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Open Cell Spray Foam over HVAC ducts in unconditioned attic

user-7709149 | Posted in GBA Pro Help on

My contractor insulated our HVAC ducts in an unconditioned & vented attic with open cell spray foam, achieving R-12 value. We live in Zone 6. Is open cell spray foam acceptable, or will I have condensation issues? I’ve attached a
photo of the insulated ductwork.

After questioning the insulation company why they used open cell instead of closed cell, they said “that’s how we insulate ductwork”.

Will this be a problem? The attic is vented. Also, if I need to fix it, what are
my options besides tearing off the ducts and building it all again?


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  1. Expert Member
    Akos | | #1

    This depends on a couple of things.

    If your those are insulated flex ducts with a vapor barrier sleeve under the SPF you are fine.

    If that is hard pipe and if your summer time dewpoint is high, consistently above about 60F you can get condensation. Open cell is very permeable at R12 and if the duct surface is well bellow outside dewpoint, you can get condensation on them. A bit here and there is not a problem as it can evaporate when the AC cycles but if continuous it can cause problems.

    1. user-7709149 | | #2

      Thanks Akos. The ducts are all metal with no vapor barrier. We’re in Toronto where it can get quite humid over the summer month, so we would be running the risk of condensation if we get a few consecutive days of high humidity.

      As such I have instructed my contractor to remove all the open cell insulation and replace it with 3.5” of closed cell getting R24. He said it can be done since open cell is easier to remove, but it will be messy. Any concerns or suggestions on the open cell removal. Could there be damage to the ducts or will it come off fairly easily?

      1. Expert Member
        Akos | | #3

        An inch or so of closed cell over the existing open cell will also work fine. This gets you a vapor barrier over the open cell plus some extra R value. Make sure the closed cell covers the ducts fully including at the ceiling registers.

        Closed cell is much stiffer than open cell, after install you can poke at all the ducts to confirm, the outside surface should feel like solid rigid insulation.

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