The center duct is lined with 3/4 inch foam insulation with foil on one side. I chose to place the foil side down as it adheres better to the foil tape. I have a layer of 1/2 inch foam, then reflectix above it as a vapor barrier, then this. These foam boards have R5.5 value and the other stuff adds about 3 more so I have about R8 in this center duct and will have R22 on the sides with the batts.
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