Section C402.5/Air Leakage
IECC C402.5 covers air barrier installation, fenestration air leakage rates, and building pressurization testing.
The building thermal envelope must include a continuous air barrier complying with one of three options: materials with an air permeance not exceeding 0.004 cfm/sf, assemblies tested to not exceed 0.04 cfm/sf, or whole-building testing demonstrating air leakage not exceeding 0.40 cfm/sf of envelope area at 75 Pa. Fenestration products must have an air infiltration rate not exceeding 0.20 cfm/sf for operable windows and 0.04 cfm/sf for fixed windows. Loading dock weather seals and vestibules at building entrances are also addressed.
Why this section exists
Uncontrolled air leakage through the building envelope is one of the largest sources of energy waste. Air carries both sensible heat (temperature) and latent heat (moisture), making infiltration especially costly in humid climates where dehumidification energy is high. A building with good insulation but a leaky envelope wastes much of the insulation benefit. The air barrier requirement ensures that the envelope controls air movement, not just heat conduction.
What plan reviewers look for
Plan reviewers check the architectural wall sections and details for the designated air barrier material. They verify continuity at all transitions (wall-to-roof, wall-to-slab, wall-to-window). They check the fenestration schedule for air infiltration rates. They also check for vestibules at building entrances in climate zones 3 through 8 and weather seals at loading dock doors.
Common violations
Related IECC requirements
Section C402.1 covers overall envelope requirements. Section C402.4covers fenestration thermal performance. ASHRAE 90.1 Section 5.4.3 provides equivalent air barrier requirements. Section C402.5.1 covers the specific air barrier installation requirements.