Major Changes to the 2021 Residential IECC

Major 2021 Residential IECC Changes EnergyLogic (1)

We'll help you understand the changes to the 2021 Residential International Energy Conservation Code (IECC) and avoid major bumps in the road.

Major 2021 Residential IECC Changes EnergyLogic (1)
Published November 15, 2021

A New Decade Ushers in Major Changes

The 2021 Residential International Energy Conservation Code (IECC) marks the most significant changes in model energy codes in the last decade at over 9% better than the 2018 IECC. It was also the most contentious code with serval challenges during the code adoption process by the National Association of Home Builders (NAHB) and other trade organizations.

This code will mean significant changes for builders and likely cost increases, covering critical changes from the 2018 IECC to the 2021 IECC selected due to cost and home design impact. For a complete list of changes, please refer to the 2021 Residential IECC in its completion, available for reference at not cost on the International Code Council (ICC)’s website

New Energy Efficiency Addition - Sections R401.2.5 & 408.2

A new section in the 2021 Residential IECC requires the inclusion of an additional efficiency package option as defined by Section 408.2. When following the total building performance pathway (previously known as the simulated performance alternative), a 5% improvement over the reference building can be substituted for an efficiency package option.  


Changes to Prescriptive Insulation Values

Information has been reduced from full table to focus on wood-framed construction.

Code Year

Fenestration U-Factor

Glazed Fenestration SHGC

Ceiling R-Value

Wood Frame Wall R-Value

Floor R-Value

Basement Wall R-Value

Slab R-value & Depth

2018 IECC Code




20 or 13 + 5ci


15ci or 19

10 ci, 2 ft down

2021 IECC Code




20 + 5ci or 13 + 10ci or 0 + 15 ci


15ci or 19 or 13 + 5ci

10 ci, 4 ft down

Changes to Cavity Floor Insulation Installation Requirements - Section R402.2.7 

Allows two alternate methods for floor insulation, explicitly helping in floors above the garage and cantilevers. There are three options:

  1. Insulation fully in contact with the underside of the subfloor (previously this was the only allowed option).
  2. Insulation is permitted only to be in contact with the top side of the sheathing, separating conditioned from unconditioned space. Additional insulation must extend from bottom to top along the perimeter. Framing must be air-sealed.
  3. Similar to option 2, insulation is permitted to be in contact with continuous insulation along the underside of the floor framing. The combined cavity insulation and continuous insulation must meet the required R-value for floors.

Source: Section R402.2.7

Basement Wall Insulation Changes - Section R402.2.8.1  

There are changes to requirements for the insulation of the ceiling above an unconditioned basement and basement walls in conditioned basements. This is only a prescriptive requirement and can be traded off when following the total building performance pathway.

  • Basement wall change requires insulation to be installed from the top of the basement wall down to 10 feet below grade or the basement floor.  

Source: Section R402.2.8.1

Addition of Requirements for Sunrooms & Heated Garages - Section R402.2.12 

The following prescriptive code requirements have been added to address the issue of heated garages that are thermally isolated from the main home:

  • Minimum R-24 Ceiling insulation and Minimum R-13 wall insulation
  • Maximum U-Factor of 0.45 for Fenestration

Source: Section R402.2.12

Changes to Air Leakage Requirements - Sections R402.4.1.2 & R402.4.1.3 

Some specific changes will impact both Prescriptive and Total Building Performance code compliance:

  • Code changes set a maximum air leakage of 5.0 ACH50 or 0.28 CFM/SF Enclosure Area.  
  • Allows for 0.30 CFM/SF Enclosure Area for attached dwelling units and buildings that are 1,500 SF or smaller.  
  • Sets a maximum ACH50 of 3.0 when following prescriptive compliance.  


Addition of Air-Sealed Electrical or Communication Boxes - Section R402.4.6

Requires boxes installed in the building's thermal envelope to be airtight boxes and meet the requirement of NEMA OS 4. This is a prescriptive only requirement.

Source: R402.4.6

Changes to Ducts Outside of Conditioned Space and Duct Leakage Requirements - Sections R403.3.2,  R403.3.5 & R403.3.6 

These changes do two specific things. First, it sets alternate requirements for classifying ducts either in or out of conditioned space. Second, it removes the exemption for not testing ducts entirely within conditioned space and requires a minimum duct leakage of 8.0 CFM/100 SF of floor area for that ductwork.


Addition of Testing Requirements for Mechanical Ventilation Systems - Section R403.6.3  

Requires testing to confirm that the mechanical ventilation system provides the minimum ventilation flow as required by the IRC or IMC. This includes bath fans, kitchen hoods, and whole-home ventilation.

Source: Section R403.6.3

Changes to Lighting Efficiency Requirements and Interior Lighting Control - Sections R404.1 & R404.2

Section R404.1 Lighting equipment requires only high-efficacy lighting sources & Section R404.2 requires additional lighting controls on most interior lighting fixtures. Areas specifically excluded include bathrooms, hallways, exterior lighting fixtures, and lighting designed for safety or security. These fixtures can be controlled with either a dimmer, occupant sensor or other permanently installed types of control.


Helping You Prepare with Ease!

At the writing of this article, over 12 municipalities along Colorado's Front Range are planning to adopt the 2021 Residential IECC in 2022. For more information about the 2021 IECC and how its change might impact your future buildings, please reach out to your EnergyLogic Business Development contact, or contact us here

About the Author
Nathan Kahre

Nathan Kahre

Nathan is EnergyLogic's Business Development Manager. An energy nerd at heart, Nathan enjoys diving deep into why buildings work the way they do and helping others understand how to build better. Learn more about Nathan!

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