worse than the XPS/RMAX sheets?
Dan