Section 5.4.3/Air Barrier Requirements
ASHRAE 90.1 Section 5.4.3 requires a continuous air barrier throughout the building thermal envelope to control air leakage.
A continuous air barrier must be provided throughout the building thermal envelope. The air barrier must be continuous across all assemblies (walls, roofs, floors) and across all joints, transitions, and penetrations. Air barrier materials must have an air permeance not exceeding 0.004 cfm/sf at 0.3 inches water gauge (1.57 psf) when tested per ASTM E2178. Air barrier assemblies must not exceed 0.04 cfm/sf. Whole-building air leakage testing per ASTM E779 or ASTM E3158 is an alternative compliance path.
Why this section exists
Air leakage through the building envelope can account for 25-40% of heating and cooling energy in commercial buildings. Even well-insulated walls lose their effectiveness when air bypasses the insulation through cracks, joints, and penetrations. The air barrier requirement addresses this by requiring a tested, continuous layer that resists air movement regardless of wind pressure or stack effect. The requirement was significantly strengthened in recent ASHRAE 90.1 editions.
What plan reviewers look for
Plan reviewers check the architectural drawings for air barrier material identification on wall sections, roof details, and floor assemblies. They look for continuity details at transitions: wall-to- roof, wall-to-foundation, wall-to-window, and wall-to-door frames. They verify that penetrations (pipes, conduits, ducts) through the air barrier are sealed. If whole-building testing is the compliance path, they check for the testing specification in the project documents.
Common violations
Related ASHRAE 90.1 requirements
Section 5.5.3 covers opaque envelope insulation that works with the air barrier. Section 5.5.4 covers fenestration air leakage rates. IECC Section C402.5 provides equivalent air barrier requirements. Section 5.4.3.2 covers the whole-building air leakage testing option.