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Are unvented attics considered conditioned for HVAC ducting?

HartiganWill | Posted in General Questions on

In a flat roof house design, the trusses typically wouldn’t have attic space to crawl and might be packed with cellulose. If the roof is unvented/sealed with either a.) spray foam insulation from below or b.) rigid foam on top, would this be considered conditioned space for the sake of running ducting work?

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Replies

  1. Expert Member
    Rick Evans | | #1

    Will,

    If the roof decking/sheathing layer is your air barrier, then having ducts up there is fine. It is within the thermal envelope as well as within your air barrier. Technically though, it might not be considered 'conditioned' space unless there is a duct run to the attic delivering conditioned air.

    These terms may appear interchangeable but there are situations when the differences matter.

    One example is using open cell spray foam in the attic in a cooling climate. In this case, the attic really should be conditioned. Merely including it within the thermal/air barrier may not be enough to prevent moisture issues under the roof decking.

    1. HartiganWill | | #2

      Thanks, Rick!

      I had seen unvented/conditioned used interchangeably and it was confusing. My concern was wondering if I needed to create secondary sealed chases/soffits. It sounds like as long as the roof sheathing is my air barrier then I should be okay to run the ducting through the flat roof trusses. In this design, there would be a couple of inches of polyiso above the roof and then remainder of insulation coming from cellulose on the attic floor, which would surround the ducting as well.

      1. Expert Member
        Rick Evans | | #3

        Will, it sounds like you might be mixing two different methods of insulating the roof. This might lead to undesirable outcomes.

        There are a number of great articles on unvented roofs on GBA. Its a tricky topic but worth getting the details right.

        1. HartiganWill | | #4

          Hmm, I’ve read quite a bit on the topic from this site. Everything pointed to the hybrid approach of rigid foam + cellulose or spray foam + cellulose to be okay. Just not both rigid+spray as one side needs the ability to dry.

          As long as the foam layer was right sized for your climate zone as to not condensate. I’m in AZ zone 2 so 1 inch is sufficient.

          1. Expert Member
            Akos | | #5

            When it comes to hybrid roof assemblies (rigid+batts or spf+batts) there is a big difference in cold climate between the ducts bellow the ceiling and ducts above the ceiling.

            In colder climates a hybrid assembly still needs a vapor retarder to limit moisture movement, which is the painted drywall. You can skip the vapor retarder but your ratio increases significantly, you would need much more spray foam or exterior rigid. The typical way of dealing with it is to use only spray foam in the rafter cavities where ducts need to run.

            Luckily, you are in much warmer climate, so in your case, as long as you have an inch or so of SPF, you can put your ducts into the attic space without issues.

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