Creating a tight envelope is essential to any building. A tight building envelope ensures air and moisture stays outside of a building, where it belongs. There are three ways moisture or water vapor can enter a building cavity: with air currents, by diffusion through materials, or by heat transfer. When moisture enters the building envelope, it can cause a variety of problems, including mold, corrosion and rotting. One way to make sure your building stays dry is to use an air and water barrier.
The key to managing moisture and air in any wall assembly is continuity. “All these different layers and parts of the walls, roofs, foundations-the building envelope-must be continuous and complimentary to manage the three fundamental building science principles of air, heat, moisture effectively,” explains Ted Winslow, product manager, building science, systems and technical marketing, insulation at CertainTeed Corp., Valley Forge, Pa.
The use of air and water barriers is one way to control the amount of air and water that seeps into a wall assembly. Tom Diamond, PE, project engineer at The Garland Co. Inc., Cleveland, says air barriers can be formulated as membranes that completely block vapor transmission, as in a vapor-closed, vapor barrier profile; allow vapor to slowly transmit through the membrane in a semipermeable vapor retarder profile; or allow vapor to transmit through the membrane at a faster rate in a vapor-open, vapor permeable profile.
Air resistance, water resistance, vapor permeability and durability are the key characteristics Winslow says need to be considered regardless of whether you are using an interior or exterior air barrier approach. Additionally, “It is important for air and moisture barriers to be able to adapt to unforeseen infiltration of moisture into the wall cavity because today’s assemblies have a much lower drying potential,” he explains.
“Air control, vapor control and water control are three very different things, but one product can serve as the air, vapor and water control layer,” Diamond says.
Choosing the Right Barrier
When it comes to air and water barriers, it is important to determine the proper application and type to use. There are a number of different types of air/water barriers in the market today, with self-adhering membranes and fluid-applied membranes the most prominent. “Fluid-applied is great for inconsistent surfaces such as masonry or CMU block wall,” Diamond explains.
“Self-adhering membranes are great products to use with new and even substrates such as exterior grade gypsum board or plywood. However, special care must be taken when using sheet good membranes, as each lap in the membrane must be sealed properly. This can be become much more labor intensive when compared to fluid-applied systems.”
Marysusan Couturier, principle scientist at W.R. Grace Co., Columbia, Md., notes that fluid-applied air barriers are seamless, provide quick coverage and require minimal substrate preparation, while self-adhering membranes have controlled thickness and require no special spray equipment. Other types are mechanically fastened with taped seams, taped boards and spray foam.
Kevin D. Nolan, technical director North America for
VaproShield, Gig Harbor, Wash., explains that all buildings need to manage moisture, regardless of where they’re located. “There are different strategies for wet climates versus drier, more arid climates,” he says. For example, wet weather climates require redundancy in their design, and Nolan says the best way to achieve that is with a drainage or cavity system designed in the enclosure.
Large portions of North America are considered mixed-climate regions, where Winslow says the moisture drive direction is balanced between winter and summer. “In these regions, buildings using traditional polyethylene vapor retarders may successfully keep moisture out of the cavity in the cold season, while essentially trapping it there during summer when the moisture drive reverses,” he explains. “This problem is further exacerbated by the use of moisture-retaining cladding, such as masonry, fiber cement and stucco, which release moisture into the building cavity.”
“Typically, colder climates create very strong vapor drives during winter months,” Diamond adds. “Warm moist air wants to drive to the colder, dryer climate on the outside of the building. As air tries to move through the building envelope, it will carry any moisture present within the building along with it.” This is why, he says, air barriers with a vapor-closed profile are more commonly used in northern climates.
However, Diamond says buildings located in warmer climates, where the vapor drive is not as intense, tend to utilize a vapor-open profile in the air barrier to increase the drying potential of the building envelope.
Couturier says the moisture storage function of the building’s exterior cladding, as well as the type and location of the insulation relative to the air barrier also plays a role in determining the type of air barrier to use. “A hygrothermal analysis will assist in determining which water vapor permeance of the air barrier is needed,” she says. “It is important to let the wall dry out and not trap moisture, which could cause mold.”
Installation Tips
Once the type of air and water barrier is determined, Diamond says to ensure its long-term performance, it is critical to ensure that product will properly adhere to the surface of the structure that it is being applied to. “If the product delaminates from the surface, air pockets and voids can lead to unwanted moisture accumulation within the assembly,” he explains. “Special care must also be taken to seal the air and water barrier around penetrations, transitions and end laps of the products. Just one minor discontinuity in the air barrier can lead to air movement, which carries heat as well as moisture in the vapor form. This may drastically affect the overall performance of the roof or wall assembly.”
