Historically, energy code requirements for building enclosures have emphasized thermal resistance: the R-value of insulation required in walls, roofs and floors. This was often achieved with insulation installed in the cavities between framing members.
Navigating airtightness requirements in IECC 2021
While this provides some thermal benefit, it also ignores key principles of building science, such as thermal bridging, enclosure airtightness and dew points risks. As a response to this oversight, new code requirements and design and construction best practices are being adopted to improve enclosure performance, particularly through improved airtightness requirements. A leaky building requires more energy to maintain comfort and increases risk of moisture and indoor air quality issues. Airtightness requirements have become more stringent with each iteration of the International Energy Efficiency Code (IECC). The IECC 2021 continues that trend, with requirements for building airtightness with two compliance options. Which is the best option for your project?
With few exceptions, the IECC requires a continuous air barrier across the entire building thermal envelope (ref C402.5.1 Air Barriers and C402.5.1.1 Air barrier construction). Okay … so what does continuous air barrier mean? We know it is required and that having it improves performance, but tangibly what is a continuous air barrier? The answer has historically been subjective. However, the 2021 IECC is providing clarity by defining air barrier continuity, at least from a code minimum perspective. This new code has two paths that satisfy continuous air barrier compliance: enclosure airtightness testing (via ASTM E779) or building enclosure air barrier commissioning. These alternatives are described as follows:
Enclosure Airtightness Testing
· For Type R construction, airtightness testing is conducted on individual units at a specified sampling rate, every unit for buildings under eight units, or 20% of the of units for buildings with more than eight. The tested units must achieve an air leakage rate of no more than 0.3 cfm/ft2 @ 50 Pa.
· For all other types of commercial construction, testing of the whole building enclosure is required. The building must have an air leakage rate no greater than 0.4 cfm/ft2 @ 75 Pa. If the test reveals leakage greater than 0.4, a diagnostic evaluation using smoker tracer or infrared imaging is required to find air leaks. If the results are greater than 0.4 but less than 0.6, only air leaks accessible without destruction needs to be sealed. If greater than 0.6, the air leaks must be remediated to get a certificate of occupancy.
Based on our experience conducting testing, this standard is fairly liement and achievable with appropriate attention to details and proper installation. For comparison, the Washington State Energy Code requires air leakage no greater than 0.25 cfm/ft2 and the Passive House Institute US (Phius) certification limits infiltration to 0.08 cfm/ft2, both significantly more stringent.
Air Barrier Commissioning
· For this option, a code official, registered design professional (engineer or architect), or other approved agency must verify the installation of the continuous air barrier. This includes three steps:
1. A review of the construction documents to assess compliance with C402.5.1, including verifying that the continuous air barrier requirements are clearly depicted on the documents, identifying any discontinuities, and confirming that the air barrier materials, assemblies, and whole-building testing requirements are included in the specifications.
2. Inspections of the air barrier during construction while materials and transitions are still visible.
3. A final commissioning report to the owner and code official outlining each item documented by the air barrier commissioning provider during the drawing review and construction inspections.
Advantages of Commissioning
If performed by an experienced professional, commissioning of the building enclosure provides tremendous value. A trained eye can verify that the air barrier continuity design intent is clearly depicted in the construction documents and critical details of transitions between enclosure assemblies (below-grade wall to above-grade wall, wall to window, wall to roof, window to roof, etc.) are included. The provider will also identify unique conditions that require supplemental details. During construction, the air barrier commissioning provider will verify proper installation of the air barrier materials and transitions. This field verification provides significant risk reduction value. Once the air barrier components are completed and/or covered up, it is difficult and costly to go back and fix them.
A qualified commissioning provider earns the trust of the architect and works with them to detail a continuous air barrier. In the field, they help to educate contractors, construction managers and installers on new materials and assemblies, and verify that transitions are executed per the manufacturers installation instructions and accepted industry practice. A continuous air barrier is a team effort, and all players need to be on the same page. This is where the commissioning provider brings the most value.
Advantages of Testing
There are two primary values to testing. Firstly, the anticipation of the test demands greater rigor by the architect in documenting air barrier continuity design intent and requires enhanced attention by the construction team to make sure it is well executed, especially at the interfaces between installations by different trades. Secondly, it provides a meaningful validation that the installed air barrier is serving its purpose. However, the testing carries less weight when there are limited consequences for failure, as is true of the IECC 2021. Significant corrective action is only required if the leakage rate is above 0.6 cfm/ft2, which is very lenient. Introducing testing is a step in the right direction, and we should anticipate more stringent requirements in future code cycles. We should be targeting 0.1cfm/ft2 or better moving forward.
Depending on your project specifics, the IECC may allow a team to choose one approach or the other. But there is an advantage to executing both testing and commissioning. Conducting air barrier commissioning during design and construction significantly reduces the risk of air barrier discontinuities, while air barrier testing provides assurance that the air tightness design intent has been achieved. Given the energy efficiency impacts, and the occupant health and comfort implications of a leaky enclosure, there are real-world benefits beyond just code compliance. A comprehensive airtightness quality assurance program is a good investment.
Alan Scott, FAIA, LEED Fellow, LEED AP BD+C, O+M, WELL AP, CEM, is an architect with over 35 years of experience in sustainable building design. He is director of sustainability with Intertek Building Science Solutions in Portland, Ore. Ish Keener, SE, PE, is an engineer with 14 years of experience and a manager with Intertek Building Science Solutions in York, Pa. To learn more, follow Scott on LinkedIn at www.linkedin.com/in/alanscottfaia/ and Keener on LinkedIn at www.linkedin.com/in/ishkeener/.