July 1, 2012
By Andrew Hunt and Monica Karamagi
Energy use, indoor air quality (IAQ), comfort, moisture intrusion, and durability are important considerations for design/construction professionals. To create a healthy, energy-efficient living space, a facility must be able to control airflow and water movement through the building envelope. The phrase “build tight, ventilate right” is used by building science experts to express the recommended building approach. One way to help address these concerns is through the use of sprayed polyurethane foam (SPF) insulation or other materials with similar properties.
In residential and commercial construction (both new and retrofit), the building envelope should be the first thing architects and builders consider when trying to create a more energy-efficient, durable, and healthy living environment. It is the ‘shell’ protecting the structure from water intrusion and uncontrolled air movement between conditioned and unconditioned spaces.
The building envelope must be complete, sealing all penetrations and gaps around the entire structure, as well as being durable and capable of controlling airflow. The key component to designing and installing a complete and successful building envelope is the air barrier. This is the physical material used to block air from moving into the facility from the outside (i.e. infiltration), or conditioned air from escaping to the outside (i.e. exfiltration).
What is an air barrier?
In simple terms, an air barrier is a material (e.g. film, sheet, coating, or membrane) that controls or stops airflow from the outside of the building to the inside. It can be made from many different materials, but to be effective, it must be properly installed and provide a complete shield around all sides of the building. When installed in a structure, the air barrier material becomes part of an air barrier assembly, which combines with the building’s windows, doors, and other design features to form an air barrier system.
The primary objective of the system is to block the random air movement into and out of a building and its walls and roof assemblies. A facility with unchecked air movement can have a host of problems, including higher energy use and costs, water intrusion, moisture issues, and poor IAQ.
There are five key attributes to a well-designed and successfully installed air barrier system:
A well-constructed air barrier that satisfies these requirements will help control moisture in a building, reduce pollutants, improve air quality, and save energy and money on heating and cooling costs.
Uncontrolled air movement between unconditioned and conditioned building spaces can lead to indoor air quality issues. As air moves from unconditioned spaces (e.g. attics, crawlspaces, utility rooms, attached garages, and the exterior), it can transfer dust, allergens, pollutants, exhaust fumes, and chemicals directly into the interior environment. When these pollutants enter the common breathing space, they can create a host of issues for occupants, including:
While these ailments may be short-term, continual exposure to indoor air pollutants have the potential for chronic health effects.
In a home, controlling air movement from an attached garage to the living space is critical. Exhaust fumes from cars contain many known pollutants, including carbon monoxide (CO). Often, a garage will contain gasoline, pesticides, paints, and cleaning products, which can release toxic chemicals that must be kept from the living environment. An improperly installed or insufficient air barrier between the attached garage and the living area can be a serious concern for occupants, and is a National Building Code of Canada (NBC) violation.
IAQ can also be compromised by the presence of excessive water vapour. Moisture moves into and out of the home either as a liquid or as water vapour. While plumbing leaks, wind-driven rain, or minor flooding may be dramatic, some water damage in homes is caused by vapour due to uncontrolled air movement.
Exfiltration in colder climates poses a problem for IAQ. Warmed interior air will often be more humid than cold exterior air, and as the humid air passes through the walls, moisture condenses on the inside of exterior walls. Humidity from cooking, laundry, and simply breathing can create large amounts of moisture, which can collect in wall cavities and unconditioned spaces. Once there, it can condense and cause damage to the insulation. Excessive water in framing members can also lead to rot, compromising structural integrity.
Standing water or water-damaged framing members and insulation can also quickly result in mould and bacteria growth. This is of special concern because spores in the air can result in headaches, breathing difficulties, allergic reactions, and aggravated asthma symptoms.
Reducing heating and cooling costs
According to Natural Resources Canada (NRCan), “air leakage represents 25 to 40 per cent of the heat lost from an older home.” To create comfortable work and living environments for occupants, and to reduce heating and cooling costs, a building should be insulated and air-sealed. However, an improperly installed, poorly sealed, or incomplete air barrier can effectively negate insulation and other energy-saving devices. Even small gaps or holes in the air barrier allow unconditioned air to move freely throughout, making the space uncomfortable.
