By Marc Tropper
Uncontrolled air infiltration and exfiltration through a building envelope can cause negative consequences, now widely known by researchers, such as moisture damage of wall components, reduced thermal comfort, poor indoor air quality, and increased energy consumption.
Although only mandated by codes in Massachusetts, Minnesota, Michigan, and Wisconsin, the need to reduce uncontrolled air leakage in buildings is becoming clear for many architects throughout the country. Many are now regularly specifying air barriers in their designs, resulting in an exponential growth in the demand for these products. The following is a cursory review of the energy savings offered when using an air barrier and what to consider when choosing an air barrier membrane.
Air Barrier and Energy Conservation
The relationship between a building’s airtightness and its energy consumption is not a new concept; however, researchers in the past have focused largely on the nonenergy impacts of air leakage such as thermal performance, moisture damage of exterior wall components, and poor indoor air quality. To address the potential energy savings offered by using air barriers, the National Institute of Standards and Technology (NIST) recently completed annual energy simulations on various building models in five different locations in the United States. The report (NISTIR 7238), prepared for the U.S. Department of Energy, was completed in June 2005 and can be downloaded at http://fire.nist.gov/bfrlpubs/build05/art007.html.The NIST investigation determined that the use of an air barrier system with a target air leakage of 1.2 L/s-m2 (0.24 cfm/ft2) has the potential to reduce a building’s energy and cooling costs by up to 36 percent based on national blended average energy prices. The findings of the investigation are significant and are being considered by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) 90.1 Envelope Subcommittee for inclusion of a continuous air barrier as a requirement in ASHRAE Standard 90.1.
If we consider that buildings are estimated to consume 70 percent of the electricity generated and 50 percent of the energy used in America, the use of air barrier membranes can have an immediate impact on the reduction of our energy consumption. Coupled with the recent policy statement set by the AIA Board of Directors in December of 2005 to “set a goal of slashing the fossil fuel consumption of buildings by 50 percent in 4 years,” the use of air barriers will be a welcome addition for architects looking to practice this policy and create more sustainable building designs.
Types of Air Leakage
When designing the building envelope, architects wanting to reduce uncontrolled air leakage in buildings should consider treating three basic types of air flow: diffuse flow, orifice flow, and channel flow.
1. Diffuse flow is air movement through the pores of the building materials themselves. For example, concrete block, although solid in nature, is not airtight because of its high porosity. The magnitude of diffuse flow will vary depending on the porosity of the materials used in the wall assembly; however, its significance should not be underestimated because it occurs throughout the building envelope. The use of an air barrier membrane fully bonded to a substrate in the wall assembly will create a plane or airtightness and can essentially eliminate airflow by diffusion.
2. Orifice flow is air leakage that results in openings or passages that span across the entire building envelope. You can think of them as holes or penetrations in exterior walls. Because the path is short and direct, the air movement will usually be fast. Water vapor in the flowing air will have little time to condensate and be deposited within the wall assembly before reaching the exterior. Damages are generally localized in the area of the orifice. In cold climates this can often be seen by the formation of icicles on the outside cladding.
3. Channel flow occurs when air can penetrate through a portion of the wall and then travels laterally within the wall assembly before escaping to the other side. This type of flow is potentially the most damaging because the channels or passages can be long in length, therefore allowing ample time for the water vapor in the flowing air to reach the saturation (dew) point and condensate within the wall assembly. Sources of channel flow are often found at junctions between different materials such as floor-slab-to-wall or wall-to-column intersections. To address this concern, designers should specifically call out the use of an air barrier transition membrane in their specifications. These membranes are flexible and reinforced to span the cracks or openings between different materials. Their compatibility and proper tie-in with the main air barrier membrane are essential to maintain continuity of the air barrier within the envelope.
Air Barrier Membrane Considerations
The air barrier membrane must first and foremost be able to stop uncontrolled air leakage through the building envelope that is caused by air pressure forces such as wind pressure, stack effect, fan pressure from HVAC equipment, and barometric cycling. The combined loads involved are generally more significant than many designers realize, and architects should ensure manufacturers have properly tested their materials to resist these forces.
Figure 1: A typical wall assembly in a cold climate using a non-permeable air barrier.
Testing for air leakage of materials is based on ASTM 2178. In this test, a mock-up wall is constructed with a liquid- or sheet-applied membrane. Testing is performed at pressure ranging from 1.6 lb/ft2 (75Pa) to 62 lb/ft2 (3000Pa). Air leakage flowing through the membrane is measured and there is a visual inspection for delamination. The Massachusetts Energy Code currently limits air leakage for air barriers to be no more than 0.004 cfm/ft2 under a pressure differential of 0.3 in. water. Although this test is readily accepted, some manufacturers have even taken it a step further by testing their material to a modified ASTM E283 and ASTM 331 procedure in order to simulate sustained winds and gust wind loads.
Designers, however, should not limit their acceptance of a product solely on air leakage test results. Other test methods related to products' physical properties should also be reviewed to determine the durability and long-term performance of the membrane. Physical properties such as tensile strength, elongation, low temperature flexibility, water vapor permeance, adhesion to substrate, and lap peel strength can be as important as air leakage results.
