# Roof Rigid Foam Insulation Thickness



## fj80 (Jan 24, 2017)

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


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## ICE (Jan 24, 2017)

The internet says R5 to R6.5 per inch depending on the type.  So 6 to 8 inches should do it.


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## Francis Vineyard (Jan 25, 2017)

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

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


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## JBI (Jan 25, 2017)

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


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## JCraver (Jan 25, 2017)

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


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## Paul Sweet (Jan 25, 2017)

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.


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## Francis Vineyard (Jan 25, 2017)

Paul Sweet said:


> 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.



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


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## tmurray (Jan 26, 2017)

Paul Sweet said:


> 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.



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


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## Francis Vineyard (Jan 26, 2017)

That 2012 Virginia amendment (as are many of them) is directly from the 2015 IRC.

Here is a copy the change proposal with dissenters calling it a loop hole:
https://www.iccsafe.org/wp-content/uploads/04-IECC-R.pdf


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