Water Vapor Control

Exterior foam sheathing provides one of the most robust and reliable methods of managing water vapor to properly balance wetting and drying potential with appropriate application of vapor retarders in coordination with insulation strategies using foam sheathing alone (e.g., the “perfect wall”) or in combination with cavity insulation.

Top Resources for Water Vapor Control:

Tools & Education

This presentation provides guidance for and examples of the new building code advancements and their effective application for design and construction of code-compliant, durable, and energy-efficient building enclosures.

This session, presented at IBS 2017, provides actionable prescriptive guidelines for integrated insulation and vapor control strategies. 

This presentation, given by Jay Crandell (ARES Consulting) and Amy Schmidt (Dow Building and Construction) at the 2018 RESNET Building Performance Conference covers topics in energy code compliance, such as calculating the R-value of a hybrid wall and determining air space performance contributions.

This presentation, given by Jay Crandell, ARES Consulting, and Amy Schmidt, DuPont, at the 2020 RESNET BUILDING Performance Conference, explores why walls should be a priority and the impact of ignoring energy efficiency, durability, comfort, and serviceability.

Best Practices

This guide from Building America addresses various means of applying foam sheathing on 2x4 wood framed walls in climate zones 3-5, including integration with other components (e.g., windows and cladding) for foam thicknesses up to 1-1/2”. It also addresses air barriers,  water vapor retarders, and use of foam sheathing as a WRB or in combination with a separate membrane WRB.

Additional Information

Read to see how foam plastic insulating sheathing can provide right water vapor permeance to prevent the inward vapor drive problem in hot/humid climates and behind stucco or other reservoir claddings in any climate

Drying potential is only one factor when evaluating the performance of a wall. Reducing wetting potential, along with selecting an appropriate vapor retarder, are crucial factors to consider when detailing durable and energy efficient wall assemblies suited for any climate.

Practical Research

This presentation to the ASHRAE 160 hygrothermal modeling committee explains why it is important to protect OSB and other moisture-sensitive sheathing materials from excessive seasonal moisture content cycling. Building codes and current hygrothermal modeling criteria do not completely address this problem. Appropriate use of exterior continuous insulation in coordination with interior vapor retarders offer the “driest” solutions.