Volume 6 Issue 4 May 2005
All are Not Created Equal
by Larry Livermore
Expanding aerosol polyure-thane foam can enhance the installation quality of prime and replacement fenestration products by sealing the rough opening against air leakage. The installer’s usual approach is to finish installing a window from the outside of a building, then go inside and run a bead of the foam in the gap between the rough opening’s studs and the window frame. The foam then expands to assume the shape of the gap and hardens to provide an effective air barrier–a straightforward approach with the added benefits of being cost-effective and easy to use.
Problems often arise, however, when installers unknowingly use off-the-shelf, general-purpose foams not intended for windows. Great for many applications such products can cause problems in window installation because of the pressure exerted by the foam as it cures and expands. The excessive pressure can induce frame deflection, which can hamper operation of the sash or cause gaps to form that actually degrade air infiltration performance. Many window manufacturers have discouraged the use of these foams for that reason.
To their credit, foam manufacturers have responded to the problem by developing a new generation of products expressly for fenestration installation. Technically known as single-component polyurethane foams, these products are formulated to exert minimal pressure on fenestration products during the cure phase.
Foams can be characterized as having high-, medium- or low-pressure build exerted by the fully cured product. Actual pressure build values can be obtained from the foam supplier or determined by tests.
Once the maximum pressure has been established, it can be translated into the resulting window frame deflection for a given fenestration product by calculation or by physical tests. Maximum theoretical deflection of a sample beam or framing member can be calculated knowing the pressure build of the foam as the applied load, the modulus of elasticity of the beam material and the moment of inertia. This allows a window manufacturer to determine which foams are suitable for the installation of their products with greater specificity.
An indicator of foam performance over the long term is its dimensional stability, defined as the resistance of the cured foam to shrinkage or expansion under a variety of temperature and humidity conditions. While excessive expansion can produce frame deflection as noted, excessive shrinkage can compromise the foam’s ability to provide an effective air seal. A foam is generally considered suitable for installation applications if its dimensional stability is no greater than plus or minus 10 percent.
A New Foam Standard
Working through AAMA, foam manufacturers have helped develop new standards for the safe use of foam products in the installation of fenestration products. AAMA 812-04, “Voluntary Practice for Assessment of Single Component Aerosol Expanding Polyurethane Foams for Sealing Rough Openings of Fenestration Installations,” is based on the properties of pressure build and dimensional stability and offers test protocols for determining the extent of these key parameters.
Pressure build is measured by filling a cavity in a wooden test fixture with foam and measuring the developed pressure with a pressure transducer as the foam cures. The maximum pressure build is determined from the average of pressure (psi) versus time data for five specimens.
To actually measure deflection, the standard recommends a test in which a sample of the material is fastened to a substrate and foam is injected between them. Actual deflection is monitored for 24 hours and maximum deflection is noted.
The test protocol for determining dimensional stability involves applying foam in the gaps formed by four layers of wood, which are then clamped together. After several days in the specified temperature and humidity conditions, the spacing between the wood layers is measured and compared to the original value.
It is also important for installers to follow the window manufacturer’s recommendations for expanding polyurethane foams. (For more information see www.dwmmag.com as portions of this column were deleted due to space constraints.)
Larry Livermore serves as technical standards manager for the American Architectural Manufacturers Association in Schaumburg, Ill.
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