Building codes require that water vapor retarders be provided on the interior side of walls (warm in winter side) for mixed and cold climates to control water vapor diffusion and potential for condensation or moisture accumulation within assemblies during the winter. However, the older rules of thumb have become outdated with newer building materials, insulation materials, and energy code insulation requirements. Thus, building code vapor control provisions have been significantly updated. The guidance provided in the Design Resources section is based on and serves as the basis for building code improvements for water vapor control found in the 2024 editions of the IBC (Section 1404.3) and IRC (Section R702.7).
Design Resources
Use the following resources to evaluate the water vapor control performance and drying potential of walls using FPIS. Be prepared to revise your insulation strategy determined in STEP 1 to achieve optimal moisture performance while also meeting or exceeding the energy code requirements for wall insulation.
- Online Wood Frame Wall Thermal/Moisture Calculator
- ANSI/ABTG FS200.1 – 2022 "Standard for Use of Foam Plastic Insulating Sheathing (FPIS) in Building Envelopes: Above-grade Walls
For additional information on water vapor control and related moisture control topics, refer to our page on Water Vapor Control.