The International Energy Conservation Code

Changes to prescriptive values for 2018
Julie Ruth
October 17, 2017
COLUMN : Code Arena | Codes & Standards

 The International Energy Conservation Code has evolved greatly since its first edition in 1998. Overall, significant strides have been made in improving the energy performance of buildings. Based upon analysis performed by the U.S. Department of Energy and others, buildings built to the 2015 IECC use less than half the energy of buildings built to the 1998 IECC.  

During this time, builders and designers have become more comfortable using the IECC and building to this code. As they have done so, more and more emphasis has been placed on alternate, performance-based design, than the prescriptive method. Performance-based design sets a certain target for the overall performance of the building. The burden is then placed upon the designer to determine how to best achieve that target. 

Currently, the IECC bases the target for performance design on the level of performance achieved when prescriptive design is used. The general sense within the International Code Council community, however, seems to be that the time has come for building designers and contractors to determine the most efficient and cost-effective way to meet these performance targets and to promote that as best practices for energy-efficient construction. 

Prescriptive SHGC

For the 2018 IECC, at least, this means the referenced maximum prescriptive solar heat gain coefficient for fenestration in residential construction will remain stable and will not change from the 2015 IECC.

Once the ICC membership is more confident that best practices for the current performance level have been determined, the prescriptive maximum SHGCs may change again. They may be changed to align with these best practices, or to respond to possible future increased levels of stringency in the performance target. 

Prescriptive U-Factor 

Although the maximum prescriptive SHGC for fenestration will remain the same for residential construction, the maximum prescriptive U-factor will be tweaked a bit in Climate Zones 3 through 8 of the 2018 IECC. In Climate Zones 3 and 4 except Marine, it will be reduced from 0.35 to 0.32. In Climate Zones Marine 4, and 5 through 8, it will be reduced from 0.32 to 0.30.  

The change was supported by both the DOE and the National Association of Home Builders during the 2016 ICC Group B Code Change Cycle and is based upon the 2013 Energy Star values. In 2013, market research firm Ducker Worldwide conducted a market assessment on behalf of the DOE regarding products complying with the 2013 Energy Star requirements. The assessment determined a 70 to 89 percent market penetration of these residential windows in Climate Zones 3 through 8. Therefore, they were considered to be “readily available” in these climate zones.

Further, a life cycle cost-effectiveness analysis conducted by DOE in 2013 estimated net savings ranging from $16 per window in Climate Zone 3 to $388 per window in Climate Zone 8 for windows meeting the criteria over the current criteria. The analysis is based upon a 30-year life for the windows. 

Maximum Prescriptive U-Factor and SHGC for Residential Fenestration – 2018 IECC

Shown above is the maximum prescriptive U-factor and SHGC in the 2018 IECC for fenestration in residential buildings. Note: First number is maximum U-factor and second number is maximum SHGC.

  • 0.35/NR [purple]
  • 0.32/0.40 [dark blue]
  • 0.35/0.25 [light blue]
  • 0.40/0.25 [yellow]
  • NR/0.25 [red]

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