The Multiple Purposes of Air Barriers
by Jay Saldana, PE
The need for energy efficient buildings is the largest driving factor behind product innovation and building science advancements in air barrier materials. However, other equally important aspects of modern construction are sometimes overlooked by building professionals. Manufacturers continue to improve their products to extend a building’s durability and lower the costs of construction. The developments in air barrier products over the years reflect a constant demand for airtight, mechanically ventilated buildings to meet ever-changing energy codes and expectations among builders and architects for higher performing facilities. There is also pressure to find ways to design a building that could function properly in any type of climate or environment. Therefore, understanding the progression of air barrier materials will help professionals at all levels of the building design process choose the best air barrier system for their project.
Evolution of air barrier materials
The importance and inclusion of an air barrier material is not questioned by today’s builders and architects, but not long ago, a structure’s air barrier was not composed of one single membrane. With painstaking detailing of joints and transitions, interior gypsum was largely employed to reduce air leakage, knowing it had the help of other materials in the exterior wall for slowing down leakage. Since then, the function, design, and construction of an air barrier, not to mention how it is specified, has been constantly changing. It is important to remember how far the products and systems have come to fully appreciate and better understand how to utilize them in present day construction. Building wraps were among the early products to act as an exterior air barrier material and had the advantage of being vapor permeable. Even after the creation of the U.S. Department of Energy (DOE) in the late 1970s, there was still a lack of advanced science and consensus of where vapor retarders and insulation should go in an exterior wall. A “breathable” exterior wall (i.e. a wall with materials facilitating drying by air movement or vapor permeability) was thought to be the only way to prevent moisture damage. Building wraps could fit this design method by allowing vapor through the material but preventing the passage of liquid water and air. Installation of the wraps was the biggest challenge. They had to be fastened frequently and attached with precision. It was not uncommon to see wraps flapping in the wind on a construction site simply because it was not taped at the joints or attached to the building correctly. The product itself worked, but the method in which it needed to be attached and the lack of high-quality workmanship in installation led to the next generation of air barrier materials.
When self-adhered membranes came into the market, they were instantly thought to be superior to wraps as they adhered to walls without the need of a fastening pattern. Some contained self-healing qualities and sealed screw penetrations or small slices made to the membrane during construction. While these products proved more reliable, the installation process was long and depended on temperature-sensitive adhesives. When these products were first introduced, there was a period of trial and error by manufacturers with adhesives—some work better in the heat, some in the cold, and very few work well in both. Dirty and dusty substrates also contribute to adhesive failure. The self-adhering membranes sometimes required a termination bar that had to be fastened down over the membrane edge as it was common for certain edges of the membranes to peel.
Originally, peel-and-stick membranes were vapor impermeable, but as the breathable exterior wall concept spread, manufacturers developed vapor permeable versions. This transformation required years of research and new technology to allow vapor through the adhesive and the facing of the membrane.
Fluid/liquid-applied barriers were another jump in the air barrier product industry. The first versions could be spread by trowel and adhere in place on the wall, thereby becoming both the air and water barrier. Spray and roll-on versions were created as thinner applications but both thicknesses accomplished the same goal of being the air and water barrier. While self-adhering membranes evolved over time, the manufacturers of fluid-applied barriers learned from their predecessors, and soon had both vapor permeable and impermeable versions. Fluid-applied barriers are thought to have even less possibility for error in installation than self-adhered membranes. They can cover corners and angles better than self-adhered membranes, which take some skill to accomplish the tricky bends and folds. While this was a huge advancement and reduced the time of installation, in the author’s experience, the higher cost of these products often resulted in them being “value engineered” out of the project. A further advantage of fluid-applied barriers over self-adhered membranes is the lack of the membrane itself. Often, compatibility with various caulks and sealants must be tested not only with the adhesive, but also the membrane. Fluid-applied barriers are more often compatible with such construction materials and capable of being applied to them. A self-adhered membrane would have a very hard time transitioning and adhering to an irregular surface such as the face of closed-cell spray foam. Conversely, a fluid-applied product can fit into the irregularities of the surface and achieve a solid bond. In constantly wet climates, such as the State of Washington, caution must be taken with self-adhered membranes as their adhesive is typically sensitive to wet surfaces. Many fluid-applied barriers advertise their applicability while actively raining so work may continue.
