In recent years, sustainability has become a predominant design consideration for new buildings. Code officials, guided by research in the building science community showing definitive relationships between air tightness and building envelope performance, are increasingly adopting a trend towards more and more air tight building construction. These requirements for air tightness are now being encountered, in many cases for the first time, by the American design and construction community. Although relatively new in the United States, similar requirements for air tightness in construction have been in effect in Canada since the early 1990s. Stateside, Massachusetts was the first to introduce a code requirement for air barriers in its energy code (780 CMR, 2001).
More recently, Washington State adopted air barrier requirements including a requirement for whole building air barrier testing of commercial buildings over five stories. With the advent of the 2012 International Energy and Conservation Code (IECC), which now requires air barrier construction in new commercial buildings, a growing trend toward air barrier construction appears inevitable for the remainder of the US while jurisdictions begin to adopt the new IECC code language.
To date, the United States Army Corps of Engineers (USACE), as directed by the US Department of Defense (DoD), has been one of the biggest drivers of air barrier requirements in the US with stringent requirements for installation and testing of all buildings under DoD jurisdiction. With the USACE’s preliminary successful results similar to those in Seattle WA (which requires whole building air barrier testing of all commercial buildings under the 2009 energy code), we are led to conclude that most any building can employ a successful air barrier provided that correct steps are taken during design and construction. Often, required steps are small but result in large benefits. Additionally, without much added cost, it is possible to include air barrier commissioning to a common building envelope commissioning plan.
There is uncertainty in the industry as to what exactly constitutes an air barrier system and what needs to be done differently compared to what is being done presently to achieve an air barrier. Not to mention concern over possible liabilities and cost implications that may come along with the new requirement. Also, some simply don’t understand the benefits of having an air tight building, having historically allowed building envelopes to breathe.
The Air Barrier Association of America (ABAA) defines an air barrier as a system comprised of a number of materials which are assembled together to provide a complete barrier to air leakage through the building enclosure. This includes components in the walls, fenestration, floors and roof. In adding to the ABAA’s definition, we venture further to state that air barriers are a means of achieving environmental separation – the ability to maintain an indoor environment that is different from that of the outdoor environment. With this in mind, an air barrier is a system of materials that limits the passage of air, also limiting the passage of heat, moisture, sound and airborne contaminants.
One common source of confusion is the relationship of an air barrier with regard to the more commonly discussed vapor barrier. Vapor barriers have been mentioned in US code language for years. An air barrier is not a vapor barrier. But, sometimes it can be.
The difference between an air barrier and a vapor barrier can be understood by looking at the components of air itself. As it occurs naturally in earth’s atmosphere, air is a mixture of gasses comprised almost entirely of nitrogen and oxygen along with other trace gases such as argon and carbon dioxide. Water vapor is also a trace gas in air but varies with humidity. This mixture we know simply as air contains earth’s dynamic weather, including heat. Understanding this, we can begin to see why control of air movement into and out of a closed building allows us to more easily control conditions within that building.
A microscopic look at air barrier materials reveals a surface that is dense enough to block the passage of most of the gasses within the air mixture. Depending on the denseness of the air barrier materials, larger molecules like nitrogen and oxygen (which make up the majority of air) may be blocked, while smaller molecules may sometimes still pass through.
As opposed to oxygen and nitrogen, water vapor is a trace gas that varies with location. Water vapor molecules are much smaller than oxygen and nitrogen molecules. Many air barrier materials that limit the passage of nitrogen and oxygen may not always limit the passage of water vapor. These barrier materials would then be considered vapor permeable air barriers, because they stop air without stopping water vapor. Most vapor barriers are suitable air barriers, but not all air barriers are vapor barriers. This is
directly a result of the fact that water vapor molecules are much smaller than oxygen and nitrogen molecules.
Water is a leading cause of damage to building envelopes which is why so much emphasis has been put on vapor barriers in past codes. That is, despite the fact that air leakage is a much greater source of water transfer than vapor diffusion alone. In looking at the way air barrier and vapor barrier materials are quantified, the difference becomes somewhat self-evident as the driving force for each phenomenon
is understood.
Air barrier assemblies are typically lab tested in accordance with ASTM E283. Because air leakage is driven by differences in air pressure, this test is performed by inducing a pressure difference across an assembly and measuring air flow over time at a constant pressure.
