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Another roof assembly question

Jayne_Torres | Posted in General Questions on

I am building a 10′ x 28′ house on a trailer.  Steel 2×4 framing, 16 on center in both the walls and roof panel.   I’m in C zone 3 which calls for R-30 in the ceiling which I’m assuming includes the roof assembly.  With the height restrictions at 14′ I have little room on top.  This is a shed roof, minimum pitch.

The architect suggests the following:
Inside to outside
1.  1/2″ (min) gypboard
2.  3 1/2 CFS Framing with Rockwool between joists . (R13)
3. 9/16 verco shallow decking 26 g.
4. 2″ rigid insulation  (R10)
5.  Waterproof membrane underlayment

It’s obvious that I could increase the R value by adding more rigid insulation on top but I’d rather not.  If anyone has some brilliant workaround solution for increasing the r value, I’d love to hear it!

Also, will I need a vapor barrier somewhere in the roof assembly or is the membrane on top sufficient?

Thanks always!!

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  1. Jon_R | | #1

    Maybe a DIY polyiso SIP (plywood on both sides of foam).

  2. GBA Editor
    Martin Holladay | | #2

    Increasing the thickness of the rigid foam layer is the way to go. You wrote, "I'd rather not," but that's what you need to do if you want the R-value.

  3. Jayne_Torres | | #3

    Thanks you both. I will add an inch. Do I need a vapor barrier or does the waterproof membrane cancel that out?

    1. GBA Editor
      Martin Holladay | | #4

      In Climate Zone 3, there are no code requirements for an interior vapor barrier or vapor retarder.

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