Understanding Key Air Barrier Terms
Employing air barriers to prevent air leakage and energy loss in building envelopes has become standard practice in the construction industry. But unless you have a degree in building science, there are likely some terms associated with air barriers and their use that may be confusing. In this blog series, we’ll define some of the most common terms and what they mean.
Let’s start with the basics:
1. Air barrier
An air barrier is a material designed to prevent or limit uncontrolled air leakage into and out of a building through the building envelope. Air leakage reduces energy efficiency and allows the passage of moisture vapor, which can condense within wall assemblies, causing damage to wall components and promoting mold growth. To be effective, an air barrier must be continuous across all sides of the building—walls, roof, and foundations.
2. Air permeance
This refers to the amount of air that can pass through a material; the more air that passes through, the higher the air permeance. Note that this is different from air leakage, which refers to the amount of air leaking through gaps in the barrier. The air permeance of air barriers is tested according to ASTM E2178 and measured as the volume of air passing through a square meter of the material at a prescribed pressure. The International Energy Conservation Code (IECC) requires an air barrier assembly to have air permeance not greater than 0.2 liters per second/square meter at 75 Pa pressure (0.04 cfm/ft.2 at 1.57 lb/ft2). ASTM 2357 sets forth a standardized method for determining the air leakage of an air barrier assembly.
3. Climate zones
Climatic conditions—including temperature and humidity—have a tremendous impact on building design. A wall assembly that performs well in the dry desert conditions of the Southwest may perform poorly in the cooler, wetter climate of New England, and vice versa. Accordingly, the International Energy Conservation Code (IECC) has defined climate zones for the U.S., dividing the country into seven distinct zones. These include temperature divisions, from warmest to coldest, and moisture divisions, defined as “moist,” “dry” and “marine” zones, plus a “warm-humid” zone in the Southeast. This enables the IECC to make recommendations regarding building components to meet the specific needs of particular climate zones. Note that the IECC (2015) specifies a continuous air barrier for all climate zones.
4. Continuous insulation
One strategy for countering thermal bridging is the use of continuous insulation. Covering the entire exterior of a building with insulation provides a “thermal break” that prevents thermal bridges. Continuous insulation is becoming more popular in both commercial and residential construction in part because the International Energy Conservation Code (IECC) began recommending higher insulation values in colder climate zones to help improve building energy efficiency.
5. Dew point
Put simply, the dew point is the temperature at which air becomes saturated with water vapor. If the air is cooled further, this water vapor will condense and become liquid water. In the case of a wall assembly, if infiltrating air cools within the wall assembly below the dew point, it can condense on the wall components and collect there, setting up conditions for mold growth and potential structural damage.
6. Hygrothermal analysis
A hygrothermal analysis looks at the movement of heat, air and moisture flow through the building envelope, including walls, roofs and foundations. Using sophisticated software models, hygrothermal analysis enables architects and building engineers to see how a particular assembly of components will perform under a variety of temperature and humidity conditions. For example, a hygrothermal analysis may be used to see how the location of insulation relative to the air barrier will affect condensation under certain weather conditions. Hygrothermal analysis is an extremely helpful tool for taking the guesswork out of decisions regarding air barriers and other building envelope components.
7. Relative humidity (RH)
RH is a measure of the amount of moisture in an air-water mixture, expressed as a percentage. When humidity reaches 100 percent, condensation can occur. Technically, RH is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at a given temperature. This reflects the relationship between temperature and humidity; at lower temperatures, the ability of air to contain moisture vapor is decreased, at higher temperatures, it is increased.
8. Thermal bridge
In the context of buildings, a thermal bridge is a building component that has a higher thermal conductivity than the materials around or contacting it, creating a path for heat transfer. For example, a metal stud directly contacting the external sheathing of a building acts as a thermal bridge, conducting indoor heat to the building exterior (in cold weather) or conducting outdoor heat into the building (in hot weather). This conductivity can impact the energy efficiency of the building, increasing the heating and cooling requirements, and also potentially cause condensation in wall assemblies.
9. Vapor drive
This refers to the fact that water vapor moves from areas of high vapor concentration to areas of lower concentration. A key factor in vapor drive is temperature—vapor moves from areas of higher temperature to ones of lower temperature, called a “temperature gradient.” The greater the temperature gradient, the stronger the vapor drive. This can push vapor through walls via diffusion. If the vapor meets a cool enough surface on its journey, this can cause condensation in a wall assembly, which can damage building materials and promote mold growth. Controlling vapor drive is a major reason to use an impermeable air and/or vapor retarder.
10. Vapor permeance
This refers to the amount of water vapor that can pass through a material. Allowing some vapor to pass through an air barrier can be beneficial in certain situations, allowing vapor inside of a building to pass through, minimizing the potential negative effects of moisture collecting in wall assemblies. Vapor permeance is often expressed as a “perm,” defined as 1 nanogram of water vapor per second, per square meter, per pascal of pressure. The 2015 IBC defines a vapor permeable membrane as one that has a moisture vapor permeance rating of 5 perms or greater when tested in accordance with ASTM E-96D.
11. Vapor retarder
People often confuse air barriers and vapor retarders. A vapor retarder is used to limit the diffusion of water vapor (rather than air) through a wall, ceiling or floor. Vapor retarders may be made of a variety of materials depending on the application, including elastomeric coatings, aluminum foil, polyethylene, and polystyrene. It is important to note that vapor retarders do not necessarily function as an air barrier though some air barriers do function also as a vapor retarder.
12. Vapor retarder classes
Building envelope materials are classified according to their ability to limit the amount of moisture vapor passing through a material. Vapor retarders are divided into three classes, as defined by the IBC: Class I (0.1 perm or less), Class II (0.1 < perm £ 1.0 perm), and Class III (1.0 < perm £ 10 perm). For example, sheet polyethylene would be in Class I; unfaced expanded polystyrene might be in Class II, while latex paint would typically be in Class III.
13. Water-resistive barrier (WRB)
is a barrier designed to prevent the intrusion of wind-driven rain through the building envelope. A WRB is normally installed between the exterior siding or cladding material and the sheathing. A WRB will not necessarily function as an air barrier, though some air barriers do function also as a WRB.
In the next post, we’ll take a closer look at some of the factors surrounding water vapor and its impact on buildings.
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