R-38 vs R30 in ceiling attic space R402.2.1 IRC2009

[joetheinspector]

I am studying for the ICC IECC residential test and I have run into a couple of questions.

2009 IECC Section 402.2.1 Ceilings with Attic spaces.

When section 402.1.1 would require R38 in the ceiling, R-30 shall be deemed to satisfy the requirement for R-38 whenever the full height of uncompressed R30 insulation extends over the top plate at eaves . .

Question: Does this mean R-30 only over top plate with R38 in the rest of the cavity?

Or: R-30 in all of attic?

R-30 in all attic does not make cents to me as you could have a 5000 sq ft attic or larger.

 

[GBrackins]

if you are able to install uncompressed R-30 insulation over the top plate of an exterior wall, then you are allowed to use R-30 throughout the attic space. This would typically require a 2x10 attic floor joist over the exterior wall (if using batt insulation)

If you must compress the installed insulation over the exterior wall then you must provide R-38 (including that area that is compressed) throughout the attic space.

hope that helps ...

 

[Sifu]

Ditto. Basically its a built in trade off.

 

[joetheinspector]

opps wrong code reference. it should have been 09 IECC