How to Determine the Right Air Barrier for You

By Lisa Petsko

It can be confusing at times to determine what type of air barrier is the best choice when constructing a home or building. According to the Air Barrier Association of America (ABAA), by definition, air barrier systems are a component of building envelope systems that control the movement of air into and out of buildings. There are many types of air barriers to choose from, and there are many important factors to consider when determining the best barrier.

WAM!: Water, Air and Moisture
Using an air barrier system will result in energy cost savings, as well as reduce the amount of airborne pollutants that result from the combustion of energy producing fuels. The United States Department of Energy concluded that up to 40% of the energy consumed to heat or cool a building is due to air leakage into and out of buildings. A properly installed air barrier system will substantially reduce the amount of air leakage of a building envelope system, thus reducing the building's energy consumption.

Water accumulation in building envelopes is a consideration to weigh when choosing the right barrier system. Water is stealthy and has many ways of penetrating the building envelope. Wind can drive rain through tiny cracks or holes in materials. Capillary action at cracks, holes or in porous materials draws water toward the interior. And water vapor transported by air or by diffusion can condense on cold surfaces hidden within the building envelope.

According to several studies completed in the last decade, air leakage in particular has proven to be a significant potential source of condensation and moisture accumulation in building envelope assemblies. Thus, in addition to preventing water intrusion with design and construction details that protect against wind-driven rainwater entry, minimizing airflow through the building envelope with an air barrier system is also important. An air barrier system that reduces air leakage and is waterproof, is an efficient way of preventing moisture deterioration of building materials.

U.S. & Canadian Codes
Energy codes in the United States have begun to require airtightness of the building envelope, but they are not specific about levels of air permeability. Various states are considering duplicating the Canadian air barrier code into their state codes. Massachusetts and Wisconsin are the only two states with air barrier requirements at this time. However, several states have passed ASHRAE 90.1, considered the definitive guideline for achieving energy efficiency, which requires that all critical details have to be airtight. The generally accepted level based on National Building Code of Canada requirements is 0.02 L/(s-m2) at 75 Pa pressure (0.004 cfm/ft2 at 1.57 psf).

While many common building materials like plywood and gypsum wallboard meet this standard, a sheathed wall assembly will not perform well as an air barrier unless the joints are treated with an air barrier material. The sheathed wall assembly with treated joints then becomes an air barrier sub-system of the total building envelope air barrier system. The total building envelope air barrier system consists of all the interconnected air barrier materials ? for example, CMU backup with joint treatment, roof membrane, foundation waterproofing, windows and doors, and the air barrier connection materials between them.

Types of Air Barriers
One choice is a new category in the market, a fluid-applied waterproofing/air barrier. Fluid-applied waterproofing/air barriers for wall assemblies have actually been manufactured in North America for more than 25 years. Only recently has their use increased as air leakage has become recognized as a potential source of moisture accumulation in walls, and some of their unique benefits have been realized.

Fluid-applied waterproofing/air barriers are rolled or sprayed onto masonry substrates or CMU backup and become part of the structural wall. Because of the way they are applied, there are no fastener holes where water penetration may occur, and there is no potential for mislapping or tearing, as with many sheet goods. During construction, a fluid-applied barrier will cover the substrate completely, and does not have to be covered immediately with a cladding, as many of them are UV-resistant.

Another important distinction of a fluid-applied waterproofing/air barrier in wall assemblies is that it can mitigate or eliminate one of the major forces that causes water infiltration into walls: pressure difference. A fluid-applied waterproofing/air barrier, in combination with venting and compartmentalizing, enables the pressure behind the cladding material to equalize with the pressure outside. This prevents rainwater penetration caused by pressure differentials. This pressure equalizing effect is only possible when the air barrier is structural, as is the case with fully adhered fluid-applied waterproofing/air barriers. Also, by designing and constructing an "airtight" building envelope, the risk of moisture problems ? mold growth, decay, corrosion, loss of insulation value and Indoor Air Quality (IAQ) problems ? that can occur because of air leakage and condensation are minimized.

Another type of air barrier is a house wrap, or sheet good product. These materials are usually applied under a home or building's veneer and over the CMU backup. It is important to consider the climate when selecting one of these types of air barriers, as some weather better, are more water-resistant, and are more resistant to tearing. They come in a variety of sizes for different purposes. They are wrapped around the exterior of a house during construction and cut around windows and doors. House wrap tape is used to seal the joints, and flashing is applied.

Key Factors for Selection
There are many factors to consider when making your choice, but there are some key distinctions between the many types of barrier systems on the market today. Ask these key questions when selecting the barrier system for your next project:

Is it an air barrier? An air barrier reduces risk of condensation caused by air leaks through the wall construction and reduces energy costs by reducing heating/cooling loads.
Is it a waterproof coating? A waterproof coating minimizes risk of water damage to CMU backup and associated repair or replacement costs.
Is it vapor permeable? A vapor permeable choice is breathable and minimizes the risk of moisture getting trapped in the wall cavity.
What about the structure? If it is applied correctly there should be no air leakage or moisture intrusion between the CMU backup and the barrier.
Is it durable? Make sure your barrier system will not tear or lose its effectiveness with exposure to weather during construction or while in service.
Does it resist UV degradation? If it does, it will give peace of mind if construction delays occur, as you won't have to worry about extreme UV rays deteriorating performance.

No matter what type of barrier system is selected, the benefits of using an air barrier system and the energy costs savings far outweigh the initial cost of installing these systems.

For more information about barrier systems, visit the ABAA web site at www.airbarrier.org, the U.S. Department of Energy web site at www.eere.energy.gov, or any of the manufacturers' web sites.