2 bedroom quad home. Gas forced air furnace. 2 bedrooms over garage. Bedrooms are cool. Vent temp in bedroom #2 is 94f. Vent temp in living room is 114f. Outside temp is 21f. Duct in garage ceiling is in joist bay. Garage is sheet rocked. Do not know how big duct is inside bay? Unless I pul down Sheetrock I will not know if bay is even insulated? How do I get bedroom vent temp higher? IF duct is sheet metal, could I replace it with flexible, insulated duct? Would that help at all?