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How do you detail a plenum space with ducts that is below an unvented/insulated flat roof ?

user-724885 | Posted in Energy Efficiency and Durability on

This is a multi-family project in zone 4b which will have a drywall ceiling. Above the ceiling is a 2′ space where ducts will run. CCF will insulate the underside of the roof deck and the walls of this plenum space. So, ducts will be in conditioned space.

I am thinking about how we build a conditioned crawl space and intentionally condition it. This does not make sense to me for the attic area but I am concerned about moisture making its way into the plenum space. My instinct is to make the drywall ceiling airtight at penetrations and insulate ductwork even though it is in conditioned space. But also the drywall ceiling should be able to allow drying to the interior.

Do you have any input on details for this type of attic space? Thanks!

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  1. GBA Editor
    Martin Holladay | | #1

    Every building needs an air barrier and a thermal barrier; in the building you describe, these barriers will be located at the underside of the roof sheathing and the walls of the attic.

    You should strive to do a good job getting these surfaces to be airtight and well insulted.

    Your ceiling is entirely inside the home's conditioned space, so there is no need to make your ceiling airtight. I'm not sure why you are worried about moisture in the attic, since the attic conditions will be close to (or identical to) indoor conditions. If you end up having a moisture problem in your attic, that means you probably have a moisture problem everywhere indoors.

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