Vapor barrier materials on the other hand are typically lab tested in accordance with ASTM E96. Vapor diffusion, unlike air leakage, requires differences in vapor partial-pressure (humidity) instead of air pressure differences. Despite the fact that there is no pressure difference induced across the test membrane in this standard, water vapor is still driven across the membrane solely because of a difference in humidity. Typically, the amount of water that is able to diffuse through a membrane is far less than what can be driven through even a tiny hole via air leakage.
Nevertheless, both air leakage and vapor diffusion are important considerations when designing a building envelope but their respective significance must be understood.
Perhaps, one of the reasons that air barriers have been met with some uncertainty in the construction industry is that past construction practice has favored leaky buildings that are able to breathe. The term breathe is difficult to discuss because there is no set definition. Sometimes breathe means to leak air while other times breathe means to allow water vapor diffusion.
Historically, buildings were designed and constructed with little thought for air tightness, permitting uncontrolled air leakage across the building envelope. As we discussed earlier, this air leakage translated into uncontrolled heat exchange as well as uncontrolled movement of moisture across the building envelope. In cold climates especially, this air leakage and associated heat loss often resulted in cold drafts within the building interior and uncomfortable interior conditions.
During winter months in cold climates, interior humidity levels are often maintained at a level that is above what the exterior cold air can support. As a result, warm, moist air leaking across the wall, from the warm interior to the cold exterior surfaces, would reach its dew point within the wall and wet the wall assembly. In an effort to maintain comfortable conditions, old buildings would make up for heat loss with increased use of heating systems. A side effect of extra heating was to dry the building wall out by heating the exterior surface of the wall to above the dew point of the leaking air. This uncontrolled heat loss is why old building envelopes often survive without water damage, despite consistent wetting and drying patterns.
In this manner, however, historic building practices were very thermally inefficient as they used excessive heating to both make up for unnecessary heat loss and continuously dry out wet walls. This method of keeping buildings dry and warm is not sustainable in an energy conscious world. Modern building design does require buildings to breathe air. What is different is that air barrier design favors control of where exactly this air exchange occurs. In this way, modern buildings can employ smaller and more efficient HVAC systems that do not need to make up for the excessive heat losses through leaky walls. This improves the long-term energy efficiency and durability of the building.
Understanding that air leakage allows for wetting and also drying of walls, we note that there are risks associated with installation of an air barrier, particularly into an existing historic building. If not installed continuously, an air barrier can sometimes limit the drying capacity of a wall assembly, resulting in potential moisture damage if the wet wall is unable to dry quickly. It is important to consult a building envelope professional especially when renovating an existing building towards air tightness.
When designed and constructed appropriately, rewards of an air barrier in terms of energy savings and occupant comfort considerably outweigh associated risks. Energy savings translate directly into cost savings for the owner and into reduced carbon emissions. Occupant comfort results from greater control of interior temperature and humidity levels and air quality.
Air barrier systems are not difficult to design or construct, but do require attention to detail. The most important consideration when designing and constructing an air barrier is continuity. Air barrier systems can be comprised of different air tight components that vary with the architectural style, often installed by multiple trades. These components can only work as an air barrier system when they are continuously sealed at their interfaces. As mentioned above, small discontinuities in an air barrier can lead to localized wetting of a wall assembly that has limited drying capacity.
Air barriers must be designed as part of a complete building envelope system with considerations made for items including, but not limited to, thermal transfer, liquid moisture management and vapor diffusion characteristics. The Air Barrier Association of America (ABAA) provides a full list of requirements at airbarrier.org; similarly, the 2012 IECC provides a list of air barrier requirements in Section C402.4. The decision on the vapor permeability of an air barrier wall membrane for a given wall assembly depends greatly on the interior and exterior design conditions as well as on the placement of wall insulation. This is where involvement of a façade engineer is valuable.
Masonry is not typically considered an air barrier material however, numerous wall profiles that include masonry materials have been successfully designed and constructed. In masonry clad buildings, air barriers can be achieved in one of two common ways:
The following two options for masonry walls with air barriers are suitable for most climate zones.
Mass Masonry Wall with Continuous Air Tight Spray Foam Insulation:
Rainscreen Veneer Cladding with Air and Weather Barrier:
In Section C402.4.2.2, the 2012 IECC includes masonry assemblies that it deems to comply with air barrier requirements that are not mentioned above such as CMU block walls with two layers of paint. We note that these deem to comply options in the IECC may indeed comply with air barrier requirements; however, they may not necessarily serve as effective weather barrier within the holistic wall system. The air barrier and its intended performance expectations must always be considered within the larger wall system.