The amount of airflow migrating from inside the home to the outside can be quantified in terms of natural air changes per hour (NACH). A NACH rating of 1 means the home’s total volume of air is replaced with outside air over the course of an hour. For example, a typical existing North American home can have a NACH rate of 0.5 to 1. This means every one to two hours, the entire air volume within the home is exchanged with air from the outside. Every bit of air entering through infiltration must be heated or cooled to keep the building at the thermostatic set point, making leaky structures expensive to heat and cool.
There are many advantages of controlling air infiltration and exfiltration. The primary benefit is energy costs can be greatly reduced by lowering thermal transfer between the home’s inside and outside. This can provide healthier IAQ by decreasing the amount of pollutants and toxins entering the living space, and also lessen the chance of structural damage to the building through moisture damage.
Successful design and installation
When properly installed, an air barrier should reduce or stop air leakage and movement caused by wind, stack effect, and air pressures from mechanical equipment and ventilation units. To successfully create an air barrier, a combination of components and materials must be properly and continuously assembled around the entire building envelope.
There are several air barrier materials commonly used today in residential and commercial construction. These include:
Each of these can reduce airflow, but will have varying results depending on the air barrier assembly’s design and installation. Some assemblies use a combination of these materials.
To create a successful air barrier assembly, one material must be selected as the main air barrier, and then be continuously connected around the facility. Often, finished surfaces or structural materials like drywall, sheathing, and decking will be specified. This can be convenient for builders because the facing materials of ceilings, walls, and floors have to be installed anyway. Without a deliberate focus on designing and installing a complete air barrier around the entire structure, gaps and holes are inevitable.
It is important to note a material’s permeance is simply a measurement of the air migrating through the air barrier. Gaps, spaces, or holes around the air barrier that result in infiltration and exfiltration destroy its effectiveness. To reduce the amount of uncontrolled air flowing through a building, the air barrier must be part of a continuous assembly. There are many ways to construct an air barrier assembly, and the type of material used often determines the most effective design.
The primary air barrier material or system can be located anywhere within the building enclosure––near the interior, near the exterior, or in between. This choice can be made irrespective of climate. The exception is when the air barrier material is also a vapour retarder, which is the case with certain types of SPF and numerous fluid-applied and peel-and-stick air barrier materials. If the material is also a vapour retarder, it should be located on the insulation’s warm side.
Air barrier materials
Air barrier materials are tested for their air permeance, or the volume of air passing through the material under given conditions. Testing is done according to standards established by ASTM, which creates standards for air barrier materials and assemblies, along with whole building performance testing. The lower the permeance, the better the material or assembly at blocking airflow.
ASTM E2178, Standard Test Method for Air Permeance of Building Materials, is the test method for determining air permeance of building materials. The industry-accepted baseline for an effective air barrier material is not to exceed 0.02 L/(s•m2) @ 75 Pa (0.004 cfm/sf @ 1.57 psf).
In ASTM E2357-05, Standard Test Method for Determining Air Leakage of Air Barrier Assemblies, wall, roof, window, and door assemblies are put through a test series to measure the volume of air passing through them. The industry-accepted baseline for an effective air barrier assembly is not to exceed 0.2 L/(s•m2) @ 75 Pa (0.04 cfm/sf @ 1.57 psf)—this is 10 times the limit for air barrier materials.
Lastly, whole-building performance testing may be done using a blower door test. In this test, the structure is either pressurized or depressurized to a standard pressure using fans. The larger the airflow required through the fans to achieve the standard pressure, the leakier the building envelope.
Gypsum drywall and plywood
Rigid building materials—such as gypsum drywall and plywood––are sometimes selected as the primary air barrier material due to their relative low cost and familiarity. An unpainted, 13-mm (½-in.) sheet of gypsum wall board has an air permeance rating of 0.002 L/(s•m2) @ 75 Pa (0.004 cfm/sf @ 1.57 psf)—the maximum allowable amount of leakage an air barrier material can have. (For more information, see “Understanding Air Barriers” by Joseph Lstiburek, PhD, P.Eng., FASHRAE. Visit www.inhomesprayfoam.com/uploads/6/8/7/1/6871103/2-understanding_air_barriers_-_all_color.pdf). Gypsum often fails, however, as the primary air barrier material because of the difficulty and time to create a continuous air barrier. A system called the airtight drywall approach (ADA) requires joints between the gypsum board and connected assemblies and materials be sealed with gaskets. While this system works as an air barrier, it can be technically challenging to install.
