In order to design and build safe, healthy, durable, comfortable and economical buildings airflow must be controlled. Why? Because airflow carries moisture that impacts a building’s long-term performance as well as it’s indoor air quality and thermal energy.
It’s no longer a question of should you use an air barrier, but how to design and install high-performance air barriers that will stand the test of time.
What is an Air Barrier?
Perhaps we should start at the very beginning: what is an air barrier?
When Joseph Lstiburek (Ph.D., P.Eng., ASHRAE Fellow) wrote his summary of air barriers for Building Science Corporation, he defined them in this way:
“Air barriers are systems of materials designed and constructed to control airflow between a conditioned space and an unconditioned space… Air barrier systems should be: impermeable to air flow; continuous over the entire building enclosure or continuous over the enclosure of any given unit; able to withstand the forces that may act on them during and after construction; and, durable over the expected lifetime of the building.”
Plainly speaking,by essentially “wrapping” the building shell, air barriers (a.k.a. air sealing) ensure that the building is protected from the effects of airflow and air leakage. Thereby controlling a building’s moisture permeability, indoor air quality, thermal energy, and preventing the spread of smoke or even fire.
The walls, floors, and roof and ceiling make up the physical shell of the home, the building envelope. Within each of these assemblies are products that create the building’s thermal envelope and air barrier. And this continuous assembly has important benefits for the building.
4 Benefits of Air Barriers
Preventing the loss of conditioned air. For most consumers, the biggest reason “why” is comfort. In summer, we normally cool and dehumidify the air to a lower temperature and humidity than the exterior environment. In winter, we typically heat and humidify the air to a higher temperature and humidity than the exterior. Controlling interior temperature is paramount to comfort.
The United States Department of Energy reports that over 30 – 40 percent of the cost of heating and cooling a home is lost to uncontrolled air leakage. This can hamper the performance of other building systems such as insulation and HVAC. Proper air sealing helps reduce uncomfortable temperature fluctuations and often allows for smaller, more efficient HVAC equipment.
Lower utility bills. Maintaining conditioned air means less energy is needed to recondition the air. Less energy means lower utility bills. And since all building systems must perform well together to optimize the energy efficiency of a home, the savings can add up. Buildings which have a properly installed air barrier system can operate properly with a smaller HVAC system as the mechanical engineer does not have to compensate for a leaky building. In some cases, the reduction in mechanical equipment size and cost can also offset the cost of the air barrier system in addition to lowering utility bills.
Preventing moisture. Wherever air moves, water vapor can follow. Proper air sealing reduces the risk of water vapor moving into the wall system where prolonged exposure can result in moisture issues such as wood rotting and mold, which can cause expensive structural or health problems. Air leakage has the ability to transport exponentially more moisture into and through the building enclosure than occurs through vapor diffusion alone.
Indoor air quality. Air barrier systems help keep out pollutants such as suspended particulates, dust, allergens, insects, odors, noise and more.
The International Energy Conservation Code (IECC), the DOE Zero Energy Ready Home program and several state energy codes (see California Title 24) now require the use of air barriers, as do a growing number of municipal authorities, federal agencies, and large owner and developer groups.
Materials and Best Practices
Let’s recap: an air barrier’s first job is to ensure that a continuous air barrier exists around the entire thermal envelope of the home, with no breaks. And any breaks that occur must be secured.
To do that, we must first identify what materials will constitute the air barrier. Ideally, construction documents should show the location of a building’s air barrier, and should explain how the builder is expected to maintain air-barrier continuity at penetrations and important intersections. But if it does not, contractors and builders would think through components of the home’s thermal envelope (walls, floors, and ceiling) and how these components will join.
A simple test? Can you draw an unbroken line, representing the air barrier, on your plans?
A note on codes and compliance: Code requires that, for residential construction, the thermal layer of insulation be in full continuous contact with the home’s continuous air barrier (see 2009, 2012, and 2015 IRC). The home’s thermal barrier of insulation must also be continuous for best performance — discussed here and here. It should be installed without misalignments, compressions, gaps, or voids. See the Building America guide, Insulation Installation Achieves RESNET Grade 1.
So what material is best? The air barrier can consist of any durable solid material that blocks air flow between conditioned space and unconditioned space, i.e. drywall, OSB, or rigid foam insulation can serve as an air barrier. While ENERGY STAR® recommends rigid air barriers, flexible air barriers such as house wrap are acceptable if they are fully sealed at all seams and edges and supported using approved fasteners.
ENERGY STAR requires that an air barrier be installed at the exterior vertical surface of the wall insulation in all climate zones and also that an air barrier be installed at the interior vertical surface in Climate Zones 4-8. For ceilings, ENERGY STAR permits the air barrier to be at the interior or exterior horizontal surface in IECC Climate Zones 1-3 and at the interior horizontal surface in Climate Zones 4-8 (ENERGY STAR 2015).
Visit the Building America Solutions Center to learn more about codes and compliance.
5 Keys to Comparing Air Barrier Systems
Air barrier systems in buildings are designated as “systems” because various materials must combine to make an airtight plane that controls airflow. The critical areas for any airtight plane are a building’s material transitions, penetrations, and joints.
So, when choosing the right air barrier materials and/or creating a system, keep the following five criteria in mind:
- Continuity. Air barrier systems must be continuous. Without continuity, there is no point in considering the system’s other performance qualities. At the buying or comparison stage of the process, it’s important to understand how easily continuity can be achieved.
- Strength. The air barrier system must be capable of enduring winds and transferring wind loads to the structure without tearing.
- Durability. The air barrier must stand up to the stresses of building construction, since it will go up—especially an exterior system—early in the construction process. Overall lifespan and ease of repairs or replacement are also important factors to consider.
- Stiffness. Air barriers must not only absorb and transfer wind loads, but they also must distribute loads evenly. Stiffness not only helps with wind pressure, but it is very important to the function of the rain-screen, the part of the enclosure that manages bulk water.
- Impermeability. As the name implies, an air barrier is impermeable by air. It’s impossible to stop all air leakage, but there are standardized leakage rates for residential and commercial properties.
Ensuring Continuous Insulation & Air Sealing
Simply speaking, for the air barrier to be continuous, any seams between sheets of material, or joints between one material and another, or holes must be sealed. For example, seams in rigid foam can be sealed with compatible tapes, and gaps around windows can be sealed with foam rods, spray foam, and self-adhesive flashing. For practical tips on seaming housewrap to achieve optimum performance, visit the Contractor’s Field Guide to House Wrap & Seaming, and 3 Steps to Seam HouseWrap the Right Way.
Suffice it to say, it’s no longer a question of should you use an air barrier, but how to design and install high-performance air barriers that will stand the test of time. And securing a continuous, unbroken barrier means that barrier will last.