February 8, 2016
by John Chamberlin
At this point, most design/construction professionals have a pretty good understanding of the need for well-designed air and moisture control layers in wall assemblies. Water-resistive barriers (WRBs) have been in use for decades; in more recent years, a number of new systems have popped up that combine these products with an air barrier function, effectively creating ‘air and moisture barriers’ as the gold standard, if not the new norm.
Aside from simply keeping liquid water on the exterior of the building where it belongs, air and moisture barriers help reduce unwanted air movement through a building, which, in turn, lowers unnecessary energy consumption, helps prevent mold and mildew growth on the interior, stops pollutants such as radon gas and allergens from entering the building, and improves indoor occupant comfort by reducing drafts and external noise. Since air movement is also the primary source of condensation and moisture damage inside the wall cavity, an air and moisture barrier system also helps reduce structural degradation by keeping out water in both liquid and vapor forms.
If these benefits are not enough motivation in their own right, most states have adopted requirements for continuous air and moisture barriers either in their commercial, residential, or energy conservation codes (or a combination thereof). Understanding the benefits and the requirements are the first steps toward ensuring all buildings constructed include these critical systems in their design.
The proof of concept already exists as continuous air and moisture barriers systems have been in place long enough one can now see the positive long-term effects they have on buildings and their occupants, and a number of studies have been done to support this.1 However, as with so many things, reality may be more complicated than theory. The most important criteria for air and moisture barriers systems is for them to be continuous around the entire building. This continuity is where special attention and consideration needs to be given.
That buildings experience a degree of movement is something that must be taken into account and designed around, depending on the project’s climate zone and the materials comprising it. While this is not new information, it is still important to be aware that for a variety of different reasons, buildings (or their components) will continue to move as long as the structure is standing.
In looking at the causes for building movement, certain recurring themes emerge. Moisture is a major cause for building movement as it causes materials to degrade more quickly. Further, building components made from wood, stone, concrete, and brick expand and contract depending on their moisture content and temperature variations.
The ground on which buildings stand may also be prone to shifting or settling. Subsoil movement may be a result of expansion and contraction of the soil due to changes in moisture content. Some soils may experience shear or loadbearing failures, or any number of other external factors may cause the ground to move resulting in the slight shifting of buildings.
Building movement is accounted for with construction elements like masonry control joints. However, certain elements of the building move separately from adjacent elements—frequently manifesting as movement in areas such as where foundations meet walls, where walls meet doors and windows, where different wall systems meet, where different substrate materials meet, and at movement joints such as masonry control joints and floor line deflection joints.
This leads to an important question: “If we know our air and moisture barrier systems need to be continuous, and we know that buildings move, how do we ensure continuity of our air and moisture barriers systems and conditions where movement occurs?”
Pinpointing the problem
In 2010, at the National Institute of Building Sciences’ (NIBS’) Building Enclosure Science and Technology (BEST) 2 conference in Portland, Oregon, Peter Poirier and Brian Stroik presented a paper on eliminating the potential for air and moisture infiltration at the window-wall interface.2 The window-wall interface is an excellent example of a critical detail where special attention needs to be given not only to the materials used within the air and moisture barrier assemblies, but also to how those materials and assemblies perform over a long period given the building does not exist in a static environment.
In the paper, Poirier and Stroik pointed to a number of conventional methodologies used to determine whether or not a material is appropriate for use in an air and moisture barrier assembly. The first standard mentioned by the duo—ASTM E2357, Standard Test Method for Determining Air Leakage of Air Barrier—is frequently used by building product manufacturers as an initial way to validate their air barrier systems. The test looks at performance of assemblies applied to an opaque wall, as well as a wall with numerous pre-defined penetrations such as windows, electrical boxes,
and pipes. While this test does a good job of determining compatibility of the different components within the assembly, it does not address real-life site conditions or construction practices, and it does not account for movement of the building.
