Joint Seals

[dumdum]

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?

[selfemployed]

Ask them to tape the joints. The current 2008 code requires R-8 for ducts in attics unless the attic space is inside the envelope of the structure.

[jrbenny]

Ask them to tape the joints. The current 2008 code requires R-8 for ducts in attics unless the attic space is inside the envelope of the structure.
Careful on quoting code...

Not all states have the same requirements.

My great hack state is still R-6.

[selfemployed]

I had put - May depend on where you live, but edited it out.