For most “roofing” consultants, the evolution from BUR, Single Ply and other more traditional roof systems (yes I did include single ply in the traditional category) to waterproofing systems, air barriers and exterior skin elements related to the “Exterior Building Envelope” has been a learning process that has taken time. Nobody got out of bed one day and knew all there is about each or any of the systems that are on the market today. So, with that in mind, I wanted to bring forth some of the basic concepts related to Air Barriers. This is as much for my own education and review as it is for my employees and others that care to read and learn as we try to do each day.

Let us start with Definitions. What is an Air Barrier?
According to the Air Barrier Association of America (ABAA) by definition, air barrier systems are a component of the building envelope systems that control the movement of air into and out of buildings. There are many types of air barriers to choose from, and there are many important factors to consider when determining the best barrier.

Different Types of Air Barriers: Fluid-Applied
One of the new systems being utilized more today is the fluid-applied waterproofing/air barrier. What is not known by most is that Fluid-applied waterproofing/air barriers have actually been manufactured in North America for more than 25 years. It is only recently that their use has increased as air leakage has become recognized as a potential source of moisture accumulation in walls, and some of their unique benefits have been realized.

Much like fluid-applied waterproofing membranes, Fluid-applied waterproofing/air barriers are rolled or sprayed onto substrates and become part of the structural wall. Because of the way they are applied, there are no fastener holes where water penetration may occur, and there is no potential for improper laps or tearing, as with many sheet goods. During construction, a fluid-applied barrier will cover the substrate completely, and does not have to be covered immediately with a cladding, as many of them are UV-resistant.

Another important distinction of a fluid-applied waterproofing/air barrier in wall assemblies is that it can mitigate one of the major forces that causes water infiltration into walls: pressure difference. A fluid-applied waterproofing/air barrier, in combination with venting and compartmentalizing, enables the pressure behind the cladding material to equalize with the pressure outside. This prevents rainwater penetration caused by pressure differentials. This pressure equalizing effect is only possible when the air barrier is structural, as is the case with fully adhered fluid-applied waterproofing/air barriers.

As discussed by Lisa Petsko, a Sto Guard Associate Market Manager, for the Sto Corp. in the Masonry Magazine, “by designing and constructing an “airtight” building envelope, the risk of moisture problems — mold growth, decay, corrosion, loss of insulation value and Indoor Air Quality (IAQ) problems — that can occur because of air leakage and condensation are minimized.” Another good reference can be located at www.buildingscience.com This is a great source for information on Air Barriers.

I know that all of these items are on the list of goals to be achieved when design and specifying the air barrier for a project. Seeking out the best fit for the project is always the challenge due to the variety of choices. None of them clear cut at first glance.

Different Types of Air Barriers: House Wraps
We all have seen the more common air barrier, House wraps, or sheet good type systems. These materials are usually applied under a home or building’s veneer and over the substrate, such as CMU, plywood, gypsum sheathing or in some cases, insulation board. It is important to consider the climate when selecting one of these types of air barriers. Some of the key characteristics include; Better weathering capability, some are more water-resistant than others, and some are more resistant to tearing. They come in a variety of sizes for different purposes. They are wrapped around the exterior of a building during construction and cut around windows and doors. Self adhesive house wrap tape can be used to seal the joints/laps, and flashing is applied at openings and penetrations. Unlike fluid-applied systems, the method of lapping, sealing and incorporating flashings become a bit more tedious, but critical to the overall performance.

Vapor Barriers are not to be confused with an air barrier. A vapor barrier is designed to restrict the flow of water vapor through a material, just the same as an air barrier material restricts the flow of air through a material. Vapor barriers or vapor retarders are intended to control the rate of diffusion into a building assembly. A vapor barrier will control the rate of moisture flow where they are placed. Therefore the vapor barrier does not have to be continuous, does not have to be free of holes, does not have to be lapped, does not have to be sealed, etc. A hole for example in a vapor barrier will simply mean that there will be more vapor diffusion in that area compared to the other areas where the vapor barrier exists.

