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I just bought a house and am evaluating my options for A/C retrofit. We have a partially finished attic, and plan to fully finish it. The roof is currently uninsulated. We have 6" roof joists.

One installer I spoke to proposed to put down 2" of closed-cell foam along the roof deck, then to run oval ducts between the joists in this partially insulated space, and then finally to envelop the ducts in additional spray foam.

Vanguard Energy expressed horror at an analogous setup that used fiberglass batt. The installer argues that this setup is much better than that because the spray foam insulation performs so much better.

Is he right?

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How big is the attic space? Maybe forget ducts and put in a mini-split system instead for the attic.

But your contractor is nuts. Two inches of foam is not enough to prevent significant heating/chilling of the ducts from the roof. And spraying foam OVER or next to the duct does nothing to insulate the duct from the roof deck.

Your contractor will probably counter with "but it's only a couple of feet, so you might get a hot/cold blast when the system starts, but then it'll be fine". No. Even when your fan is not running, air is flowing through the system due to convection. So your attic ducts will continually heat/cool the whole house.

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  • It's worse than that: these were ducts for a lower level, which he was going to run across the entire up-down of the pitched roof before dropping down into the other side. So the air would be running 2" from the exterior for about 40 feet before making its way downstairs.
    – David Bruce Borenstein
    Commented Dec 22, 2016 at 15:41

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