Insulating metal square ductwork in the attic.

[Themp]

I live in North Carolina and had my heating/cooling system replaced. It is a 1984 house with a long square metal duct work running the length of the house(60 feet or so) insulated at R4 on the inside. Flexible branch lines were replaced off of this metal duct at R8. The metal duct work was also sealed at all the joints, but no added insulation was done. I would like to add some insulation to this square duct work to at least bring it up to R8. What would be the easiest and most cost efficient way to do this? One other note is that the metal duct is laying on the ceiling joists which have insulation under it, so I was not planning on raising the duct to insulate under it. Just do something on the other three sides. Also, the duct sealing was only done on the exposed joints, not underneath.

 

[Dana]

How much attic floor insulation is there?

If it's less than R49 (13" of mid-density batts or blown cellulose 15-16" of low density blown fiberglass) it's usually worth air-sealing the attic floor and topping over the floor insulation with blown cellulose (3" minimum added, if over fiberglass, to prevent convective loss of performance of low density good), and at the same time bury the ducts in cellulose. With the R4 interior insulation the risk of condensation/dripping into the insulation on the exterior surfaces of the ducts goes way down, and cellulose would safely wick and redistribute any incidental condenstation on the most extremely sticky days, whereas fiberglass would not.

Some pictures might be useful.

 

[Sponsor]

 

[Themp]

Hello Dana, I thought you would respond... Thanks. Here are some pictures:

https://www.dropbox.com/sh/xt3p521kn4yapq5/AADEpS9s2OuvaFX1jAgda7a0a?dl=0

The duct runs to the other end of the house thru a bedroom and then the playroom.