Designing the overlooked transition between air barrier and fenestration

• C402.5.1.2.1 Materials, which states, “Materials with an air permeability not greater than 0.004 cfm/ft² (0.02 L/s•m²) under a pressure differential of 0.3 inch of water gauge (75 Pa) when tested in accordance with ASTM E2178 (Standard Test Method for Determining Air Leakage Rate and Calculation of Air Permeance of Building Materials) shall company with this section. Materials… shall be deemed to comply with this section, provided that joints are sealed and materials are installed as air barriers in accordance with the manufacturer’s instructions.”
• C402.5.1.2.2 Assemblies, which states, “Assemblies of materials and components with average air leakage not greater than 0.004 cfm/ft² (0.02 L/s•m²) under a pressure differential of 0.3 inch of water gauge (75 Pa) when tested in accordance with ASTM E2357 (Standard Test Method for Determining Air Leakage Rate of Air Barrier Assemblies), ASTM E1677 (Standard Specification for Air Barrier (AB) Material or Assemblies for Low-Rise Frame Buildings Walls) or ASTM E283 (Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Skylights, Curtain Walls, and Doors Under Specified Pressure Difference Across the Specimen) shall company with this section. Assemblies… shall be deemed to comply with this section, provided that joints are sealed are the requirements of Section C402.5.1.1 are met.”

The air leakage of fenestration assemblies is determined in accordance with AAMA/WDMA/CSA 101/I.S.2/A440, North American Fenestration Standard/Specification for windows, doors, and skylights (NAFS), ANSI/NFRC 400, Procedure for Determining Fenestration Product Air Leakage, or ASTM E283. Along with storefront and curtain wall, fenestration in architecture refers to openings in the building envelope that include windows, doors, skylights, vents, and more.

A sampling of maximum air leakage rates for fenestration assemblies per the 2018 IECC C402.5.2 Table include:

Key considerations: Curtain wall

Storefront and curtain wall systems share similar functions and appearance. Both can protect the building and its occupants from weather, while providing daylight to the interior and views to the outside. Beyond these commonalities, these fenestration systems have numerous differences in how each best suits its intended application.