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Air Barrier vs. Vapor Barrier- What Contractors Need To Know

Insulation being put in the house

Air barrier, vapor barrier - though the terms are often used interchangeably, they're actually responsible for very different tasks on a work site. Using the right product can mean a huge difference between a successful job and one that is plagued with mold, mildew and moisture problems. In this post, we'll discuss the differences between air and vapor barriers and why it's important to know and the differences on your work site.

Air Barrier

An air barrier does exactly what it says it does - it blocks air leakage both out of and into the building envelope. Air leakage on a completed structure often ends up being a much more significant problem than the architect and engineers anticipated in their design, with a variety of materials used in the past as air barriers often falling far short of what was intended for the project. Building felt, blocks of concrete, different building wraps and gypsum drywall were all believed to be good air barriers in the past, but can actually allow significant amounts of air into the building envelope, so a better solution was needed.

Foam InsulationBecause warm, humid air carries significant amounts of water vapor along with it as it enters or exits the building envelope, air barriers often block the majority of moisture that can come into the structure. Air barriers can include building wraps that are nailed down, sprayable membranes, brushed- or rolled-on sealant materials, insulating foam boards, non-insulating boards, polyurethane spray foam, cast concrete, a wide variety of metals, glass, and many other building materials.

In addition to air leakage, you also need to deal with air permeance. Air permeance refers to the amount of air that actually goes through a material, rather than taking advantage of holes or gaps. A material with an air permeance that is below 0.02 L/(s·m²) at a 75 Pa (0.004 cfm/ft2 at a pressure difference of 1.56 lb/ft2) pressure difference when it is tested under ASTM E 2178 meets air barrier standards. A material with a higher air permeance may not meet your client's needs and will need to be considered carefully for sealants and similar products.

Vapor Barrier

By comparison, a vapor barrier lowers how much water vapor can diffuse through wall materials due to a gradient of different vapor pressures. Air leakage is responsible for moving the majority of the moisture into a building. Because of this, a vapor barrier often isn't nearly as vital to a building's health as a solid air barrier proves to be in many cases.

Insulation in an atticA vapor barrier blocks or at least slows down the movement of water vapor through the material itself, rather than being carried in through holes and gaps.  Vapor barriers are placed on the side of a building's insulation that is more often in contact with warmth, whether from a hot exterior or a hot interior. Because water vapor condenses out of the air at cooler temperatures, keeping the warm, moist air, and specifically water vapor, away from a colder space is vital to preventing mold, mildew and moisture problems in your structure.

Vapor barriers come in similar formulations to those seen as air barriers, but the thickness of the material also makes a big difference with regards to how much moisture can diffuse through the material, with thicker material blocking more of the air and vapor from transmitting through the material. The transmission of water vapor through material is measured and the calculation of the vapor permeance is the end result. Testing using ASTM E96 provides measurement of how much water vapor diffuses through a material.

To properly integrate materials correctly in constructing a building, designers have to understand how the materials' properties will intersect with their design to create a successful structure. At Wallboard Supply Company, we're happy to provide a variety of materials and a highly-experienced team to help meet your construction needs. Contact us today for more details on vapor and air barriers, to get a quote or to schedule a delivery.


3m's Air Barrier Solutions Guide CTA

Photo credit: Logantech / Foter / CC BY

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Wallboard Supply Company is a third generation, family run business that has been serving New England's building needs since 1970. Bob Filion started the company with a commitment to provide quality drywall and finishing products with unmatched customer service. The company has grown over the years, expanding its' product range, but never wavering from its' core values.