Roof Rigid Foam Insulation Thickness

[fj80]

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

 

[ICE]

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

 

[Francis Vineyard]

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

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

 

[JBI]

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

 

[JCraver]

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

 

[Paul Sweet]

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.

 

[Francis Vineyard]

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?

 

[tmurray]

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.

 

[Francis Vineyard]

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