ASHRAE 90.1-2019 Air Barrier Requirements: Leakage Limits and Verification

ASHRAE 90.1-2019 §5.4 requires a continuous air barrier across the commercial building envelope. The 2019 edition states three leakage criteria, all at 75 Pa: 0.004 cfm/ft² for air barrier materials, 0.04 cfm/ft² for assemblies of materials and components, and 0.40 cfm/ft² for the whole building. The values below are for the 2019 edition; confirm the edition your jurisdiction has adopted, because later editions can differ.

What are the ASHRAE 90.1-2019 air leakage limits?

An air barrier is a continuous assembly of materials and components in the building envelope that limits uncontrolled airflow. Standard 90.1-2019 §5.4.3.1 sets leakage criteria at three scales. The U.S. Department of Energy’s Building Energy Codes Program lists them in its 90.1-2019 webinar slides:

The same slides note that the whole-building criterion carries exceptions: testing of portions of buildings, an allowance up to 0.60 cfm/ft², and compliance by inspection and verification of materials and assemblies. The verification provisions sit in §5.9. Read the standard for the exact conditions of each exception.

A material that meets the 0.004 limit does not make a wall compliant. Penetrations, joints and transitions add leakage that the assembly or whole-building criterion captures. Ask the building envelope consultant which path the authority having jurisdiction (AHJ) accepts before permit submission.

Why does ASHRAE 90.1 require a continuous air barrier?

Uncontrolled air movement through the envelope adds heating and cooling load that the mechanical design may not account for. Isolated sections of membrane do not control airflow if gaps remain at penetrations, transitions or seams. Continuity is checked at four kinds of locations:

• Joints between envelope assemblies: wall-to-roof, wall-to-foundation, wall-to-window
• Seams between sheets of barrier material
• Penetrations for pipes, conduit, ducts and structural connections
• Transitions such as parapets, eaves and balcony connections

Infiltration is also one of the two largest uncertainties in an HVAC load calculation, which is why an air barrier detail affects mechanical sizing (DOE 90.1-2019 webinar, mechanical section).

How is whole-building air leakage tested?

Whole-building testing pressurizes or depressurizes the envelope and divides the measured airflow at 75 Pa by the envelope area. Read §5.4.3.1 and §5.9 of the adopted edition for the required test method and the reporting requirements. For orientation, Oregon’s compliance form for its energy code (which adopts ASHRAE 90.1 as its base) requires an independent third-party tester and reports results in cfm/ft² and cfm at 75 Pa; it also shows that a continuous air barrier design and installation verification program is an alternative path for larger buildings. Oregon amends the standard, so use it as an example of a reporting form, not as the 90.1-2019 limit.

ACH50 converts from airflow as: ACH50 = (measured cfm at 50 Pa × 60) ÷ building volume in cubic feet.

How do residential and commercial air barrier requirements compare?

Commercial buildings under ASHRAE 90.1 are measured in cfm per square foot of envelope at 75 Pa. Residential buildings under the IECC are measured in air changes per hour at 50 Pa (ACH50). Under the 2021 IECC, R402.4.1.3 sets the prescriptive-path maximum at 5.0 ACH50 in climate zones 0, 1 and 2 and 3.0 ACH50 in climate zones 3 through 8. R402.4.1.2 (Testing) separately caps every compliance path at 5.0 ACH50 or 0.28 cfm per square foot of dwelling unit enclosure area, with testing by ANSI/RESNET/ICC 380, ASTM E779 or ASTM E1827, reported at 50 Pa (model text as quoted in a Minnesota Department of Labor and Industry handout). Other editions and local amendments differ, so check the code your jurisdiction adopted.

What air barrier installation failures should drawings anticipate?

1. Unsealed penetrations. Pipe sleeves, conduit and structural brackets each need individual sealing. Membrane on the flat wall does not make the barrier continuous.

2. Missing transitions between envelope assemblies. Wall-to-roof, wall-to-foundation and wall-to-window transitions need compatible materials and adhesives, and the barrier must lap continuously across each. Detail them on the drawings, not only the field of the wall.

3. Tape seam failure. Self-adhered and mechanically attached sheet barriers can fail at tape seams applied over dust, wet substrates or outside the manufacturer’s temperature range. Inspect seams before the barrier is covered.

4. Applying an older or newer edition’s values. Air leakage criteria and verification rules changed between editions. Check the adopted edition before writing the specification.

Frequently asked questions

What is the maximum air leakage rate for commercial buildings under ASHRAE 90.1-2019?

Standard 90.1-2019 §5.4.3.1 lists 0.004 cfm/ft² at 75 Pa for materials, 0.04 cfm/ft² for assemblies, and 0.40 cfm/ft² for the whole building, with exceptions that include testing portions of a building, an allowance up to 0.60 cfm/ft², and compliance by inspection and verification (DOE Building Energy Codes Program, 90.1-2019 webinar).

What is the residential blower-door limit by climate zone?

Under the 2021 IECC R402.4.1.3 (prescriptive path), 5.0 ACH50 in climate zones 0, 1 and 2 and 3.0 ACH50 in climate zones 3 through 8; R402.4.1.2 caps every path at 5.0 ACH50 or 0.28 cfm/ft² of enclosure area. Check your jurisdiction’s adopted edition and amendments.

When must a commercial building undergo whole-building air barrier testing?

It depends on the adopted edition, the building size and the path chosen. Under 90.1-2019 the whole-building criterion is 0.40 cfm/ft², with a verification alternative. Ask the AHJ which path it accepts and read §5.4.3.1 and §5.9 of the adopted edition.

Where do air barrier failures occur?

At penetrations, at transitions between assemblies, and at tape seams. Failures behind finishes are hard to find, so inspect before closing walls or test the building.

Sources

• U.S. DOE Building Energy Codes Program, Standard 90.1-2019 webinar slides (air leakage criteria, §5.9 verification).
• Oregon Building Codes Division, Measured Air Leakage Results Reporting form (§5.4.3.1 reporting, third-party tester).
• Minnesota Department of Labor and Industry, Residential code TAG handout (model text of 2021 IECC R402.4.1.2 and R402.4.1.3).
• ASHRAE 90.1 itself is paywalled; read §5.4 and §5.9 of the adopted edition before specifying.