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Air Barriers are Essential – How to Determine the Right Air Barrier for You
By: Lisa Petsko - Tuesday, May 9, 2006
Source: Air Barrier Assoc.

It can be confusing at times to determine what type of air barrier is the best choice when building 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.

Using an air barrier system will result in energy cost savings, as well as reduce the amount of air borne pollutants that result from the combustion of energy producing fuels. The United States Department of Energy has 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. To prove this, a recent study conducted by the National Institute of Standards and Technology (NIST) determined that air barriers are a cost effective measure to be installed in buildings, showing that the energy savings ranged from 30 – 40% for heating climates and 10 - 15% for cooling climates.

Water accumulation in building envelopes is also 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 towards the interior. And water vapor transported by air or by diffusion can condense on cold surfaces hidden within the building envelope.

Air leakage, in particular, has proven to be a significant potential source of condensation and moisture accumulation in building envelope assemblies, according to several studies completed in the last decade. Thus, in addition to preventing water intrusion with design and construction details that protect against wind driven rain water entry, minimizing air flow 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.

Energy codes in the United States have begun to require air tightness of the building envelope, but they are not specific about levels of air permeability. 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, wall sheathing with joint treatment, roof membrane, foundation waterproofing, windows and doors, and the air barrier connection materials between them.

So, just what are your choices among air barriers? One choice is a new category in the market, a spray-on waterproofing/air barrier. Spray-on or roll-on waterproofing/air barriers for wall assemblies have actually been manufactured in North America for more than 25 years. Only recently has their use become more popular as air leakage becomes recognized as a potential source of moisture accumulation in walls, and some of their unique benefits are realized. Spray-on waterproofing/air barriers are rolled or sprayed onto masonry substrates or sheathing 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. There is no need for tape or flashing because of the way it is applied. During construction, a spray-on 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 spray-on 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 spray-on waterproofing/air barrier, in combination with venting and compartmenting, 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 spray-on waterproofing/air barriers.

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. Spray-on waterproofing/air barriers distinguish themselves from many sheet good products by providing structural integrity, UV resistance, and waterproofing, in addition to air tightness, and the ability to design a pressure equalized wall assembly.

Another type of air barrier is a house wrap, or sheet good product. These materials are usually applied under a home or building’s siding and over the sheathing. 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 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.

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:

1. Is it an air barrier? An air barrier reduces risk of condensation caused by air leaks through the wall construction; reduces energy costs by reducing heating/cooling loads.

2. Is it a waterproof coating? A waterproof coating minimizes risk of water damage to sheathing and associated repair or replacement costs.

3. Is it vapor permeable? A vapor permeable choice is breathable and minimizes the risk of moisture getting trapped in the wall cavity.

4. What about the structure? If it is applied correctly there should be no air leakage or moisture intrusion between the sheathing and the barrier.

5. Is it seamless? You don’t want tears, holes, or mislapped joints that can compromise performance in service.

6. 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.

7. 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 will far outweigh and offset the initial cost of installing these systems.

For more information about barrier systems, visit www.airbarrier.org www.eere.energy.gov (U.S. Department of Energy), or any of the manufacturers’ web sites.

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