To account for building movement, Poirier and Stroik pointed to the 2009 International Energy Conservation Code (IECC); in Chapter 5, “Commercial Energy Efficiencies: Sealing of the Building Envelope,” it states:
openings and penetrations in the building envelope shall be sealed with caulking materials or closed with gasketing systems compatible with the construction materials and locations. Joints and seams shall be sealed in the same manner or taped or covered with a moisture vapor-permeable wrapping material. Sealing materials spanning joints between construction materials shall allow for expansion and contraction of construction materials.
Armed with some guidance regarding how materials should perform, and what is expected of them in the service of the building, the next step is to explore some of the most commonly used products for these applications.
Unquestionably, the most popular products used for continuity of air and moisture barrier systems are self-adhered membranes. Among these products’ many advantages is their simplicity. The concept of taping over an opening to stop air and water moving in is straightforward, resonating on a very basic level—tears and holes can be fixed with tape.
Beyond the obvious, there are numerous features that make sense with self-adhered membranes. Typically the most economical option, they come in all different shapes and sizes, but are usually very manageable and not too heavy. They have a convenient uniform thickness throughout the roll to ensure one knows exactly how much material is being used. The carrier sheet is usually flexible so it can be bent, folded, and stretched; the sheet itself can be made up of any variety of materials to offer different performance characteristics. Further, the adhesives themselves have come a long way over the years, and now feature different chemistries boasting their own pros and cons.
Once the self-adhered membrane is put in place, a very aggressive bond should occur. Therefore, regardless of the type of building being designed or the climate it will endure, there is likely some version of suitable self-adhered membrane.
Concerns about self-adhered membranes do exist, though. In particular, installation is not as simple
as it seems. In most cases, self-adhered membrane manufacturers recommend a primer on the substrate before installation of the membrane begins. Once the surface is primed, the self-adhered membrane can be laid flat onto the surface to create a bridge in the air and a moisture barrier assembly allowing for continuity. Unfortunately, most self-adhered membranes are not meant to be used with first joints larger than 13 mm (1/2 in.), so additional pieces may need to be lapped onto each other to properly move across the transition.
Typically, self-adhered membranes bond well to each other, so there is no need for a primer where two pieces of tape meet. However, many manufacturers recommend an extra step to seal the edges, meaning a mastic may be needed around all outside edges of the self-adhered membrane pieces.
It is important to remember that buildings are geometrically complex, and the more complex they get, the more difficult it is to fit a piece of tape or several pieces of tape together to fit the critical details and keep air and moisture out. When considering inside and outside corners, recessed windows, and any number of pipes sticking out of the side of the building, there may be a few conditions where one must splice, fold, stretch, and counter-flash to not only ensure water is being kept out, but also to ensure there are no wrinkles or fish mouths in membranes that might inadvertently create new problems.
In Joe Lstiburek’s classic article “Stuck on You,” he refers to this need of trying to fit self-adhered flashing tapes to complex geometries as a trade’s need to learn origami to properly wrap openings.3
Further, these products are rarely designed to account for thermal or seismic movement. So, while an aggressive bond between the membrane and the other building components will likely exist if the membrane can be properly applied, the long-term effectiveness of this method may still be questionable.
If self-adhered membranes are the most well-known means of maintaining continuity in air and moisture barrier systems, then the combination of silicone sealants and silicone extrusions are probably the most well thought of in terms of durability, function, and overall performance. Silicone sealants are extremely common in construction in areas where a sealant is needed to prevent air and moisture infiltration. While ‘silicone sealant’ is often used as a very broad term, most designers and contractors know these sealants come in a number of different varieties, including high- and low-modulus formulations that may be used for transfer of structural loads or for use in high movement areas, respectively.
Of course, silicone sealants by themselves cannot bridge wider gaps and joints, so they are frequently coupled with silicone extrusions. These extrusions are pre-formed rubberized silicone membranes available in a variety of sizes, styles, and, in some cases, even pre-formed shapes. Silicone extrusions provide excellent flexibility in movement conditions with less likelihood of tearing or loss of adhesion which may be seen when self-adhered membranes are used.
