Creating a home that withstands the test of time involves more than just a solid structure. The integrity of your home’s building envelope plays a crucial role in its longevity and health. Imagine a scenario where the unseen forces of air and moisture threaten to compromise this envelope. To safeguard your home, it’s essential to understand the roles between an air barrier and vapor barrier.
The Basics: Air Barrier vs Vapor Barrier.
An air barrier, situated on the exterior of the building envelope, acts as a shield against the infiltration of air through the structure. It can be fashioned from various materials such as house wrap, plywood, or gypsum board sheet products. In contrast, a vapor barrier, located on the interior or “warm side” of the building, focuses on preventing moisture from permeating the building envelope. Materials like polyethylene sheeting or foil-faced kraft paper are commonly used for this purpose.
Why Distinguish Between An Air Barrier and Vapor Barrier?
While both barriers are crucial components of a building envelope, they serve different purposes. Air barriers aim to prevent the flow of air and the moisture it carries into a building, while vapor barriers solely focus on preventing moisture transport through vapor diffusion.
Air barriers take precedence because the amount of moisture carried by airflow is significantly greater than that carried by vapor diffusion—50 to 100 times greater, to be exact. Impermeable vapor barriers, if not installed correctly, may lead to mold and rot issues.
The Mold Factor:
It’s crucial to clarify that vapor barriers don’t cause mold when installed correctly without gaps or holes. Moisture becomes a problem only if the vapor barrier is improperly installed or in the wrong location, creating ideal conditions for mold growth.
Mold needs food, water, oxygen, and warmth to thrive. Allowing a building assembly to “breathe,” meaning it can dry in both directions when wet is important. This significantly reduces the likelihood of moisture problems and microbial growth. If you run into mold, try reaching out to Healthy Building Science in the San Francisco Bay Area for an inspection or testing.
Rating Air Barriers and Vapor Barriers:
Air barriers are rated based on their air permeance, measured as the quantity of air that permeates through a product. The recommended minimum air permeance for air barrier systems is 0.02 L/(s·m²) at a pressure difference of 75 Pa, as per ASTM E 2178. Here are some examples:
- Gypsum board: 0.5 L/(s·m²) at 75 Pa
- Plywood: 0.5 L/(s·m²) at 75 Pa
- Extruded polystyrene insulation: 0.0002 L/(s·m²) at 75 Pa
- Closed-cell spray foam insulation: 0.0001 L/(s·m²) at 75 Pa
Vapor barriers, on the other hand, are classified by their vapor permeance or perm rating. Here are the classifications:
- Vapor impermeable (0.1 perm or less): Glass, sheet metal, polyethylene sheet, rubber membrane.
- Vapor semi-impermeable (1.0 perm or less and greater than 0.1 perm): Unfaced expanded or extruded polystyrene, 30-pound asphalt-coated paper, plywood, bitumen-coated kraft paper.
- Vapor semi-permeable (10 perms or less and greater than 1.0 perm): Gypsum board, fiberglass insulation (unfaced), cellulose insulation board lumber, concrete block, brick, 15-pound asphalt-coated paper, house wrap.
- Vapor permeable (greater than 10 perms): Unpainted drywall and gypsum board, Tyvek™, Typar™, and other house wrap materials.
Conclusion:
Understanding the roles and proper installation of air barriers and vapor barriers is vital to the health and longevity of your home. For more details on vapor and air barriers, Fine Building has a great blog discussing the topic in detail. If you need assistance with your project in the Napa, Sonoma, or Marin Counties, don’t hesitate to call the pros at Healthy Building Science.