The two air barrier wall assemblies described would form very tight air barrier systems on their own. Air barrier installation is most successful when both design and construction teams understand the concept of an air barrier and understand the ultimate goal for sealing. Possible leaks are introduced into the system when one considers that the walls will be penetrated with components such as windows, doors and vents. Not to mention, interfacing with different air barrier systems at the roof and floor. In keeping a continuous air barrier, it is important to understand and identify which components need to seal to which.
For example, one of the most commonly forgotten interfaces is the roof-to-wall. A perfect air barrier at the wall will be ineffective if it does not tie into an equally continuous air barrier at the top of wall and again at the base of wall.
Similarly, discontinuities are also common at window and door interfaces. Best practice is to employ a commissioning process that helps identify air barrier components and how they transition at windows, doors, penetrations such as ducts and pipes, roof and floor.
Testing of an installed air barrier is an important step in verifying its performance. Several methods of testing can be employed, both quantitative and qualitative. In many cases testing is not required. Increasingly, however, the trend in the industry is for buildings to undergo and pass whole building air barrier testing. Quantitative testing provides an indication of the amount of leakage that may be occurring in an air tight building. This type of testing typically includes pressurization and depressurization in an effort to estimate the air leakage rate at a standard reference pressure point.
A test protocol jointly developed by The Engineer Research and Development Center (ERDC) and the ABAA has been established. Requirements for acceptable levels of air leakage may vary by standard and jurisdiction. The USACE requirement for a 4-story, 13,200 sf building is less than or equal to 0.25 cfm/sf for exterior envelope at 0.3'' of water gage (75Pa) pressure difference, while ASHRAE 189.1-2009 is 0.40 cfm/sf. Qualitative testing provides an indication of locations of leakage pathways and typically includes thermal imaging or use of a smoke pencil in combination with building pressurization in accordance with ASTM E1186.
USACE requires testing of all new buildings with the most stringent air tightness requirements currently in the US. Washington now requires testing of all new commercial buildings over a certain size. In future codes, Seattle will likely require passing of this test before issuing a certificate of occupancy, as is required for residences. Whether testing is required or not, it is beneficial because it allows the installer to target any required minor or major repairs to the air barrier as seen in image 2.
Along with the requirement for air barriers, whole building air barrier testing is a rather new requirement in the US. To date, however, USACE has taken a leading role in testing of all new DoD buildings with air barriers and has tested a large and growing number of buildings.
To estimate achievable savings from reduced air leakage in newly constructed and retrofitted buildings, ERDC and National Renewable Energy Laboratory (NREL) researchers conducted simulation studies using EnergyPlus 3.0 building energy simulation software. The baseline building was assumed to be an existing barrack, dormitory or multi-family building built to meet the minimum requirements of ASHRAE Standard 90.1-1989 (ASHRAE 1989) by climate zone.
The barrack is three stories high with an envelope area of 30,465 sf (2,691 m2) and includes 40 two-bedroom apartment units, a lobby on the main floor and laundry rooms on each floor. Benne1 includes further details on the barrack and the baseline heating, ventilation and air-conditioning (HVAC) systems used. Note that the energy costs used in this study are based on Energy Information Administration 2007 average data for commercial rates in each state and may not reflect utility rates at a specific location.
Four representative air tightness levels were modeled: 1.0, 0.5, 0.25 and 0.15 cfm/sf (@75 PA pressure difference). The first value, which was used as the baseline, was derived from expert opinion of existing buildings based on pressurization tests. Three other values are considered to represent reasonable performance improvements achievable with a low, medium and best effort for sealing existing buildings.
Energy savings are based on total building site energy consumption. Energy savings range between 2% and 16% with the air tightness improvement to 0.4 cfm/sf at 75 PA; between 3% and 31% with the air tightness improvement to 0.25 cfm/sf; and between 8% and 44% with the air tightness improvement to 0.15 cfm/sf. Results are highest in the coldest climates and decrease in warmer climates. These savings translate to roughly $0.10-0.50 per sf. Results can vary with the change of baseline building air tightness, types of HVAC systems used and energy rates.
Simulations proved the cost and performance value of further testing and in the establishment of regulations. Recent USACE white papers that have analyzed the results of this testing show that buildings of various types and sizes when designed and constructed with air barriers are able to easily pass USACE mandated air leakage requirements, with an average of more than 300 buildings’ test results coming in at 0.17cfm/sf @ 0.03" wg.
1Benne, K and Deru, M (February 2009). Reference Barracks Building. National Renewable Energy Laboratory, Golden CO, in preparation.
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