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Roof Rigid Foam Insulation Thickness

Discussion in 'Residential Energy Codes' started by fj80, Jan 24, 2017.

  1. fj80

    fj80 Sawhorse

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    I'm designing a house with a cathedral ceiling with exposed structural beams, with structural decking above, and then rigid foam insulation above the decking. Per the 2012 Virginia Residential Code Table N1102.1.1 I know that I need to have R-38 roof insulation. Where do I find out how thick my rigid insulation needs to be to reach that R value?

    2012 IRC and 2012 Virginia Residential Code
     
  2. ICE

    ICE Sawhorse

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    The internet says R5 to R6.5 per inch depending on the type. So 6 to 8 inches should do it.
     
    #2 ICE, Jan 24, 2017
    Last edited: Jan 24, 2017
  3. Francis Vineyard

    Francis Vineyard Sawhorse

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    N1102.2.2 permits a limited area with R-30.

    Might consider SIP or faux beams depending on your clients desires.
     
  4. JBI

    JBI Sawhorse

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    Product manufacturer specifies the R value for the foam, based on testing.
     
  5. JCraver

    JCraver Sawhorse

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    Yes, and all the SIP manufacturers will have different specs, some even if they use the same foam.
     
  6. Paul Sweet

    Paul Sweet Sawhorse

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    I've wondered why the Virginia amended section N1102.2.1 allows R-38 to be reduced to R-30 if there is an attic and the full height of uncompressed R-30 insulation extends over the wall top plate at the eaves, but N1102.2.2 only allows a limited area to be R-30 if there is no attic.

    I would think that foam insulation would be far more effective than fiberglass of an equal R value, because the slightest breeze would drive cold air in winter most of the way through fiberglass, reducing its effective R value.
     
  7. Francis Vineyard

    Francis Vineyard Sawhorse

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    Perhaps the stack effect of heat collecting at the ceiling peak suffers loss of efficiency?
     
  8. tmurray

    tmurray Sawhorse

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    That does sound counterintuitive. I wonder if they made the change without looking at the other sentence.
     
  9. Francis Vineyard

    Francis Vineyard Sawhorse

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    #9 Francis Vineyard, Jan 26, 2017
    Last edited: Jan 26, 2017
    my250r11 likes this.

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