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.

Successful moisture control requires an integrated approach to 5 key building science concepts:

  1. Control Rain Water Intrusion (e.g., continuous water-resistive barrier)
  2. Control Air Leakage (e.g., continuous air barrier)
  3. Control Indoor Relative Humidity (e.g., building ventilation & de-humidification)
  4. Control Initial Construction Moisture (e.g., prevent enclosure of wet materials)
  5. Control Water Vapor (e.g., optimized balance of wetting and drying through strategic use of insulation and vapor retarders)

Tools & Education

This wall calculator is a tool to help coordinate energy code thermal insulation compliance and building code water vapor control compliance for a proposed wood frame wall assembly on a commercial or residential building.

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

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.

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.

This Research Report provides actionable guidelines to effectively consider, evaluate, and manage the balance of wetting and drying potential in common above-grade, light-frame wall assemblies in a broad range of climate conditions.

A comprehensive assessment of the state-of-the-art of water vapor control for modern, code-compliant, light-frame wall assemblies.

Overview of key building science concepts, U.S. & Canadian code requirements, accepted methods of design to control water vapor, and design solutions for all climate zones and walls with or without continuous insulation.

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.