Selecting an Air Barrier:
Which one should you choose? That is the big question. In today’s world cost is the ruling factor. However, with the competitive nature of the systems and the similarity of all the systems it is still important to identify the one that will work best for you regardless of cost. As a designer it is critical that you review all the components and choose one that addresses all the potential issues for the project. Code compliance is critical to understand. Cities and or government requirements dictate the use of Air Barriers in many cases. Make sure to check with the local building officials and verify what is required prior to writing or specifying a material.

Checking to see whether the Air Barrier has been tested and approved for the area where the project is located is important. Reviewing systems that have been approved by the Air Barrier Association of America would be a good place to start. Listed below are the materials which have been through the evaluation process as performed by the ABAA. Note, this is not a complete list of Air Barrier products by any means. Just an example of the number of systems that you can start with. The chart is taken from the ABAA website www.airbarrier.org for use in understanding what they have done to evaluate the systems.

ABAA Listed Materials
Air Barrier Materials which have Completed the ABAA Evaluation Process can be found on this page.

To continue the thought, some of the following key design aspects should be considered when choosing the system for your project as offered by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Journal:

• Design the exterior envelope and its components to withstand the combined design wind, stack and fan pressures in an airtight manner.
• Design an air-barrier system into the building envelope that can take this pressure, both positive and negative, without displacement or failure. For areas within a building with significantly varying climates, include an air-barrier system into the separation between the two areas, such as between pools and offices, or humidity-controlled areas and adjacent uncontrolled areas.
• Separate shafts—elevators, stairs, ducts and atria—from the floors they serve by airtight assemblies. Provide vestibules and gasket doors and access panels to control transfer of stack pressure.
• Separate pollutant areas such as photocopy rooms, chemical or cleaning storage areas, toilets and garages with gasket doors, and make the surrounding partitions airtight at the deck and floor.

In addition, Wagdy Anis, AIA , LEE D AP, principal of Boston’s Shepley Bulfinch Richardson and Abbott, was quoted in the March 2005 issue of the ASHRAE Journal, recommending that the air-barrier system should be:

• Constructed of relatively air-impermeable materials and assemblies, interconnected with flexible joints.
• Continuous throughout the enclosure.
• Structurally supported to withstand positive and negative air pressures (including design wind pressures and gusts, as well as persistent low pressures such as stack effect and fan pressurization) without displacement or failure.
• Durable to last the life of the enclosure if inaccessible, or maintainable if it can be accessed.

In addition, Anis, who also chairs the Building Enclosure Technology and Environmental Council (BETEC ) of the National Institute of Building Sciences (NIBS), points out that the air-barrier system should be clearly identified by the architect in the construction documents on the building-enclosure details, with a strong focus on intersections of different enclosure systems and transitions. “You have to be able to trace through from one identified plane of airtightness in the first assembly through a sealed joint to the plane of airtightness in the adjacent assembly,” Anis explains.

In general, it’s important to emphasize the fact that the building envelope is part of the mechanical system, even though it is designed by architects (who may not be aware of this) and built by contractors (who may not be particularly focused on airtightness). That being said, we still need more education, greater knowledge and much more informed collaboration about this critical function of the building envelope to the continue to expand on the best practices principles.

Conclusion:
As you can imagine, having just read through this information, there are a myriad of thoughts, recommendations, guidelines and even code requirements that need to be understood. Do not choose lightly. Be diligent in verifying which system will be best for your project. Seek out the experts. As I indicated above, nobody got out of bed this morning with all the answers. What we do know is that with the properly designed and installed Air Barrier system for a project, it will provide significant benefits including energy savings, moisture control, HVAC efficiency to name a few. In today’s market using the proper system will save dollars in the long run. So don’t be afraid to spend a few on the upfront cost of design and installation in order to save long term cost in the building maintenance and management.