Architect’s Corner: Back to the Basics

An Introduction to Air & Water-Resistive Barriers in Wall Assemblies

By Karine Galla, Director, Product Management for Sto Corp.

(Editor’s Note:  Karine Galla is Director, Product Management for Sto® Corp.  She has more than 20 years of experience in product marketing in EIFS, stucco, air and water-resistive barriers, and other materials.   Galla has a master’s degree from the University of Lyon, France, and holds AWCI’s EIFS Doing it Right and Building Envelope Doing it Right certifications, as well as the ISO Internal Lead Auditor certification from Georgia Tech.)

Uncontrolled airflow through the building enclosure can significantly degrade a building’s performance in several ways, contributing to reduced energy efficiency, water damage and mold, and poor indoor air quality and thermal comfort for occupants.  An effective air and water-resistive barrier can help prevent these problems by protecting the building against both air leaks and water intrusion, and hence is becoming widely used in all types of residential and commercial constructions. 

         The Air Barrier Association of America defines an air and moisture barrier or water-resistive barrier as a system of building assemblies within the building enclosure that is designed, installed, and integrated in such a manner as to stop the uncontrolled flow of air into and out of the building enclosure.  Using an air and water-resistive barrier benefits the building in two ways.  The air barrier seals leaks and creates an airtight building envelope, while the water-resistive barrier is designed to protect building components against bulk water intrusion while allowing water vapor to diffuse and dry out.

Air Leakage and Energy

         Air leakage through the building envelope can account for up to 50% of the energy consumption used to condition the building.  An airtight building envelope can significantly reduce the energy needed to heat and cool the building.  Reduction of energy use is receiving a lot of attention and building codes across the country have taken note, recommending air barrier systems as a critical component for increasing the operating efficiency of buildings.  Many states now have a code requirement for air barriers that is at least recommended in new construction, if not mandatory.  Some states even require blower-door testing to ensure that the building’s envelope is airtight.  However, not all states have adopted the ASHRAE/IES Standard 90.1-2022 and the commercial provisions of the 2018 International Energy Conservation Code. 

Air Leakage and Moisture

         Most moisture is carried by air, and when this moisture gets into the building’s walls, it can condense and cause water damage and mold.  Air and water-resistive barriers protect the wall from moisture by minimizing the airflow through the wall, thus reducing the risk of water damage and associated repair costs.  Condensation, leaks, and insufficient drainage are the most common causes of moisture problems in a building.

         Drafts from leaky building enclosures, both cold and warm, significantly reduce the ability to control indoor temperatures, which directly affects occupant comfort.  In addition, uncontrolled airflow allows for external pollutants, odors, and noise to enter the building.  Finally, mold and mildew growth within the wall assembly can drastically affect the quality of the indoor air, and can cause serious health issues for occupants.

Installing an Effective Air and Water-Resistive Barrier

         Air and water-resistive barriers are being installed not only in new construction, but on buildings that are being renovated as well, making them more energy efficient and improving the overall longevity of the buildings.  Not surprisingly for such a critical component of a building, proper installation is the key to performance.

         Air and water-resistive barrier systems are designed to control air infiltration and exfiltration in buildings.  They accomplish this by sealing the wall with a continuous membrane.  This includes sealing joints, penetrations, and rough openings to create an airtight assembly.  Air and water-resistive barrier systems should meet three key criteria.  First, they should be continuous so as not to allow any opportunities for air leakage.  Second, they should be structural in the sense that they should be permanently secured to the supporting structure.  Third, they must be able to withstand wind pressure, stack pressure, and any pressure caused by mechanical effects, which is ultimately transferred to the structure.

Types of Air and Water-Resistive Barriers

         There are numerous types of air and water-resistive barriers available on the market today, including wraps, self-adhered membranes, fluid-applied membranes, and pre-coated sheathing.  The wrap is the most widely used, especially in the residential market.  It is the least expensive, can be applied in low and high temperatures, is not affected by rain, and is easy to install.  The downside is that they lack in durability and there can be leaks due to perforation by the fasteners.

         Self-adhered membranes can also be applied in low and high temperatures.  Unlike the wraps, they do not rip or tear, and do not require fasteners.  As with all air and water-resistive barriers, there are also downsides.  Applying a self-adhered membrane can be a two-step process, using a primer with the self-adhered membrane.  They can be hard to handle when dealing with critical detailing and the peel-and-stick membrane needs to be cut to size.

         Fluid-applied air and water-resistive barriers offer seamless protection, do not require fasteners, and are ideal to handle critical details and reach hard to get to places, and they don’t rip or tear.  A primer is not needed with a fluid-applied air and water-resistive barrier, and it is easily applied using a roller, a trowel, or a sprayer.  The downside to these types of membranes is that they do need time to dry and cure prior to applying the next layer of the wall assembly, typically not more than 24 hours depending on conditions.    The latest technology to enter the market is the pre-coated sheathing.  This air and water-resistive barrier provides added protection for the sheathing board against rain and moisture even before installation and saves on time and labor.  The applicator, however, still needs to treat the joints and fasteners with an air and water-resistive barrier. 

         Regardless of what type of air and water-resistive barrier you use, the benefits of including an air and water-resistive barrier system in your building are abundant.  The key to having a successful air barrier system is to make sure that it is continuous, structural, and durable, while remembering to account for thermal changes as well as movement.

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