ASHRAE has proposed that portions of ANSI/ASHRAE Standard 90.2-2004, "Energy-Efficient Design of Low-Rise Residential Buildings," be adopted in the IECC published by the International Code Council.

ASHRAE's proposal addresses two key areas where Standard 90.2 provides greater energy efficiency than the current IECC provisions, according to Chris Mathis, vice chair of ASHRAE's Code Development Committee who oversaw the proposals. These are fenestration solar heat gain coefficients (SHGC) in Southern climates and modeling assumptions to quantify the benefits of exterior shading, as these are applied to code compliance using the performance path.

The first proposal seeks to require a lower SHGC, 0.37 from 0.40, for the reference house fenestration assumptions in climate zones 1 and 2. These zones are the most southerly of United States climates as defined by code.

This makes the prescriptive envelope requirements consistent with those of Standard 90.2 and the reference house in the code equal in efficiency assumptions to Standard 90.2 when seeking compliance using the simulated performance alternative in the code.

For example, under the current 0.40 SHGC reference case in the IECC, up to 20 percent of the improvement over code needed to qualify for Energy Star Homes in hot Southern climates can be obtained by improved fenestration, such as typical, low-E windows with a 0.32 SHGC, according to Mathis.

"The 0.40 maximum SHGC requirement was originally established in 1997 when the ratings by the National Fenestration Rating Council were in their infancy and few products were rated," he said. "Today hundreds of thousands of products are rated, many with significantly lower SHGC values. The 90.2 committee's analysis showed these values as cost effective for these climates, so we felt this element of the standard should be considered for the IECC."

The second proposal regarding Standard 90.2 deals with modeling assumptions related to external shading. Currently, the IECC assumes that the reference house design in the simulated performance alternative has no external shading of any type. However, many residential buildings have permanent exterior shading from overhangs and other architectural features, nearby trees, and adjacent buildings.

The ASHRAE proposal seeks to make sure that the external shading assumptions of the reference house design are the same as those of the proposed house design. This would eliminate any "false credit" from manipulation of shading assumptions when seeking compliance with the IECC, according to Mathis.

The proposed changes are scheduled to be evaluated in September 2006 for possible inclusion in the 2007 IECC Supplement.

Publication date: 05/01/2006