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IECC 2021 & Building Enclosures

Keeping up with energy codes is complicated. The adopted code can vary state-to-state and even cities may require different code adherence than the state they’re located in. It’s important to know which code the local jurisdiction your project is in is required to follow. 


Code Adoption 

The International Code Council (ICC) develops and publishes the International Energy Conservation Code (IECC), a model building code that sets minimum efficiency standards in new construction for a structure’s walls, floors, ceilings, lighting, windows, doors, duct leakage, and air leakage. These codes are released on a three-year cycle; the last code update was 2021 and changes, updates, and modifications are expected to be released in 2024. Once the codes are released, it’s up to each state to adopt the codes. Because the codes are not uniformly adopted at the time of release, it’s important to know which codes are required by which state, and what cities or counties are under a different jurisdiction from the state, meaning they require different codes.  


The IECC 2021 introduces several key changes to improve a building’s enclosure performance. The changes impact two key areas: enhancements to the enclosure and air leakage standards. 


Building Enclosure Enhancements 
  • Adjusted climate zones: Under IECC 2021, 10% of the counties in the US moved to a different climate zone, the majority into a warmer zone. This impacts multiple large, urban areas that are also experiencing population growth (including Dallas/Fort Worth and middle Tennessee). The warmer climate zones tend to have fewer energy efficiency requirements and less stringent insulation and air leakage requirements. They are also areas with a need for cool roofs. (C301.1) 
  • Reduced fenestration U-factors and solar heat gain coefficients: Windows and glazed areas play a crucial role in energy efficiency. The IECC 2021 sets stricter limits on U-factors (heat transfer through windows), solar heat gain coefficients (solar radiation entering the building), and the elimination of orientation as a factor in these values. (C402.4) 
  • Expanded air leakage requirements: IECC 2021 extends requirements for air leakage testing. Ensuring a tight building envelope minimizes drafts, improves comfort, and reduces energy consumption. 


Air Leakage Standards 

The IECC 2021 made significant changes to air leakage (C402.5).  

Air leakage requirements include mandatory non-residential air leakage testing or building envelope performance verification for buildings not tested. There are two paths for performance verification: 

A. Conduct Whole Building Air Leakage Testing in accordance with ASTM E779.  


B. Third-party Verification consisting of the following: 

  1. Review of construction documents (check for material air permeability or the materials must be deemed to comply) and assembly test;  
  2. Inspection of continuous air barrier components and assemblies conducted during construction while air barrier is still accessible and repair to verify compliance with requirements
  3. Final commissioning report provided by inspections completed by design professional or approved agency. The commissioning report shall be provided to owner, owner’s agent, or code official. The report shall identify deficiencies found during review of construction documents and inspections and details of corrective measures taken. 



There are a few differences between IECC 2021 and ASHRAE 90.1.  

  • ASHRAE requires both WBAT and limited building enclosure commissioning. 
  • ASHRAE does not require BECx related to moisture/water. 
  • Commissioning requirements are more robust (air barrier, thermal insulation, fenestration air leakage) in ASHRAE. 
  • New ASHRAE process requirements for new projects that surpass certain square footage. This also applies to renovations and additions. 


The adoption of IECC 2021 brings several benefits to projects, for owners, operators, and occupants. 

  • Energy Savings: Improved insulation, tighter envelopes, and efficient fenestration lead to reduced energy consumption, translating into lower utility bills. 
  • Resilience: A well-designed building enclosure enhances resilience against extreme weather conditions, moisture, and air infiltration. 
  • Environmental Impact: By minimizing energy use, we contribute to reducing greenhouse gas emissions and combatting climate change. 

The IECC 2021 emphasizes better building enclosures, promoting energy efficiency, and environmental stewardship. As builders, architects, and policymakers embrace these standards, SSR is prepared to ensure building enclosures are designed and installed to code.  

For more detailed information, refer to the official IECC 2021 documentation.