One of the most common kinds of air barrier material is building wrap. This is usually made of fibrous-spun polyolefin plastic with additional materials woven in to increase strength and durability. A building wrap is delivered to the construction site in large rolls and attached to the structure using mechanical fasteners like staples. The material, once applied to the facility’s frame, is sealed using tape between each seam. Depending on the manufacturer, style, and material quality, building wraps can be effective water barriers, especially against wind-driven rain. However, they are less successful as an air barrier.
If unsupported on both sides, as is the case with a brick cavity wall, a building wrap can be challenged by negative wind loads, which can rupture the fabric or tear at the mechanical fasteners. When evaluated by the basic criteria for a successful air barrier, a building wrap is not the best choice as the primary material. These products can be easily damaged during construction, are not completely impermeable to airflow, and will be effective only as long as the adhesive on the tape used to seal joints and seams is maintained.
Similar to a building wrap, some building contractors use a single polyethylene sheet as an air barrier. Installed on the wall assembly interior, the polyethylene sheet is usually stapled to the wooden framing members and rests against fibreglass batt insulation. Although polyethylene has one of the lowest air permeable ratings—about 0.005 L/(s•m2) @ 75 Pa (0.001 cfm/sf @ 1.57 psf)—it is almost impossible to successfully secure the material without holes, gaps, and tears around the fasteners, transitions, and wall penetrations.
Self-adhering, or ‘peel-and-stick,’ membrane sheets are another type of air barrier material used in both residential and commercial construction. These large sheets of modified bituminous (mod-bit) material are usually made out of styrene-butadiene-styrene (SBS) rubberized asphalt that has been reinforced with a cross-laminated polyethylene. They are mostly impermeable to air, water, and water vapour.
Membrane sheets can provide a continuous air barrier and adhere well to most substrates. Sheet installation can be difficult, labour-intensive, and usually requires a skilled craftsman––particularly around penetrations and gaps. They must be properly supported across all voids or gaps in the wall structure to avoid undue stress. Additionally, care should be taken to follow the manufacturer’s instructions to minimize the chance of the sheets pulling away from the substrate material.
Spray-applied or painted-on air barrier materials are liquid membranes made from synthetic rubbers or elastomeric bitumen. As the name implies, in this case the material is sprayed onto the exterior of the building substrate before drying in order to form a continuous air barrier. These spray-applied air barriers may require several coats to ensure that the proper thickness is being achieved once the material has dried.
When properly installed, the spray-on membranes can perform well. They are also faster to install and generally less expensive than peel-and-stick membranes. However, spray-applied air barriers have marginal bridging capabilities around small gaps and cracks in the wall assemblies.
Sprayed polyurethane foam insulation is made by reacting two liquid components to form a urethane foam. The resulting foam matrix expands to fill gaps and cracks. It is often used as a water, vapour, and air barrier material in the building assembly. SPF can be sprayed between framing members or over substrate material walls on the exterior. It can also be sprayed as a roofing material. It immediately adheres to the surface, expands 30 to 120 times it size, and then solidifies into a foam matrix that fills gaps, cracks, and other air penetration points.
There are two kinds of SPF used in construction today: closed-cell SPF (ccSPF)—32 kg/m3 (2 lb/cf)—and open-cell SPF (ocSPF)—8 kg/m3 (½ lb/cf). Besides having an almost zero air permeable rating, ccSPF has a high insulating property with an R-6 rating per inch. When properly installed, SPF can be an ideal air barrier material because it forms a continuous air barrier around penetrations like pipes, door assemblies, and windows. Additionally, ccSPF is structurally strong enough to withstand significant air pressures from inside and outside the structure, and is very durable during and after construction. In addition to thermal and air barrier performance, ccSPF offers unique attributes in stormy climates by adding structural strength. It can also be used in commercial roof replacement as a simple, effective way to replace an old roof, while adding energy and water durability performance.
