Pull insulated flex duct over rigid duct

ductsinsulation

The problem

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.

Some options

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.

The questions

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?

Best Answer

Going from R-4 to R-8 isn't a very big difference, and I doubt your plan would result in measurable savings, even if it were feasible. And it's not really feasible, since pulling flex duct around rigid duct is going to be nigh-impossible without removing and reinstalling the rigid duct and ripping the inner plastic liner of the rigid duct to pieces.

There are better solutions for the problem of ducts in an attic. Easy and cheap solutions:

  • Spray foam new insulation around ducts. This would give them R-15+, depending on thickness, and with a perfect air barrier, too
  • Wrap the insulated ducts in even more insulation
  • Bury the ducts in a new layer of cellulose covering the entire attic floor (easiest if they're resting on the attic floor)

Hard and expensive solutions:

  • Spray foam the entire attic at the roofline to bring it into the conditioned space of the house
  • Build entirely new ducts within the conditioned space of the house, possible hidden in new chases and bulkheads
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