Advent of continuous insulation
The changes and advancements in energy codes, specifically the 2009 International Energy Conservation Code (IECC), started to require exterior continuous insulation (ci) in most of the eight climate zones. Knowing vapor permeable air barrier membranes were the newest and fanciest product, and the exterior wall still needed ci, it was not uncommon to find both installed on the exterior side of the wall. If the ci was rigid foam plastic insulation and installed after the air barrier membrane, as it typically is, the vapor permeable properties of the membrane are rendered useless. A wall is only as breathable as its least breathable component and in this case, it is the vapor retarding rigid foam plastic insulation. This multilayered approach did not allow for vapor permeance to the exterior because the insulation was preventing it from exiting. While this is not necessarily a poor design, it just means the owner paid a premium for an individual air barrier product that is not being used to its full potential. It also means if the mechanical engineer was counting on a permeable exterior wall, some of the HVAC calculations may be off. Continuous insulation as an air and vapor barrier means the old practice of using interior vapor retarders is no longer necessary in cold climates, and this material can be removed from the assembly. Those vapor retarders were meant to keep warm, humid air from getting into the stud cavity and touching a cold surface. Continuous insulation reduces the chances of having a surface below dewpoint temperature inside the stud cavity and, along with the wall no longer being leaky, the humid air in the cavity will normalize with the interior and be part of the HVAC system’s conditioned air. If any moisture was to leak in the wall cavity, it would now dry to the interior. As architects and designers were learning the building science of why and how to use ci properly, they were also looking at the increasing cost and number of materials employed in the exterior wall. The need for one multipurpose product on the exterior wall created a business opportunity for rigid foam plastic manufacturers.
Extruded polystyrene and polyisocyanurate
Extruded polystyrene (XPS) and polyisocyanurate (polyiso) insulation manufacturers began to successfully test their products to show they could act as the air and water barrier, vapor retarder, and ci of the building. One product performing all these controls allows the removal of one or more materials from the assembly, such as the individual air barrier, saving installation time and money.
To accomplish this level of performance, rigid boards need to be mechanically fastened to the walls to resist negative wind pressure. They must also seal every board joint and fastener and through-wall penetrations. Typically, tape is employed to seal the board joints and fasteners, while flashing is used for through-wall penetrating objects. If installed correctly, tape can be effective, but the thousands of foam board joints and tens of thousands of foam board fasteners create potential direct paths for leakage. If the rigid foam board is used as the air and water barrier, it is imperative every joint and fastener treatment be installed correctly.
Closed-cell polyurethane spray foam
Closed-cell polyurethane spray foam has recently been approved as an exterior air, water, and vapor barrier. Additionally, spray foam does not have any joints or fasteners to seal. Rigid polyiso foam plastic insulation and closed-cell polyurethane spray foam have similar performance characteristics. They each have a closed-cell structure that can utilize a blowing agent to achieve some of the highest R-values per inch on the market. Spray foam tops the insulation R-value charts at around R-7.1 per inch. They both perform as an air, water, and vapor barrier. However, spray foam is installed very differently. It is sprayed onto the exterior sheathing (or masonry back up wall), adhering to the surface, and expanding outward to create the desired thickness. Spray foam will self-support and adhere to common construction materials, including itself. This ability allows spray foam to be installed much more quickly than rigid foam board and it does not have any joints to treat. Spray foam can also allow for eye-catching architectural curves in walls to be insulated and sealed easily whereas rigid foam board would require thin vertical strips to make it around the radius. Spray foam insulation helps expand the horizon of design opportunities for commercial architects beyond what was feasible with fibrous or rigid board insulation options. Architects can allow their creativity to show with the confidence spray foam will help meet the energy efficiency, overall performance, and functional needs of a building. Closed-cell spray foam insulation tackles design challenges and problem areas that are difficult or nearly impossible with other types of insulation, including arches, curves, cathedral ceilings, and transitions from the exterior wall to fluted roof deck over hangs. Air barriers have evolved from encompassing several different materials with labor-intensive processes to single, multipurpose products making construction faster and more cost efficient when combined with the right design and building science.