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Community and Q&A

Opinions on this Roof Insulation Plan

jabloz | Posted in GBA Pro Help on

Im planning to insulate my roof
1. layer wood sheating 6 inch T&G boards
2. layer  2 layers Polystyrene 3 inch foil faced boards glued seams with can foam
3. metal roof installed on purlins (2 inch total height)attached witch long screw to wood sheeting

what can go wrong
Styrofoam would be vapor barrier. On the bottom any water can evaporate thru T&G crack inside. On top it will be 2 inch ventilation channels
between metal and foam boards
project is located in Pennsylvania

Tere will be vertical boards
1 inch by 2 inch wide layed on top of foam attached to joists thru 6 inch foam  and horizontal purlins same size on top of vertical boards
It will be grid approx 2ftx2ft


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  1. Jon_R | | #1

    Not clear where the idea that can spray foam is a long-term effective air barrier for rigid foam comes from. Use a good tape.

    The 2" ventilation channel may not be necessary.

  2. Expert Member


    - Your stack-up seems to be missing a roofing underlayment.
    - The purlins will be running horizontally to attach the roof panels. They will not provide a ventilation channel as they have no entrance or exit.
    - To understand the assembly we really need to know what occurs below the sheathing too.

    1. jabloz | | #3

      What is the purpose of roofing underlayment
      Any water passes metal will drain down on top of Styrofoam
      Any water that enters around long screws will evaporate thru gaps between woodboards

  3. Expert Member


    You are asking a lot of your styrofoam, remembering that it shrinks and moves. That's what the underlayment is for - to provide a secondary layer of protection. The other reason to use it is it is required by code.

  4. maine_tyler | | #5

    Usually your purlins would be attached to the rafters with long structural screws, and your roofing attached to the purlins.

    Is the board sheathing an existing situation?

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