A third consideration that is getting more attention is the thickness of the membrane being applied. With increased competition, some manufacturers are reducing their recommended application thicknesses. Although their product may still pass required air leakage values when tested in a laboratory, this can be misleading. The thickness of the installed membrane is often directly related to its ability to seal pores in materials, span cracks, elongate and recover with a building's natural movement, and to gasket or “self-seal” itself around fasteners. Many manufacturers claim their product can self-seal when penetrated with fasteners. These claims, however, are often not backed with testing. Designers should keep in mind that the ability of a fastener to compress the air barrier membrane in order to seal around the fastener is related to the thickness of membrane installed. It’s simple: The more material you have around the fastener the better chance you have of creating a compression gasket. Architects should therefore consider, and even specify, minimum application thicknesses of the air barrier membrane regardless of whether a manufacturer supports air leakage values at thinner applications.
Classes of Air Barrier Membranes
It has become clear that moisture problems in walls often result from air leakage and/or vapor diffusion. Although these two mechanisms are different, historically many people in the industry have confused the function of the air barrier and the vapor barrier. Furthermore, many products can act as both the air barrier and the vapor barrier, thus adding to the confusion. For this reason, air barriers are now separated into two classes: non-permeable and vapor permeable.
Non-permeable air barrier membranes can act as an air barrier, a vapor barrier, and a rain barrier. They have been tested to resist air leakage, have very low vapor permeance, often less than 0.1 perms, and are watertight. Because they also function as vapor barriers their position within the wall assembly should be verified by doing dewpond analysis or hydrothermal analysis to prevent moisture condensation. Figure 1 shows a typical wall assembly in a cold climate using a non-permeable air barrier. Because the membrane also functions as a vapor barrier it should be positioned on the “warm” side of the insulation. This limits the designer's choice of insulation and its placement within the wall assembly. Walls using non-permeable air barriers also have less “drying” potential if moisture becomes trapped within the cavity.
Vapor permeable air barrier membranes can act as an air barrier and a rain barrier; however, they are not vapor barriers. They have been tested to resist air leakage, can be watertight, but have a high vapor permeance. The standard CAN/CGSB 51.32-M77 recommends vapor permeable products have a minimum of 3 perms when tested to ASTM E96. Because they allow the diffusion of vapor they can be positioned anywhere in the wall assembly. This offers architects more flexibility in their choice of insulating materials. Figure 2 shows a typical wall assembly in a cold climate using a vapor permeable air barrier. Note that insulation is placed within the interior stud wall. Additional insulation can also be placed within the cavity to reduce thermal bridges and increase the wall's thermal value, or R-value. Compared to the non-permeable wall design, the wall profile for the vapor permeable can be “thinner” since not all the insulation is in the cavity. By reducing the width of the cavity the overall thickness of the wall can be reduced. Brick ties or cladding anchors are not as long, shelf angles are not cantilevered out as far, and in zero lot line situations the space saved can mean an extra couple of inches of usable floor space within the building. A thinner profile exterior wall can represent significant cost savings.
Figure 2: A typical wall assembly in a cold climate using a vapor permeable air barrier.
Vapor permeable air barrier membranes are also effective in mixed-humid climate zones like the Carolinas where heating degree-days and cooling degree-days are almost equal. Water vapor can escape through this class of air barriers, thereby reducing the occurrence of condensation as the dew point changes within the wall assembly throughout the year.
Summary
Research is demonstrating that air barriers can provide energy savings by reducing uncontrolled air leakage through the building envelope. Coupled with the architectural community's goal to reduce the overall energy consumption of buildings and create more sustainable design, the use of air barriers in more building types will continue to increase. Air barrier membranes can create a plane of airtightness within the building envelope, preventing air flow in both positive and negative pressure. When choosing an air barrier membrane, designers should not solely rely on air leakage results but need to review the physical properties and application thicknesses of the product to ensure long-term durability. Many membranes exist in the market today and are separated into classes: non-permeable and vapor permeable air barriers.
Marc Tropper, P.Eng., is director of product management for Henry Company and has been involved with the development and specification of building envelope systems and air barriers for 15 years.
References
- Anis, Wagdy, Commissioning the Air Barrier System, American Society of Heating, Refrigerating, and Air-Conditioning Engineers, 2005.
- Rousseau, Jacques, Air Barriers: Assemblies and Construction Materials, National Building Envelope Council, 1994.
- Emmerich, McDowell, and Anis, Investigation of the Impact of Commercial Building Envelope Airtightness on HVAC Energy Use, National Institute of Standards and Technology, 2005.
- Snoonian, Deborah, "AIA Board of Directors Sets Ambitious Agenda for Sustainability," Architectural Record, 2005.
- Ganguli, Wind and Air Pressures on the Building Envelope, Building Science Insight, 1986.
- Quirouette, The Difference Between a Vapor Barrier and an Air Barrier, BPN 54, National Research Council, 1985.