Codes and standards
International Energy Conservation Code (IECC)
With IECC, designers can demonstrate building envelope requirements by using the following methods:
Specific insulation requirements (Prescriptive)—Insulating materials found in opaque assemblies must have a minimum thermal resistance as listed in the code.
ASHRAE 90.1—IECC gives designers the option to use ASHRAE 90.1 instead of the prescriptive method contained in the code.
Note: 2006 IECC references ASHRAE 90.1-2004; 2009 IECC references ASHRAE 90.1-2007; 2012 IECC references ASHRAE 90.1-2010 and 2015 IECC references ASHRAE 90.1-2013; 2018 IECC references ASHRAE 90.1-2016 and 2021 IECC references ASHRAE 90.1-2019.
EnergyWise Roof Calculator Online includes the specific insulation requirements for roof assemblies. For more specific details and information, refer to the appropriate version of IECC. To purchase a copy, go to
ASHRAE Standard 90.1
With ASHRAE 90.1, designers can demonstrate building envelope compliance using the following methods:
Energy Cost Budget Method—Designers determine the "design energy cost" and illustrate it does not exceed the "energy cost budget."
Prescriptive Method: Building components (e.g., envelope, HVAC, power, lighting) have to meet the minimum requirements set forth in ASHRAE 90.1. Under the prescriptive method, designers have two options:
Building Trade-off Option: The envelope performance factor of the "proposed" building is less than or equal to the envelope performance factor of the "budget" building.
Prescriptive Building Envelope Option: Roof assemblies shall meet a maximum U-factor and minimum insulation R-value as specified in ASHRAE 90.1. This option only can be used if the skylight fenestration area does not exceed 5 percent of the gross roof area.
EnergyWise Roof Calculator Online is based on the Prescriptive Building Envelope Option. For more specific details and information, refer to the appropriate version of ASHRAE Standard 90.1. To purchase a copy, go to
ASHRAE Standard 189.1
ASHRAE 189.1 provides minimum requirements for high-performance green buildings, so it contains more stringent criteria than ASHRAE 90.1. ASHRAE 189.1 replaces the minimum R-value requirements contained in ASHRAE 90.1 with higher insulation values, so the information also is based on the Prescriptive Building Envelope Option. For more specific details and information, refer to the appropriate version of ASHRAE Standard 189.1. To purchase a copy, go to
Note: ASHRAE 189.1-2009 references ASHRAE 90.1-2007 and ASHRAE 189.1-2011 references ASHRAE 90.1-2010.
International Green Construction Code
IgCC is intended to be used as an "overlay code," meaning it is not intended to replace existing codes but should be used with IECC. With IgCC, designers using prescriptive-based compliance methods will need to increase an assembly's required U-value by no less than 10 percent using the 2012 IgCC and no less than 5 percent using the 2015 IgCC.
Note: 2012 IgCC references 2012 IECC and 2015 IgCC references 2015 IECC.
IgCC's Chapter 6—Energy Conservation, Efficiency and CO2e Emission Reduction provides specific requirements for buildings' energy efficiencies, including building envelopes' thermal performances, which include roof assemblies.
Alliance for Progress
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