The U.S. Department of Energy recently issued a Proposed Determination on the 2009 IECC, and found that the 2009 IRC was not equivalent in energy savings, largely due to the weaker 0.35 SHGC requirement. The Department has recommended that states adopting the 2009 IRC energy chapter (Chapter 11) adopt a handful of strengthening amendments to make the code equivalent to the 2009 IECC – including an amendment to strengthen the SHGC requirement to 0.30 for climate zones 2-3.
The discrepancy between the two codes will no longer be a problem in the 2012 International Codes because the Chapter 11 (Energy Efficiency) of the 2012 IRC will simply require compliance with the 2012 IECC. This will bring complete consistency in code requirements, even in states like Texas that have traditionally adopted the IRC for single-family construction.