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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?

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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
  • It might be impossible, but I have to replace a section of rigid duct anyways with flex as it was crushed to make room for a bath fan, so I'll try it out anyways. As for spray foam, it's a good idea, but rather expensive at over $600 for just 1.5" of ccSPF. As for double wrapping, I thought it was bad to have a double vapor barrier? I planned to bury the ducts after reinsulating all ducts to R8. Those last two are definitely too expensive, with spray foaming roof deck at over $6000 easily. – joshdoe Nov 2 '16 at 18:20
  • Btw, I'm basing my R8 insulated ducts buried in cellulose based on this report I read from January 2016: Compact Buried Ducts in a Hot-Humid Climate House. With less than R8 I'm worried about condensation issues. – joshdoe Nov 2 '16 at 18:22
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    Condensation is a problem of air infiltration through the insulation, not the level of insulation. Airtight = no condensation. Air permeable but high-R = condensation. If you have a bulletproof air and vapor barrier on the outside of an assembly, and the inside is not expected to be humid, then you can have as many vapor barriers as you want, though of course it's not ideal. – iLikeDirt Nov 3 '16 at 19:45
  • Thanks for your advice, I'll let you know what I end up doing. I've asked another related question FYI. – joshdoe Nov 7 '16 at 13:14
  • I got a good source for inexpensive R8 flex duct, so have pulled out the old rigid duct and will replace it with this. All the runs are less than 12 ft and are very straight, so if pulled tight air flow loss shouldn't be bad. – joshdoe Nov 9 '16 at 12:36
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You can easily pull flex jacket over rigid metal duct. Start by pulling flex over metal duct , including wire liner. After you have pulled it a foot or so, pull on outside plastic jacket and insulation only. It will pull through without pulling inner wire liner. Cut to size and you can easily pull out inner metal liner. I tool it a step further by adding another flex 2 inches wider using the same technique. It gives me two moisture barriers that should not be a problem in dry Bakersfield California. Not sure if double barrier is good idea in humid southern states

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