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Is this a good roof assembly?

Mixed_Beans | Posted in General Questions on

I’ve read a number of articles and I feel like I have read some conflicting information. Here is what I was planning to do for our roof in a new build. 

 – Home in climate zone 5

 – Single slope parallel trusses. Not sure of the size of these yet since we are waiting on engineering but they will be 3/12 pitch

– Interior insulation would be 9.5 inches of blown in wood fiber (~R-32).

– Sheathing would be 5/8″ ZIP

– Exterior insulation would be 7.25″ of wood fiberboard (~R-26)

– On top of all this would be a vented roof deck (strapping and another layer of ZIP)

– Standing seam metal roof to top it off.

Is this a reasonable assembly? I’ve read where the insulation should be up against the sheathing in one place but then I read that you don’t need to do that if that space in unvented. Which is it? Having to put dense pack or batts into that roof assemble might be a bit more challenging compared to blown in.

Also curious about that vented roof deck. Is that the best way to go? If so are those the right materials to use or is there a cheaper but just as effect a way to do that?

NOTE: We may or may not have a PV array on this roof. right now it is planned for the garage roof. However, that roof is lower and may not be optimal for a PV array.

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Replies

  1. Expert Member
    Akos | | #1

    Usually for any reasonable clear span those trusses will be 18" to 24" tall. That is plenty of space for fluffy only with simple vented space above it. No need to complicate your life with exterior insulation and 2nd roof deck.

    If you must go unvented, the bellow deck insulation needs to be tight under the sheathing. You can't have a gap here as it creates this space that is neither inside or outside your house, hard to predict moisture levels there and could lead to condensation and mold.

    If you don't want to dense pack, you can do batts under trusses held in place with straps. Take a look at the build photos here:

    https://www.jlconline.com/how-to/insulation/why-and-how-to-build-an-insulated-overroof_o

    1. Expert Member
      MALCOLM TAYLOR | | #2

      Akos,

      "Usually for any reasonable clear span those trusses will be 18" to 24" tall."

      Or just call them out for whatever height you want. I'm part way through the design of a small ADU, and have some parallel chord trusses spanning 12 ft that are 30" deep.

      1. Mixed_Beans | | #3

        What are you doing for insulation?

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