Air barrier materials and systems are primarily rated by their ability to control air leakage and, for membranes, their permeability to water vapor. Understanding these ratings is crucial for designing energy-efficient, durable, and healthy buildings.
What are the Ratings for Air Barriers?
Air barriers are materials or systems designed and constructed to control the movement of air across the building envelope. Their effectiveness is quantified through various ratings, with vapor permeance being a key characteristic for air barrier membranes and air leakage rates for the overall air barrier system.
Understanding Air Barrier Ratings
An air barrier's primary function is to resist the uncontrolled flow of air into or out of a building. This air movement can carry moisture, significantly impact energy consumption, and reduce indoor air quality. Therefore, air barrier ratings help professionals select appropriate materials and design robust building envelopes.
Vapor Permeance: A Key Material Rating
Vapor permeance measures how readily water vapor can pass through a material. This rating is particularly important for air barrier membranes, which can also serve as vapor retarders or allow for drying depending on their permeance.
The unit for vapor permeance is the U.S. perm (1 perm = 1 grain of water vapor per hour per square foot per inch of mercury pressure difference).
- Air barrier membranes are assigned permeance ratings from 0 to above 75 U.S. perms.
- The industry recognizes materials that have a vapor permeance of 10 U.S. perms or higher as being “permeable.”
Based on their permeance, materials are classified into different categories, which are critical for managing moisture within wall assemblies:
| Permeance Class | U.S. Perms (ASTM E96) | Description | Common Use Cases |
|---|---|---|---|
| Class I | 0.1 or less | Vapor Impermeable: Highly resistant to vapor diffusion. Often used where no drying to the interior is desired or where specific vapor drive conditions necessitate a strong vapor retarder. | Polyethylene film, rubber membranes, foil-faced insulation. |
| Class II | 0.1 to 1.0 | Vapor Semi-Impermeable: Offers significant resistance to vapor diffusion but allows for some limited drying. | Some asphalt-coated felts, unfaced extruded polystyrene (XPS). |
| Class III | 1.0 to 10 | Vapor Semi-Permeable: Provides moderate resistance to vapor diffusion. Often suitable for walls designed to dry in at least one direction, commonly used in cooler climates. | Latex paint, some gypsum wallboard, fiberglass insulation (not an air barrier), specific house wraps. |
| Vapor Permeable | Above 10 | Vapor Permeable: Readily allows water vapor to pass through. These materials primarily function as air barriers and water-resistive barriers, allowing assemblies to dry out effectively. | Spunbonded polyolefin house wraps, many liquid-applied air barrier membranes, building paper/felt (asphalt-impregnated kraft paper), some oriented strand board (OSB). |
It's important to note that a material's vapor permeance does not inherently dictate its effectiveness as an air barrier; rather, it informs moisture management strategies within the building envelope.
Air Leakage: The Primary System Performance Rating
While vapor permeance is a material property, the overall performance of an air barrier system (which includes materials, assemblies, and connections) is primarily rated by its air leakage rate. This measures the volume of air that passes through a given area of the air barrier at a specified pressure difference.
Key standards and metrics include:
- ASTM E2178: This standard measures the air permeance of individual building materials used as part of an air barrier system. It provides data on how much air a specific material allows to pass through.
- ASTM E2357: This standard evaluates the air leakage of entire air barrier assemblies (e.g., a wall section including sheathing, insulation, and membranes). It assesses how well the combined components and their connections prevent air movement.
- Maximum Air Leakage Rate: Building codes and green building standards often specify a maximum allowable air leakage rate for air barrier materials or systems, typically expressed in cubic feet per minute per square foot (cfm/ft²) at a pressure difference of 75 Pascals (Pa). A common requirement for high-performance air barrier materials is 0.004 cfm/ft² at 75 Pa or less. For whole-building airtightness, targets are often expressed in air changes per hour (ACH) at a specific pressure.
Other Important Characteristics and Considerations
Beyond permeance and air leakage, other characteristics contribute to the overall "rating" and effectiveness of an air barrier system:
- Durability and Longevity: Ability to withstand UV exposure, temperature fluctuations, moisture, and aging without degrading.
- Adhesion and Compatibility: How well the air barrier material adheres to adjacent components and its compatibility with other building materials.
- Structural Integrity: Resistance to wind loads and other pressures that could compromise its continuity.
- Continuity: The air barrier must be continuous across the entire building envelope, free of gaps, holes, and unsealed penetrations. Detailing at junctions, penetrations, and transitions is critical.
- Flexibility and Movement Accommodation: Ability to accommodate building movement without cracking or tearing.
- Fire Resistance: Compliance with local fire safety codes and regulations.
Why Understanding These Ratings Matters
Correctly specifying and installing air barriers based on these ratings leads to:
- Enhanced Energy Efficiency: Reduced uncontrolled air movement minimizes heat loss or gain, lowering heating and cooling costs.
- Improved Moisture Control: Prevents moisture-laden air from entering wall cavities, reducing the risk of condensation, mold, and rot.
- Superior Indoor Air Quality: Blocks pollutants, dust, and allergens from entering the building.
- Greater Thermal Comfort: Eliminates drafts and maintains more consistent indoor temperatures.
