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I've heard mixed information on this.
I am replacing all my old ducting with new r6 flex duct. Continuous runs with no leaks.
Some recommend suspending it above the blown cellulose insulation, but I'm not sure I understand why I should do this. My reading suggests that the insulated ducts will sweat and get my insulation wet. I find this hard to believe but I see it mentioned a lot.
It will be much easier to blow insulation over the ducting, burying it.
I have not had problems with flex duct sweating but the humidity in Oregon is low compared to the east coast. If the duct is totally encapsulated in insulation it cannot sweat. The humid air has to come in contact with a cold surface to condense moisture so a duct that is fully covered won’t sweat in the first place.
If you can keep the ducts 100% on the occupied side of the attic insulation, it will enhance the performance of your heating/cooling system.
However, if you lay the ducts on the ceiling and install loose fill insulation up around the ducts, you’re creating a gap 6”-10” wide (the width of your duct) in your attic insulation.
If you provide sufficient “cover” of insulation over each duct, then you’ll have the minimum R-value required for attic insulation. I’ve seen cardboard “tents” built over ducts so insulation is uniform over the ducts and pieces of insulation does not settle down between ducts, etc.
