Understanding Key Air Barrier Terms

Employing air barriers to prevent air leakage and energy loss as a key piece of the building envelope 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 article, we’ll define some of the most common terms and what they mean.

Let’s start with the basics:

1. Building envelope

The building envelope is the physical barrier between the conditioned and unconditioned environment of a building that protects the structure from elements, such as precipitation, wind and humidity. The building envelope includes the foundation, wall assembly, roofing systems, windows, doors, skylights as well as penetrations such as chimneys or vents. 

2. Air flow 

Air flow is the movement of air from one area to another. Air flow across a building enclosure is driven by wind pressures, stack effect, temperature gradients and mechanical air handling equipment such as furnaces. 

3. Air barrier

An air barrier is a material designed to minimize 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, including the walls, roof and foundation.

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4. Primerless air barrier

Primerless air barriers are sheet-applied air barriers with an adhesive backing that allow the membrane to be applied to a substrate without having to apply primer beforehand. This can significantly reduce installation time.

5. ASTM International

ASTM International (formerly known as the American Society for Testing and Materials) develops and publishes technical standards for a variety of materials and systems, including air barrier products. 

6. Air permeance

Air permeance refers to the amount of air that can pass through a material; the more air that passes through, the higher the air permeance. 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 2012 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.

7. 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 eight 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 except zone 2B.

8. 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 IECC began recommending higher insulation values in colder climate zones to help improve building energy efficiency.

9. 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.

10. Dew point calculators

These tools, many of which you can find online for free, compute the dew point or frost point temperatures at any given ambient temperature and relative humidity. This information can help architects and builders determine whether a certain building component of a wall, ceiling or roof will stay warm enough during the winter to avoid condensation problems.

11. 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.

12. Relative humidity (RH)

RH is a measure of the amount of water vapor in air, 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.

13. 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 cladding attachment screwed into an interior metal stud can lower the temperature of the stud in the wintertime. This conductivity can impact the energy efficiency of the building. 

14. Vapor drive

Vapor drive refers to the fact that water vapor moves from areas of high vapor concentration to areas of lower concentration. Key factors in vapor drive are vapor pressure and temperature. The greater the pressure and 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 barrier.

15. 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.

16. Vapor retarder

Vapor retarders are materials that reduce the rate at which water vapor can move through a material. Vapor barriers or vapor retarders can be part of a moisture control strategy in all buildings. They control moisture vapor through the wall, floor, ceiling or roof assemblies of buildings to prevent condensation. 

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 or polyethylene. 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.

17. Vapor retarder classes

Building envelope materials such as vapor barriers 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.

18. Impermeable air barriers/impermeable vapor barriers

A vapor impermeable air barrier is one that air or water cannot get through. 

19. Vapor permeable air barriers

A vapor permeable barrier is one that controls the flow of air but allows a certain amount of moisture vapor to pass through it. It will have a vapor permeance of > 10 perms per ASTM E96B.

20. Water-resistive barrier (WRB)

A water-resistive barrier is 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.

21. Through wall flashing 

Through wall flashing extends through a masonry wall and is used to divert moisture that has entered into the wall to the outside before it can cause damage. 

22. Aluminum flashing 

Aluminum flashing is a thin, typically impervious material, installed at transition areas, such as windows, to prevent the passage of water into a building.