Protecting Your Home: Understanding Air Barriers vs Vapor Barriers

Jan 25, 2024General Contracting, Home Maintenance, Indoor Air Quality, Insulation, Remodeling, Special Projects, Sustainable Building

Creating a home that lasts involves more than just building a sturdy structure. How well your home holds up over time depends on its protective layer. Imagine a situation where air and moisture sneaks in and causes internal problems. To keep your home safe, it’s important to know about air barriers and vapor barriers—kind of like the shields that help keep your home strong and healthy.

Understanding the Fundamentals:

An air barrier, positioned on the exterior of the building envelope, serves as a shield against the infiltration of air through the structure. Various materials, such as house wrap, plywood, or gypsum board sheet products, can be used to create an effective air barrier. On the other hand, a vapor barrier, situated on the interior or the “warm side” of the building, focuses on preventing moisture from permeating the building envelope. Common materials for vapor barriers include polyethylene sheeting or foil-faced kraft paper.

Why Distinguish Between Air Barriers and Vapor Barriers?

While both barriers are integral components of a building envelope, they have distinct 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, precisely. Improperly installed impermeable vapor barriers may lead to mold and rot issues.

Navigating Mold:

It’s crucial to clarify that vapor barriers don’t cause mold when installed correctly without gaps or holes. Moisture becomes problematic only if the vapor barrier is improperly installed or in the wrong location, creating ideal conditions for mold growth.

Mold requires food, water, oxygen, and warmth to thrive. Allowing a building assembly to “breathe,” meaning it can dry in both directions when wet, significantly reduces the likelihood of moisture problems and microbial growth. If you encounter mold, consider reaching out to experts in your area for inspection or testing. Call the pros at Healthy Building Science.

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.

In Conclusion:

Understanding the roles and proper installation of air barriers and vapor barriers is vital for the health and longevity of your home. For more details on vapor and air barriers, consider exploring Fine Building’s comprehensive blog on the topic. If you need assistance with your project in the Napa, Sonoma, or Marin Counties, feel free to reach out to the professionals at Home Stewards.


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