I recently had my a/c system completely replaced by a reputable dealer, including all ductwork. They did a great job except for one thing: they did not tape the bottom of the joints where the 4-foot sections of plenum join. The other 3 sides are sealed with mastic tape. The Unit sits on the roof trusses which is probably why the bottom was not taped.
How much leakage can I expect by not having the fourth side of each joint taped? Should I try to get them to fix this?
Also, the plenum is insulated with one inch duct insulation on the inside of the sheet metal (dealer says it has a value of R-4). I was looking at maybe adding 2" foil backed polystyrene rigid board to the outside of the plenum with glue and/or plastic straps. This would give me an additional R-8 of insulation. Would this be feasible or worthwhile?