For compliance with energy code requirements to control whole-building air leakage and to protect assemblies from the potential damaging effects of moist air intrusion, it is necessary to consider and air barrier strategy for your building walls and the building as a whole. Controlling air leakage around windows also lessens the effect of pressure differentials at window-to-WRB flashing interfaces such that water leakage risk is reduced.
Like continuous insulation and the WRB system, an air barrier system must also be continuous to be effective. This means seal the air leaks at joints, interfaces with various components that penetrate the wall, and at interfaces between the wall and other building assemblies (e.g., floors, roofs, and foundation).
Modern energy code requirements for residential and commercial construction provide a detailed list of air barrier and air sealing requirements. For one- and two-family dwellings and multi-family buildings ≤3 stories, the air barrier and whole building air leakage testing requirements (i.e., blower door test and maximum air leakage rates) are found in the 2024 IECC Section R402.5. For commercial and Group R buildings (>3 stories), the applicable requirements are found in 2024 IECC Section C402.6. Similar requirements are found in ASHRAE 90.1-2022, Section 5.4.3.
There are many choices for air barrier materials and methods and also the location of the air barrier in a wall assembly. One of these material choices is to use the FPIS-ci as an air-barrier on the exterior side of the assembly. This is particularly efficient when the FPIS-ci is also used as the WRB since this would already require all joints to be sealed.
Design Resources
For a review of air barrier options and practical design and installation guidance, refer to the following series of articles:
- Code Compliance Report: Foam Plastic Insulating Sheathing Used as an Air Barrier Material in an Air Barrier Assembly
- Additional Air Barrier resources