According to Winslow, there are a variety of strategies that can be employed to address the air and moisture resistance of an assembly. The most common approaches for installation include using tapes and sealants (i.e., caulks, adhesives, etc.), or stapling. Winslow says the method chosen is usually be preference of the installer and the type of framing system, such as metal framing, which may require the use of sealant and/or tape only.
In metal buildings, Diamond says that moisture barriers need to be provided by rigid board insulation or insulated metal panels. “In these scenarios, the sealing of the joints of the boards or panels is very critical,” he explains. “Special care must be taken when installing the sealants and tapes required to provide the moisture barriers. Thousands of linear feet of joints may be present. Even one foot of the seal being inconsistent has the potential to create issues.”
Nolan recommends having a complete set of shop drawings for the air and water barrier so the contractor understands how it is to be installed and what penetrations need to be interfaced with it to make it continuous. He also recommends installing the system in a cavity or rainscreen fashion so it allows redundancy and provides a method to drain water that gets past the exterior cladding. “That alone provides a capillary break that really limits the amount of moisture that can penetrate through the fastener locations,” he adds.
To verify the leakage of an assembly, Winslow says it is important to conduct testing (e.g., blower door testing), to verify leakage rates in the wall assembly. Ideally, this should be done at different phases of the construction by professional certified and rated contractors.
Effective communication between the various trades on a project is also important. “All too often the effectiveness of the air barrier can be compromised due to ineffective communication or responsibility of the trades involved,” Winslow notes. “Plumbers, electricians, insulation contractors, drywall contractors, general contractors and many more trades are all responsible for maintaining the airtightness of a wall and all of them must be on the same page when it comes to understanding their impact for a successful airtight assembly.”
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All-In-One Systems
Traditionally, air barriers are installed as part of a multi-component wall system. However, some manufacturers offer air barriers as part of a singlecomponent system, such as Moon Township, Pa.-based CENTRIA’s Formawall Dimension Series. The all-in-one system is made up of architectural insulated metal panels (IMPs) that include factory foamed-in-place foam insulation sandwiched between a metal face and liner and acts as the exterior rainscreen, air and moisture barrier, and thermal insulation. CENTRIA’s Advanced Thermal and Moisture Protection “A vented horizontal joint is designed for pressure equalization, and, even in the presence of an imperfect air barrier, the pressure-equalized joinery maintains the systems performance integrity,” explains Greg Lusty, director of product management and R&D at CENTRIA. “With a barrier system, imperfections can lead to moisture infiltration.” Installed using concealed clips and fasteners attached to structural supports, Formawall panels don’t require any exterior gypsum or weather barriers as they act as the building’s weather barrier. Insulated metal composite panels have a sandwich-type design with metal facers surrounding a closed-cell insulation core. Sealed to create a continuous barrier, the materials are not conducive to water retention. “Metal is also impervious to vapor diffusion, and closed-cell insulation has a much denser and more compact structure than most other insulation materials and functions well in air and vapor barrier designs,” explains Lusty. “It is nonporous and does not allow moisture to become entrapped.” To learn more about CENTRIA’s Formawall System, visit www.centriaperformance.com. |
Energy Codes
When it comes to building and energy codes, Nolan says the most important thing to know is that they are changing quickly. As different states and municipalities are adopting building and energy codes, he notes that there may be changes, especially within the area of air barriers, which can change the way buildings are assembled. For example, he notes that in Seattle there are requirements for both air barriers and continuous exterior insulation, which play into how they are installed, avoiding thermal breaks and creating a truly continuous barrier.
According to Winslow, airtightness is rapidly becoming central to new building codes, affecting all aspects of the building envelope performance. However, he says, as the airtightness of a building increases, moisture creates another new challenge. “The same continuous seal that keeps air leakage out of the building could be trapping harmful moisture in the cavity,” he explains.
Winslow goes on to say that airtightness without proper moisture management can eliminate moisture’s escape path, trapping it within the walls. “The hidden consequences of this moisture buildup could include loss of thermal efficiency, along with potential mold and rot issues,” he adds.
The International Energy Conservation Code (IECC) requires a continuous air barrier be provided throughout the entire building thermal envelope for most climate zones, while the International Building Code (IBC) provides requirements for vapor retarders to be present in an exterior wall assembly depending on the climate zone.
Couturier says that in 2005 the National Institute of Standards and Technology (NIST) reported results of an investigation that showed the use of an air barrier can save an average of 33 percent in energy savings. “We are striving to improve the energy efficiency of buildings as seen in the more stringent requirements for air barrier and continuous insulation in the 2012 IECC codes,” she adds. “Because of this increased insulation, which can be combustible, there are more stringent fire codes such as NFPA 285. Air barriers must now also meet this fire requirement if they are used on exterior wall with other combustible components.”
