Chapter 7 Interpretation: 2020 NGBS Envelope Leakage

Published Friday, October 27, 2023

This interpretation allows buildings seeking 2020 NGBS certification and using the Chapter 703 Prescriptive Path to comply with the testing requirements of 2021 IECC R402.4.1.2 and R402.4.1.3.

The 2020 NGBS is based on the 2018 suite of I-Codes: 2018 IECC; 2018 IRC; and 2018 IBC. Most NGBS mandatory practices are derived from these codes. The Chapter 7 Prescriptive Path requires the building thermal envelope to be in accordance with 2018 IECC R402.4.1.2 (buildings under 4 stories) or C402.5 (building 4 stories and above). R402.4.1.2 establishes an air leakage limit of 5ACH50 for Climate Zones 1-2 and 3ACH50 for Climate Zones 3-8. No exceptions are provided. Low-rise multifamily buildings with relatively small air volume to envelope ratios struggle to meet the testing metric even when they are properly sealed and caulked as opposed to larger single-family homes with larger air volume to envelope ratios. The 2021 IECC addresses this problem by providing an option for 0.3 cfm per square foot of enclosure area for multifamily dwelling units 1,500 sq ft or smaller.

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