The importance of air tightness in design and construction has been stressed for more than 25 years. Air leakage can lead to reduced occupant comfort, increased heating and cooling loads, damage to the building envelope, and even indoor air quality issues. As such, air barriers-a system of building envelope components that stop airflow into and out of buildings-have been code-required for commercial construction in Canada since the mid-1980s and in Massachusetts since 2001.
As other U.S. local codes consider adopting similar requirements, there is a heightened focus on practical design elements for air barriers. Aerosol expanding polyurethane foams-a cost-efficient, durable, and easily applied plastic foam material-can be of great advantage when used appropriately.
Controlling Air and Water Leakage
Building envelope enclosures are not continuous from material to material unless the gaps between them are connected. Most air intrusion occurs through the many joints and penetrations between wall products. Some polyurethane foam plastic sealants have shown to be a durable and efficient seal for these discontinuous areas.
Polyurethane foam plastic sealants can assist in controlling air leakage through building envelope penetrations such as windows, utilities, or interfaces between building envelope materials. When used to complete a continuous air barrier plane, polyurethane foam plastic offers an additional benefit-neutralizing the pressure difference across the building shell, which can help reduce water intrusion. Gap tests using a 0.58 kPa (12 psf) pressure difference have confirmed this air and water resistance in certain plastic foam sealants. These tests are in accordance with ASTM International E 331,Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference.
When seals are selected to join critical junctions in building envelopes, vapor diffusion should also be considered. Most plastic foam sealants are semipermeable, so they do not act as a vapor barrier.
Modeling can be employed for moisture accumulation predictions if desired. One example is WUFI-an advanced hygrothermal model that predicts heat and moisture transport (and accumulation) in building envelopes for many cities (see Oak Ridge National Laboratory site atwww.ornl.gov/sci/btc/apps/moisture). Designers should consider water and air intrusion issues for all components used in the building envelope.
In addition to preventing air and water intrusion, polyurethane foam plastic sealants also offer the potential for energy savings, improved comfort, weather resistance, sound mitigation, and reduced exterior noxious gas infiltration.
Structural and adhesion advantages
In certain cases, structural enhancements for building assemblies can be increased with polyurethane foam plastic sealants. Most foam plastic sealants adhere well to nearly all substrates, adding structural strength in some sealing applications. Windows are one example where a polyurethane foam plastic sealant can prevent side jamb rotation, raise window design pressure ratings, and help increase the anchorage of the window or door during high wind events.
The DP rating system measures the amount of pressure a window or door is designed to withstand when closed and locked, along with other performance factors, such as structural pressure, water penetration, and air infiltration. The higher the DP, the better the performance. This rating system is established in AAMA 101/I.S.2/NAFS-02 and AAMA/WDMA/CSA 101/I.S.2/A 440-05,Voluntary Specification for Aluminum, Vinyl, and Wood Windows and Glass Doors.
Certain foam plastic sealant products can perform these structural functions better than others; when these functions are critical, evaluations should be run using independent, third-party testing.
Adhesion and the attachment between building materials is another developing use for polyurethane foam plastic adhesives. In the aftermath of recent hurricanes, plastic foam roof tile adhesives were noted to perform well.
Plastic polyurethane foam sealants can be good for reducing sound transmission through gaps in wall, floor, or roof assemblies, helping minimize noise pollution. ASTM International C 919,Standard Practice for Use of Sealants in Acoustical Applications, quantifies sound reduction when gaps are sealed in building enclosure assemblies.
Testing Objectives and Standards
Early plastic polyurethane foam sealant testing focused on plastic product material properties borrowed from standard ASTM D 20 Committee on Plastics, tests intended for preformed cellular products. Current testing of foam sealants now focuses on building assemblies or subassemblies mirroring the actual end-use. Therefore, sample preparation for testing should be specific and simulate the foam plastic sealant geometry evident in the final application.
In 1997, the ASTM Committee on Aerosol Foam Sealants (C 24.61) began the task of developing germane standards for foam plastic sealants. ASTM C 1536 (“Standard Test Method for Measuring the Yield for Aerosol Foam Sealants”) and ASTM C 1620 (“Standard Specification for Aerosol Polyurethane and Aerosol Latex Foam Sealants”) have been published.
- a maximum leakage limit per ASTM E 283, Standard Test Method for Determining Rate of Air Leakage through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen
- a maximum allowed flame-spread index and smoke-developed index requirements per ASTM E 84, Standard Test Method for Surface Burning Characteristics of Building Materials
- a minimum requirement for R-value
- mandates the reporting of several additional foam plastic sealant properties (including a reporting requirement for foam plastic sealant yield measured exclusively by ASTM C 1536)
The American Architectural Manufacturers Association (AAMA) develops and publishes standards for fenestration products and installation practices, including AAMA 812-04,Voluntary Practice for Assessment of Single Component Aerosol Expanding Polyurethane Foams for Sealing Rough Openings of Fenestration Installations. The AAMA Foam Sealant Committee is dedicated to developing a standard for the minimum moisture performance requirements for window and door installation foams. Plastic foam sealants with pressure-build values as low as 0.55 kPa (0.08 psi) have recently been reported. When specifying this type, the pressure-build value should be quoted, per AAMA 812-04. Thus, the window manufacturer can select or specify a foam plastic sealant suitable to his or her window/door product.
Since the International Building Code does not specifically reference foam plastic sealants (or other sealants and most adhesives), local codes are often left to various interpretations. Unlike the denser tube sealants or adhesives they resemble in use, some code officials treat foam plastic sealants as if they were cellular plastic insulation. This can place excessive thermal barrier protection requirements on products for many sealant applications. Some manufacturers have employed Underwriters Laboratories 1715, Fire Test of Interior Finish Material, to obtain acceptance when this issue is in doubt. As such, diversified testing and International Code Council Evaluation Service reports are used to confirm the fire safety of existing applications or help gain acceptance for new ones.
The National Building Code of Canada specifically references foam plastic sealants. The Canadian National Standard, Underwriters Laboratories of Canada (CAN/ULC) S 710.1,Thermal Insulation-Bead: Applied One-component Polyurethane Air Sealant Foam, Part One: Material Specification, is a foam plastic sealant material requirement published in January 2005. Few construction products meet such rigid demands; CAN/ULC S 710.1 includes an air barrier assembly durability test using a full wall section with rapid thermal cycling from –20 to 66 degrees C (–4 to 150 degrees F) for 60 cycles. Pressure cycling is simultaneously employed from –1,000 to 1,000 Pa (–21 to 21 psf).
Clearly, polyurethane foam sealants can be of great advantage in today’s modern building projects. In addition to preventing air and water intrusion through the building envelope, they also offer the potential for energy savings, increased occupant comfort, weather resistance, sound mitigation, and improved indoor air quality. For more information on their use in sustainable design and construction, visitwww.GreenBuildingSolutions.org.