With the 2008 hurricane season dawning, we are reminded that resistance to wind-driven rain penetration is one of the primary functions of fenestration systems. While concerns naturally focus on exposed coastal regions, thunderstorms anywhere can briefly produce a credible imitation of a hurricane. So, to what level of wind and rain exposure should a window designer aim?

Water leakage is a complex phenomenon involving the interaction of gravity, capillary action, surface tension and pressure differentials. Fortunately, there are accepted practices and performance standards to provide design targets and laboratory tests to verify the designed performance. 

Door and window products that meet AAMA/WDMA/CSA 101/I.S. 2/A440-08 NAFS—North American Fenestration Standard/Specification for windows, doors and skylights (and applicable code-mandated predecessors) must pass water penetration tests of increasing stringency depending on their performance class and performance grade (PG); i.e., applicability to residential, commercial and architectural (typically high-rise) structures. 

Structural Integrity
In the 2008 standard, a primary consideration is the structural integrity, i.e., the ability of a product to resist design wind pressure. Water penetration resistance is linked to structural integrity through the Design Pressure (DP), based on the maximum wind velocity likely to be experienced at a given geographical location as determined by the well-known ASCE wind speed contour maps. In the United States, exception of side-hinged exterior doors, R, LC and CW Performance Class products are water-tested at 15 percent of positive DP to a maximum of 12 psf, and AW performance class products at 20 percent of positive DP to a maximum of 12 psf in the United States.

The 12-psf pressure cap for the water tests represents a solid trade-off among cost, performance, aesthetics and functionality. Actually, a 12-psf wind pressure level may be considered quite extreme. Even within the 90 mph ASCE contour, the 50-year, five-minute wind event of approximately 65 mph creates less than a 12-psf inward-acting pressure.

While designers and specifiers may stipulate higher laboratory static water test pressures than the 12-psf cap, this can negatively impact cost, operating force and aesthetics. Increased water test pressure almost inevitably results in larger, bulkier components. Maximum test pressure should be selected to optimize design trade-offs for the application intended.

Performance to the DP-related water pressure levels is verified by laboratory tests for code compliance and as the basis for product certification in the AAMA Certification Program.

Note that these laboratory tests are conducted under ideal, controlled conditions and can only simulate the actual conditions experienced in the “real-world” environment. However, this is not to say that the laboratory tests treat fenestration products like delicate hothouse plants.

A Look at the Tests
The test methods specified by 101/I.S. 2/A440-08 for determining water resistance (American Society for Testing and Materials [ASTM] E 547 and E 331) require zero water penetration when the exterior surfaces of the fenestration product are subjected to a static or cyclical (respectively) water application rate of 5 gal/ft2/hr – roughly equivalent to 8 inches of rain per hour in a basically horizontal direction – for a total of 15 minutes at the DP-related test pressures. However, like the 12-psf water test pressure cap, this is quite extreme compared to typical real-world conditions. For example, ASCE/SEI 7-02 cites the design rainfall intensity for a 100-year, 1-hour rainfall in Birmingham, Ala., as 3.75 inches per hour, and 1.5 inches per hour in Los Angeles. A 15-minute duration, 100-year design event typically ranges from 2 to 4 inches per hour for areas within the 90 mph Basic Wind Speed Contour. Therefore, for most applications, the required ASTM E 331 15-minute static water test is the most stringent of the various water tests that can be applied, and provides reasonable assurance that the design as tested will perform satisfactorily.

Dean Lewis serves as product certification manager for the American Architectural Manufacturers Association in Schaumburg, Ill. He may be reached at dlewis@aamanet.org. Mr. Lewis’ opinions are solely his own and do not necessarily reflect those of this magazine.

DWM
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