Where in the IRC 2009, Chapter 11, is the cathedral ceiling limited to 500 square feet or 20%?
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I can find no reference to 20%. This requirement is, I read it, for ceilings that can’t meet the specified thermal resistance and are allowed to downgrade. Obviously this limit would not apply if the ceiling is fully insulated.
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You're right, this does not limit any kind of ceiling that meets the prescriptive requirements for your zone. But it falls within the exemptions from prescriptive measures:
■ 500 ft² or 20% of ceiling area of cathedral ceiling, whichever is less, is allowed to have R-30 insulation
Peter,
I don't have a copy of the 2009 IRC, but in the 2006 IRC the cathedral ceiling limitations are in N1102.2.2.
Peter, and all,
The 20% limitation is in the IECC in Chapter 4 (Residential Energy Efficiency). The IECC is, of course, an option for the builder. So, if they select that code, they would need to comply with THAT requirement. If they use the prescriptive requirements of the IRC (Chapter 11), they could avoid this requirement. Sorry for the confusion. It allows me a chance to make a good point: Remember this may change ... again. The 2012 Code may reverse this exception or add the requirement to the IRC. Keep your eyes open.