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The problem

My vented attic in Virginia has a straight 28ft of 12" x 8" rigid rectangular trunk duct with seven 6" branches. The insulation is quite low around R-4, and also has many tears and rips, and the ducts need to be air sealed. 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.

This question is related to a previous question.

Standard solutions

From what I've read, there are two standard solutions recommended. The first is to wrap the duct in something like Owens Corning SOFTR Duct Wrap FRK, but it can only be purchased from HVAC supply houses that usually don't sell direct to homeowners. They also sell only in 50 ft rolls for 3" R-8, and for 2016 run around $100 a roll. Wrapping the ducts would entail disconnecting all the branch lines, air sealing with mastic, lifting the duct up to pull the insulation underneath, cutting holes for all the branch starting collars, then sealing the seams with UL181 foil tape. Lifting the duct and cutting the holes just right I feel would be rather difficult.

The second solution is to encase the ducts in 2-part spray foam. This has the advantage of air sealing and doesn't require a thick layer, but it is very expensive even if done yourself, requires special safety precautions, and it would be very difficult to seal the bottom of the ducts. At around $1 per board foot (1 sq ft 1 in thick, about R6), this would require about 150 bd ft, and DIY kits come in 200 bd ft for about $300.

Proposed solution

Surround the ducts with 1.5" (R-9.6) polyisocyanurate rigid foam insulation board. These run about $30 for a 4x8ft board, and only four would be needed for my installation with very little waste. As the duct is all the same dimensions and straight, the cuts would be very simple and could be done outside the attic. The seams would be sealed with UL181 foil tape, perhaps with some caulk or construction adhesive where the boards overlap at the corners. If needed the foam board could be cut to fit around the starting collars without even taking off the branch ducts. Near the plenum where it might be difficult to lift the duct, the foam board could be cut to fit between the joists, with simple spray foam or caulk sealing where it meets the joists.

Questions

Fiberglass duct wrap would be pressed right against the duct, but the foam would leave a slight and variable gap. Is there any problem with this? I suppose there could be some convection, but if the ducts are sealed and the insulation is sealed, how could this be a problem?

Are there any problems with my proposed solution?

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Your plan to use polyiso foam board will work, but you must be absolutely sure to perfectly air-seal the insulation so that humid summer air cannot reach the cold metal duct surface. The only advantage of the spray foam approach is that it's easier to get this air seal. But you can get it with foam board using canned foam, tape, and caulk, it's just harder.

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    Turns out a friend could get me material from an HVAC supply house, so I ended up getting the duct wrap at about half the cost of the polyiso, plus I'll have 20 ft left over if needed elsewhere. Thinking about all the seams needed sealing with polyiso didn't seem very easy after all. – joshdoe Nov 9 '16 at 12:34
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A previous owner used the SOFTER duct wrap, it looks terrible with age as the backing has been punctured leaving the fiberglass to hang out, so I'd advise against it.

Polyiso is going to talk a long time to hang and seal, but will leave you with a nice smooth and paintable surface, so is probably preferred in any area there might be a lot of eyeballs.

If you don't mind the texture and value your time more than the extra $ expanding foam seems like a clear winner.

Alternatively, as recommended to me in my similar question, you can also get foil backed peel-n-stick neoprene. I imagine if you are careful and have straight runs and rectangular ducts, this might look okay as well. However the previous owner of my house used this on a curve and thus had many overlapping patches making it look almost as bad as the fiberglass wrap.

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