The 2021 IECC: A Breakdown of Residential Energy Efficiency

Matt, building performance expert, from southern energy management leading an on-site training at a new construction build about building performance

In This Article

ENERGY STAR 3.2 now aligns with the 2021 IECC, as other municipalities talk about adopting it as well. We'll help you understand the Residential Energy Efficiency section so you can stay prepared for any changes.

The industry is buzzing with talk about potential updates to North Carolina’s Building Code following HUD and USDA announcing their adoption of the 2021 International Energy Conservation Code (IECC) for single family homes starting late 2025. As of now, North Carolina Building Code adheres to the 2018 NCECC. We’ll continue to track these updates, but in the meantime, we want to share a breakdown* of the 2021 IECC to prepare your HUD and USDA projects for the shift in late 2025. 

This breakdown only covers the Residential Energy Efficiency chapter and is not a full breakdown of all the code changes.

Climate Zone Realignment


Most of North Carolina (NC) will now be categorized as Climate Zone 3, except for the mountainous regions in Western North Carolina (WNC)

Additional Energy Efficiency


Buildings complying with section R401.2.1 must install one of the additional efficiency package options specified in section R408.2

Buildings complying with section R401.2.2 must meet one of the following:

  • An additional efficiency package from R408.2 without including these measures in the proposed design under section R405
  •  A proposed design under section R405.3 that achieves an annual energy cost equal to or less than 95% of the standard reference design
  • For the ERI alternative, the ERI must be at least 5% lower than the targets specified in Table R406.5


Buildings complying with R401.2.1 must install one of the following additional efficiency packages:

R408.2.1: Enhanced Envelope Option

The total building thermal envelope UA must be < 95% of the total UA resulting from U-factors in table R-402.1.2

R408.2.2: HVAC Equipment Option

All HVAC systems in homes with multiple systems must meet or exceed:

  • 95 AFUE gas furnace and 16 SEER air conditioner
  • 10 HSPF/16 SEER air source heat pump
  • 3.5 COP ground source heat pump

Learn all about HVAC Design & Grading → 

R408.2.3: Water Heating Option

Hot water systems must meet one of the following:

  •  .82 EF gas water heating
  • 2.0 EF electric water heating
  • 0.4 solar fraction solar water heating

Learn more about water heating options → 

R408.2.4: HVAC Location Option

100% of ducts and air handlers must be within the building’s thermal envelope

R408.2.5: Air Leakage Option

Measured air leakage rate must be ≤ 3.0 ACH with either an Energy Recovery Ventilator (ERV) or Heat Recovery Ventilator (HRV) installed

Our HERS Raters can help you test air leakage, learn more →


R402.1.1: Vapor Retarder

In Climate Zone 4, all exterior walls must have an interior vapor retarder complying with R702.7 of the IRC, requiring kraft-faced batts

R402.1.3: Fenestration U-Factor and SHGC

Windows in Climate Zone 3 must have a U-value of .30 or less and a SHGC of .25 or less (down from .35/.30 in the current NCECC)

R402.1.3: Ceiling Insulation

Climate Zone 3 now requires R-49 for all ceiling insulation

R402.1.3: Wall Insulation

Climate Zone 3 requires R-20 or R-13+R-5 continuous insulation for exterior walls, necessitating 2×6 framing for the R-20 option

R402.1.3: Basement/Crawlspace Walls

Climate Zone 3 requires R-5 continuous insulation or R-13 cavity insulation for crawl space walls. Basement walls are exempt due to reclassification as Climate Zone 3a – Warm Humid (previously R-10 required). Learn more about crawlspace insulation → 

R402.1.3: Slab Insulation

Climate Zone 3 mandates R-10 continuous insulation to a depth of 2 ft. Learn more about slab insulation → 

Air Sealing and Leakage

R402.4.1.1: Sealing of Electrical and Communication Boxes

An air barrier (foam) must be installed behind all electrical and communication boxes on exterior walls, or air-sealed boxes can be used

R402.4.1.2: Air Leakage Testing

All homes must undergo a blower door test with a maximum allowable leakage rate of 5 Air Changes per Hour (ACH). Learn all about air sealing tips, tricks, and best practices → 

Duct Leakage and Ventilation

R403.3.6: Duct Leakage

Homes must have a duct leakage test with a maximum rate of 4 cfm/100 sq ft of conditioned space. If all ducts and air handlers are within the thermal envelope, the total leakage must be ≤ 8 cfm/100 sq ft. Learn more about duct leakage → 

R403.6: Whole House Mechanical Ventilation

Homes must have mechanical ventilation compliant with the International Residential Code (IRC) or International Mechanical Code (IMC)

R403.6.2: Whole House Mechanical Ventilation Efficacy

In-line supply or exhaust fans for whole house ventilation must have an efficacy of 3.8 cfm/watt

R403.6.3: Whole House Mechanical Ventilation Testing

Mechanical ventilation systems must be tested and verified to meet minimum flow rates as required by R403.6.2


R404.1: Efficient Lighting

All permanently installed lighting fixtures must use high-efficacy lighting, excluding kitchen appliance lighting

R404.2: Interior Lighting Controls

Permanently installed lighting fixtures must have dimmers, occupant sensors, or built-in controls. Exclusions include bathrooms, hallways, exterior fixtures, and safety or security lighting

R404.3: Exterior Lighting Controls

Exterior lighting over 30 watts must:

  • Be controlled by a manual on/off switch
  • Automatically shut off when daylight satisfies lighting needs
  • Not allow overrides unless they return to automatic control within 24 hours

What’s Next?

We will continue to monitor the HUD/USDA initiative and provide continual updates, so in the meantime, sign up for our newsletter or give us a call to learn how these changes may affect your projects.

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