Open-cell SPF insulation also has a low air permeability rating (at a thickness around 127 mm [5 in.]) and an R-value of 3.7 per inch. It offers different advantages than ccSPF. For example, open-cell SPF is vapour-open, allowing wall assemblies to dry in both directions. It also has stronger acoustical benefits, and can be less expensive. Having ocSPF installed within the stud cavities can successfully create high-performing air barrier assemblies. It is not applied to the exterior wall or in a cavity wall assembly as it will not withstand bulk water ingress.
Other materials do not qualify as air barrier materials because they fail to stop an acceptable volume of air when put under pressure. These include:
Air barriers and building codes
Since a well-designed and properly installed air barrier system can dramatically improve a building’s energy efficiency, durability, and IAQ, it is unsurprising governing bodies require air barrier standards in building codes.
Building energy codes are minimum requirements for energy-efficient design and construction for new and renovated residential and commercial facilities. The expected trend is for all building codes to migrate toward standards mandating more energy-efficient design and construction.
The 2011 National Energy Code of Canada for Buildings (NECB) provides minimum requirements for the design and construction of energy-efficient structures and covers the building envelope, system, and equipment for HVAC. This code is similar to American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, which is already in use in some Canadian jurisdictions.
NECB is for new facilities, as well as substantial renovations in existing ones. It does not apply to farm buildings or to housing and smaller structures covered in NBC Part 9, “Housing and Small Buildings.” Further, Model National Energy Code for Houses (MNECH) provides technical requirements for energy-efficient house construction. This code applies to single-family houses of three stories or less and to additions of more than 10 m2 (108 sf).
An example of air barrier language found in ASHRAE 90.1 specifies air barriers be designed and noted in the following manner:
It also requires certain areas of the continuous air barrier be “wrapped, sealed, caulked, gasketed, or taped in an approved manner,” such as joints, junctions, penetrations, seams, transitions, and connections between different air barrier materials.
SPF as an air barrier
When trying to conform to Canada’s airtightness standards, it can be difficult to comply with the progressive requirements when using traditional air barrier materials. However, for many reasons, SPF insulation can be an ideal air barrier material because it consistently addresses the major concerns in both air barrier and insulation systems.
Some of the direct advantages of using SPF as an air barrier material instead of traditional materials include:
When buildings are tightly constructed, one can no longer count on ‘fresh air’ ventilation to naturally occur through gaps and cracks in the structure. Therefore, HVAC contractors must take control of the ventilation through mechanical means. One good place to consult for ventilation guidance is in ASHRAE 62, Ventilation for Acceptable Indoor Air Quality. Tight construction and mechanical ventilation that can be filtered and pre-conditioned will provide superior IAQ compared to random, uncontrolled natural ‘ventilation’ through air leakage.
Air barrier materials, assemblies, and systems are a critical part of energy efficiency in residential and commercial construction. Without a well-designed and complete air barrier, buildings can suffer from excessive air infiltration and exfiltration that can compromise the air quality, durability, and comfort of living environments.
Although there are many air barrier options, many traditional materials no longer satisfy the increasingly stringent requirements for energy efficiency because they are often not installed well enough to be airtight. Sprayed polyurethane foam insulation, however, continues to prove suitable due to its flexible installation and ability to create a continuous air barrier around the entire structure.
Andrew Hunt is the vice-president of Confluence Communications, which creates outreach and communication materials specifically designed to promote energy-efficient building practices and technologies in the residential and commercial construction market. He can be reached via e-mail at email@example.com.
Monica Karamagi is the regional marketing and industry affairs manager for the polyurethanes division of Huntsman Corp. She has a bachelor’s degree in chemical engineering from Texas A&M University, and a master’s degree in chemical engineering from Howard University (Washington, D.C.). Karamagi is currently a member of the Spray Polyurethane Foam Alliance (SPFA) board of directors, and leads the Energy Efficiency Task Group in the rigid foam committee of the Center of the Polyurethanes Industry (CPI). She is also an active member of North American Insulation Manufacturers Association (NAIMA). Karamagi can be contacted at firstname.lastname@example.org.
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