To evaluate this, specific tests have been designed to assess the effectiveness of silicone extrusion and silicon sealant assemblies. ASTM C1518, Standard Specification for Precured Elastomeric Silicone Joint Sealants, is a very rigorous set of tests requiring the silicone sealant and silicone extrusion assembly be tested in various climate conditions. It looks to ensure long-term adhesion and elongation of the assembly without tears or other failures after extensive movement of the assembly.
Installation is fairly easy with silicone assemblies. In this case, silicone sealants may be used as the adhesive for the extrusion membrane. Once the sealant has been applied to the surface, the extrusion is pressed into the sealant and smoothed out to remove air bubbles. Additional sealant may be used along the edges of the extrusion to ensure an air- and watertight seal similar to how the mastic is used with the self-adhered membranes. The difficulty with silicone-based assemblies comes from their compatibility with other components within the air and moisture barrier system.
Unfortunately, not all silicones are created as equals. That means before constructing an air and moisture barrier assembly with silicone sealants and extrusions, compatibility between the components must be verified. Typically, this is not a problem when considering the silicone components because a single manufacturer will very likely offer both the sealant and a compatible extrusion. What about the other components of the system, though?
The drawback of silicone systems is typically the cost of the materials. A silicone extrusion can cost more than five times the price of a similarly sized self-adhered membrane. While higher performance comes from the silicone extrusion, it is often more than what is necessary, and the self-adhered membrane (or any number of other products) will suit the detail well enough.
For this reason, silicone sealant and extrusion assemblies are frequently connected to components composed of different chemistries. Unfortunately, silicone does not adhere well to a large number of these components—therefore, compatibility between the various different components must be proven before application can occur.
Elastomeric cores in fabric facings
A new option in maintaining continuity of air and moisture barrier systems in movement conditions comes in the form of an elastomeric core encased in a fabric facing. Due to the elastomeric core, these fabric-faced transition membranes feature a high degree of flexibility similar to silicone extrusions, allowing for continuity of the air and moisture barrier systems as the building expands and contracts over time. The fabric facing on these products becomes critical for proper installation as they are used exclusively in fluid-applied air and moisture barrier systems.
The process may seem familiar, but in many ways it is much simpler than what is done with both self-adhered membranes and silicone extrusions. In fluid-applied systems, a coating or liquid membrane is used to cover the field of a wall, as well as all critical details. These liquid membranes form a structural bond with the substrates to which they are applied, creating fully adhered air and moisture barriers once cured.
To maintain continuity at movement areas, the same coating used on the field of the wall can be used as a ‘bed coat’ to press the fabric facing into. Using the same coating, the edges of the membrane can be sealed with a top or edge coat. Since liquid barrier products fully bond with themselves, the transition membrane becomes completely embedded within a monolithic air and moisture barrier system. Loss of adhesion is unlikely to occur because the cured membrane has formed a structural bond with the substrate. Tearing is unlikely to occur because the fabric facing transition membrane will expand and contract with the building. Further, unlike self-adhered membranes, these products do not have problems with tenting or wrinkles and their cost is a fraction of the cost for silicone extrusions.
Of course, there are still factors that need to be considered with this type of product. Due to the fabric facing, the membrane is sealed to the building via a liquid—the properties of this liquid membrane itself may dictate limitations in regards to application sequencing and timing. Additionally, any material that interacts with the liquid membrane must be evaluated to ensure adhesion occurs at the time of installation and will not be compromised over time. As with all assemblies, mockups should be constructed and tested to verify the wall system will function properly and that components are compatible with one another.
There are numerous ways to keep air and moisture barriers intact in building envelopes, even at those areas experiencing movement. There is no one product ideal for every situation, but understanding the need and understanding the options allow us to make good decisions from both a design and installation standpoint.
John Chamberlin is the director of business development at Sto Corp. Prior to this position, he served as product manager for StoGuard and StoEnergy Guard, focusing on heat, air, and moisture management within the building envelope. Chamberlin earned a Master’s Degree in Business Administration at Atlanta’s Emory University and is a graduate of the University of Tennessee, with a Bachelor of Science degree in Marketing. He can be reached by e-mail at email@example.com.
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