The multiple purposes of air barriers

by Samantha Ashenhurst | July 6, 2018 9:27 am

Photo © Jay Saldana

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.

Closed-cell spray foam installed on building radius as continuous insulation (ci), air barrier, weather-resistive barrier (WRB), and vapor retarder. Photos courtesy Icynene-Lapolla[2]
Closed-cell spray foam installed on building radius as continuous insulation (ci), air barrier, weather-resistive barrier (WRB), and vapor retarder.
Photos courtesy Icynene-Lapolla

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
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.

Self-adhered membranes
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.

Closed-cell spray foam insulation tackles design challenges and problem areas like oddly shaped structures and curves that are difficult or nearly impossible with other types of insulation.[3]
Closed-cell spray foam insulation tackles design challenges and problem areas like oddly shaped structures and curves that are difficult or nearly impossible with other types of insulation.

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-applied barriers
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.

The ability to self-support and adhere to common construction materials, including itself, allows spray foam to be installed much more quickly than rigid foam board and it does not have any joints to treat.[4]
The ability to self-support and adhere to common construction materials, including itself, allows spray foam to be installed much more quickly than rigid foam board and it does not have any joints to treat.

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.

Closed-cell polyurethane spray foam insulation installed on top of a self-adhered membrane.

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.

Closed-cell spray foam can utilize a blowing agent to achieve around R-7.1 per inch, one of the highest values in the market.[6]
Closed-cell spray foam can utilize a blowing agent to achieve around R-7.1 per inch, one of the highest values in the market.

Conversion to multipurpose products
Even before energy codes required an individual air barrier material, ci was already being implemented in construction. It drove the movement to remove other materials from the wall and rely on insulation to act as an all-in-one air, water, and vapor barrier. Today, ci continues to grow.

The advancements in ci materials helped engineers and manufacturers explore how to build a stronger, better wall. It provides several benefits to a building besides the elimination of thermal bridges. It increases the overall durability of the wall assembly and the building’s energy efficiency, thereby reducing the facility’s energy bill over time. If also installed as an air barrier, it can reduce the risk of condensation and moisture infiltration. The popularity of multipurpose ci products is directly related to the rise in desire from designers, owners, builders, and end-use occupants for high-performance, low-maintenance, energy efficient buildings.

Commercial buildings account for 18 percent of the nation’s energy use and greenhouse gas (GHG) emissions, according to an Energy Star Commercial Building Design[7] report. Along with the consistently rising gas and electricity prices, builders, owners, and architects are willing to spend and invest more in their designs and buildings now in order to save on future energy bills, driving the demand for ci and air barrier products.

While product innovation in air barrier materials has evolved dramatically in the past two decades, the demand for greater efficiency will continue to drive new developments in years to come.

Benefits of using air barriers
There are several advantages to a tight building beyond just energy efficiency. Cooperation of the whole design team is required to achieve these advantages. The insulation needs to be positioned in the correct place on the exterior wall to the correct R-value per climate zone. It requires the mechanical engineer to not overdesign the HVAC system with the understanding the building will be tight. The adage of “build it tight, ventilate it right” must be implemented. It also requires the proper controls and monitoring of the interior environment so that the HVAC system can respond as needed.

Closed-cell spray foam insulation acts as an all-in-one air, water, and vapor barrier, thereby saving time and money.[8]
Closed-cell spray foam insulation acts as an all-in-one air, water, and vapor barrier, thereby saving time and money.

Controlling humidity is a large part of indoor environmental quality (IEQ) and is crucial in commercial buildings. Uncontrolled humidity in a stud cavity can lead to mold, condensation, and even deterioration of structural components (e.g. rust). Exterior water vapor (humidity) can get into a building by either air movement or diffusion. Humidity travels most easily though air, so by first applying an air barrier in the proper place (i.e. the exterior side of the exterior wall), vapor intrusion can be prevented from getting into the wall cavity. If the air barrier also happens to be a vapor retarder, like spray foam ci, the vapor intrusion is all but eliminated. This allows the HVAC system to be designed at a lower humidity load and amounts to less work for the system. If accounted for correctly, a proper air barrier and ci combination may even lead to downsizing of the HVAC system.


Vapor pressure drive is from high to low just as high temperatures migrate to lower temperatures. In climate zones with more cooling-degree days than heating-degree days (mostly hot weather) the general vapor drive is from the exterior to the interior. In extreme locations (e.g. Miami and New Orleans), it makes sense to keep the air barrier and vapor retarder on the exterior side of the wall to prevent warm, humid air from entering the building.

This same design works in climate zones with more heating-degree days than cooling-degree days (mostly cold weather) when ci is employed. The proper amount of ci ensures the wall components interior of the insulation are warm and above the dewpoint temperature. Of course, ci works best if it is also an air barrier. Cold air bypassing ci is no different than using a loose-knit sweater to stay warm in windy, cold weather. The sweater is pointless if you can feel the cold air through it.

Close-up of foil-faced polyisocyanurate (polyiso) insulation.[9]
Close-up of foil-faced polyisocyanurate (polyiso) insulation.

Air barriers work in all climates to help manage humidity, especially when they are integral with the ci. Better control over humidity means the prevention of condensation and mold, and its negative health effects.

Air barriers also help reduce sound transmission through the wall, which is a little thought of benefit. Reducing sound transmission makes for improved IEQ. Sound travels through air, but if air movement can be stopped from the outside, it essentially slows down sound. An air barrier by itself will not stop sound altogether, but it will reduce it, leading to further indoor comfort.

Air barriers are more than just a required control in building codes. While improving energy efficiency is the key purpose, their other benefits, such as helping control the interior environment, providing a durable design, and creating a high-performing building, should all be closely considered in air barrier material selection. There are many products that do not limit design, cost, or time. Understanding the options of these materials and systems will help architects and specifiers make informed decisions on selecting the right air barrier for creating high-performing buildings.

Jay Saldana, PE, is senior engineer at Icynene-Lapolla, specializing in commercial construction. Saldana brings his building science experience to architects, engineers, and designers looking to employ spray foam to help ensure building enclosures function properly and at their full potential. Saldana received his bachelor of science degree in civil engineering from Texas Tech University. He can be reached at[10].

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