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Community and Q&A

Do we insulate the built-up, sloping space above an insulated roof deck?

erksuzuki | Posted in Green Building Techniques on

We have a flat roof deck above living space, and are building the surface drainage slope up with sleepers, etc., above the plywood deck (with more plywood, and waterproofing above the sloped area). This creates an at-present uninsulated (and unvented) area between the 2 layers of plywood. Should this be insulated?

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  1. GBA Editor
    Martin Holladay | | #1

    1. What is your climate? Where are you located?

    2. How much insulation is there in your roof now? What type and thickness?

    3. Is there venting between the current insulation (if any) and the lower layer of plywood?

    4. What is the thickness of the new air space that you are creating between the two layers of plywood?

  2. erksuzuki | | #2

    Sorry - forgot to include the critical info. Climate Zone 3C (north of San Francisco) and we had specified closed cell R30. No venting and the air space ranges from 3 1/2" at the building perimeter and down to 1 1/2" around the center drain.

  3. GBA Editor
    Martin Holladay | | #3

    Well, the space isn't that thick, so insulating it won't make a huge difference. And insulating a tapered space is fussy work. I wouldn't bother.

  4. erksuzuki | | #4

    thank you very much. That was our inclination but the insulation sub felt otherwise.

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