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Community and Q&A

Unvented roof assembly in Climate Zone 3

Hinton308 | Posted in GBA Pro Help on

Finishing up drawings of new single home construction. The attic space will be unvented. Planning on ducts being placed in attic using the Buried and Encapsulated method. Will use loose fill on top of this. My questions are:
1– Should I use foam board (what type and thickness) attached to the roof rafters with insulation (what type) in the cavities, or no insulation inside at roof, or Insulation (what type) on exterior roof deck?
2– Should I consider a raised heel? I have been told that roof needs to be stick built vs. truss system due to complications of roof design.
3–Does vapor diffusion come into consideration with this roof insulation design?

Notes–Planning on using Zip insulated on exterior walls with Zip on roof deck. Exterior will be combination of stone, brick and some stucco

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  1. GBA Editor
    Martin Holladay | | #1

    I'm confused. If you are building an unvented attic, that usually means that the insulation follows the sloped roofline, rather than being placed on the attic floor. So why do you want to bury your ducts with insulation? That makes no sense.

  2. Hinton308 | | #2

    Thank you for the reply. This was planned by Allison Bailes. I was just trying to figure out if I need any insulation at the roof. Maybe I am wrong in saying unvented, maybe it should be vented??

    1. GBA Editor
      Martin Holladay | | #3

      If you aren't designing these attic details -- if the designer is Allison Bailes -- then you should have a conversation with Allison Bailes to understand what's going on. You might ask:

      1. Will my attic be vented or unvented?
      2. Will my attic be conditioned (at room temperature) or unconditioned (closer to outdoor temperature)?
      3. Will there be insulation along my sloping roofline, or only insulation on the attic floor?

      If you are designing these details yourself, you need to be able to answer these questions. If these questions are confusing, you might want to read these articles:

      "Creating a Conditioned Attic"

      "All About Attic Venting"

  3. Hinton308 | | #4

    Thank you Martin

  4. Jon_R | | #5

    If you are creating a unconditioned attic with ducts in it, then there is a legitimate question about whether it makes sense to also put some insulation under the roof sheathing. I'm not aware of data on this issue.

  5. Hinton308 | | #6

    This is the assembly looking at for insulating the ducts. I was just wondering if it made sense to add any more insulation to roof. Since the ducts are at a R22- to possible R31 then additional insulation my not be needed.

    1. GBA Editor
      Martin Holladay | | #7

      If you insulate along your sloping roofline to create an unvented conditioned attic, you don't need to bury your ducts.

      The "buried ducts" approach is only used in vented unconditioned attics.

      For more information on this issue, see these two articles:

      "Burying Ducts in Attic Insulation"

      "Solutions to the Attic Duct Problem"

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