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 moisture 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.
StoGuard VaporSeal. Modera Central in Orland, Fla. Photo courtesy of Sto Corp.
What is an Air and Moisture Barrier?
The Air Barrier Association of America (ABAA) defines an air and moisture barrier as a system of building assemblies within the building enclosure—designed, installed and integrated in such a manner as to stop the uncontrolled flow of air into and out of the building enclosure.[1] Using an air and moisture barrier benefits the building in two ways. The air barrier seals leaks and creates an airtight building envelope, while the moisture 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. But not all states have adopted the ASHRAE/IES Standard 90.1-2010 (90.1-2010) and the commercial provisions of the 2012 International Energy Conservation Code (2012 IECC).[2]
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 moisture 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.
Main Sail at Shelter Cove, Hilton Head Island, S.C. Photo courtesy of Sto Corp
Air Leakage and Occupant Comfort
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 Moisture Barrier
Air and moisture 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 moisture 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 moisture 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. Air and moisture barriers that are not a permanent part of the structure may move independently of the structure, stressing the structural materials and claddings as well as displacing air in the cladding cavity. In addition, the system must be durable. Air and moisture barriers are typically installed behind the cladding of a building and may require removal of the cladding for any type of maintenance or repair. For this purpose, a highly durable air barrier will always be preferred so that maintenance can be avoided.
Photo courtesy of Sto Corp.
Types of Air and Moisture Barriers
There are numerous types of air and moisture 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 moisture 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 moisture 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 moisture 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. Silicone and STPE based products, a hybrid of silicone and polyurethane technology, are less susceptible to wash off and they dry faster in wet conditions than more traditional acrylic based membranes. When using a fluid-applied air and moisture barrier you rely on the applicator for the proper thickness.
The latest technology to enter the market is the pre-coated sheathing. This air and moisture 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 moisture barrier. Using a pre-coated sheathing is typically more expensive and is still so new to the market that there needs to be more time to assess the durability of this choice.
Regardless of what type of air and moisture barrier you use (and they all have their use), the benefits of including an air and moisture 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.
[1] Source: The Air Barrier Association of America (ABAA) http://www.airbarrier.org/
[2] Source: Building Codes Assistance Project (BCAP) http://bcapcodes.org/code-status/commercial/
About the author
Karine Galla
Senior Product Manager for Sto Corp.
Karine Galla is Senior Product Manager for Sto Corp. She has more than 17 years of experience in product marketing in EIFS, stucco, air and moisture barriers, and other materials. Karine has a master’s degree from the University of Lyon, France. She is multilingual 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.
