My vented attic in Virginia has rigid 6" ducts with insulation that is quite low around R-4, and also has many tears and rips. I would like to replace the insulation with R-8.
After reinsulating all ducts to R8, I plan to bury them in blown cellulose insulation, based on the report Compact Buried Ducts in a Hot-Humid Climate House.
A company makes JPL Thermal Sleeves that pull right over hard pipe like this, but it is not sold anywhere nearby.
Duct wrap such as Owens Corning SOFTR Duct Wrap FRK could be cut to fit, but that is a lot of cutting and taping, and it can only be purchased from HVAC supply houses that usually don't sell direct to homeowners.
The proposed solution
What I'm hoping I can do is to purchase R-8 insulate flex duct that is readily available at big box stores, and simply pull the flex duct over the existing rigid duct that has been air sealed with mastic. This assumes the inner diameter of the flex duct is very near the outer diameter of the rigid duct. I prefer to reinsulate the rigid duct rather than replace it with the flex duct because of the lower friction loss of rigid.
Since the flex duct has an inner plastic liner, are there any issues with two vapor permeable surfaces being sandwiched together, with a variable air gap? I tend to think there shouldn't be any problems, as I don't think condensation can form in the gap as the rigid duct will be sealed and the flex duct is sealed, so no moisture can enter this space.
Are there any problems with my proposed solution?