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Ductwork and insulation

GBA Editor | Posted in Energy Efficiency and Durability on

I am building a home. I live in the same mixed-humid climate zone as Louisville, Kentucky.
My HVAC contractor has installed ductwork in the attic. (I did not want this, but he insisted it was necessary for air conditioning).

Steel ductwork runs up a chase from the basement to an attic manifold. Flexible insulated tubes branch of this to each rooms ceilings. Also, return air flexible tubes to another ductwork manifold that runs back down to the basement geothermal unit.

I am thinking of covering this metal ductwork, which sticks up above the attic floor about 3 feet, with rigid foam. The question is, how thick? I don’t want condensation to occur.

Also, I was thinking of installing rigid foam channels on either side of each flexible attic ductwork tube, and filling it in with cellulose to insulate these. I figure the rugid foam channels, perhaps 2 feet on either side of the ductwork and two feet over the top, could be filled with blown in cellulose to insulate them.

Any thoughts on my plans? Any information appreciated!

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  1. GBA Editor
    Martin Holladay | | #1

    You've got the right idea. If it's not too late, you might consider creating an unventilated conditioned attic by installing closed-cell spray foam between your rafters.

    Failing that, you impulse is a good one. Here are a few pointers:
    1. Be sure that all duct seams -- including longitudinal seams in the galvanized ductwork -- are sealed with mastic. All joints should be secured with sheet-metal screws before installing mastic.
    2. Your duct insulation needs to be vapor-tight. (If any warm humid air reaches the galvanized ductwork, the moisture in the air will condense and drip.) Seal all seams in the rigid insulation used to insulate your ductwork with housewrap tape. Be generous with the tape and do a good job of air sealing your insulation.

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