What's the Difference Between Design Pressure, Performance Grade, and Performance Class?
Standards in the door and window industry are constantly evolving due to changes in technology, building codes and rating system performance requirements. This has led to some confusion around the differences between Design Pressure, Performance Grade, and Performance Class.
Design Pressure* (DP) - The Design Pressure Rating represents the maximum positive and negative wind load that a window or door can experience without breaking, deglazing or permanently distorting. The test results are valid for the size unit tested and smaller.
DP Rating is expressed in positive and negative pound per square foot (+/- psf) The test is conducted at 150% of the rating achieved. i.e. a DP + /- 45 psf rated window was tested to a structural load of 67.67 psf.
A Couple of Key Points Regarding Design Pressure:
• Design Pressure should not be confused with Performance Grade and Performance Class. Design Pressure Rating (DP) is the anchoring test and one of several test criteria that make up a window’s overall performance grade. See Ken Benden's article in DWM magazine for more information regarding this topic.
• A building code or architect may only specify DP and not specify an overall performance grade. In these cases the spec is only addressing the wind loads expected to affect the product.
• It is not unusual for a window’s DP Rating to be greater than the overall performance grade rating. In these cases the performance grade was limited by one of the other test criteria such as the water penetration resistance which is conducted by uniformly applying water spray at a constant rate of 5.0 U.S. gal/ft2 • hr to the test unit at 15% of a specified design pressure. The maximum rating is exceeded when water is able to be drawn or pushed over the sill under positive or negative pressures. i.e. a window’s overall rating could be limited by the water test at a DP-30 (15% of 45 psf). That same window could achieve a DP-45 (67.5 psf) in the structural load test. In this case the window’s overall performance grade is PG-30 but the window would also have a secondary rating of DP-45.
• Window Test Size is a critical variable when comparing the DP and overall performance grades of various windows. AAMA 101 specifies minimum gateway sizes to qualify for a Performance Class Ratings of R, LC, C and AW. i.e. the minimum gateway size for a single hung LC performance class window is 44” x 75”. The test results qualify that size window and below. Generally smaller windows will achieve higher results than the same window at a larger size. Some windows are tested at multiple size to satisfy the variety of customer needs. i.e. a window may be tested to DP-45 only at 36” x 60”; a larger sized unit could be tested LC gateway size (44”x75”)at an overall performance of LC-30. In this example a 44”x75” window and below would have a rating of LC-30 and at 36" x 60" and below the DP would be enhanced to a DP-45.
Performance Grade* (PG) - a numeric designator that defines the performance of a product and is a set of performance requirements corresponding to a Design Pressure range.
Performance grade (PG) is achieved only upon successful completion of all applicable tests:
1) Water Penetration Resistance - tested in accordance with ASTM E547 with the specified test pressure applied per AAMA/WDMA/CSA 101/I.S.2/A440-08. The test consists of four cycles. Each cycle consists of five minutes with pressure applied and one minute with the pressure released, during which the water spray is continuously applied. The water spray shall be uniformly applied at a constant rate of 5.0 U.S. gal/ft2 • hr.
2) Air Infiltration Resistance - tested in accordance with ASTM E283 at a test pressure of 1.57 PSF. The allowable air infiltration for R, LC & CW is 0.3 cubic feet per minute per square foot of frame (cfm/ft2).
3) Uniform Load Defection Test - tested in accordance with ASTM F588 (Windows), F476 (Swinging Doors) and F842 (Sliding Doors) at a performance level 10 rating.
4) Operating Force (Operable windows only) - Minimum and maximum operating force vary by product type and performance class.
5) Forced Entry Resistance (Operable windows only) - tested in accordance with ASTM F588 (Windows), F476 (Swinging Doors) and F842 (Sliding Doors) at a performance level 10 rating.
Performance Class - Performance class describes the likely target application for a window. This classification system provides for several levels of performance. Product selection is always based on the performance requirements of the particular project.
There are four performance classes:
R: (Residential), commonly used in one- and two-family dwellings
LC: (Light Commercial), commonly used for low- and mid-rise multifamily dwellings and other buildings where larger sizes and higher loading requirements are expected
CW: (Commercial), commonly used in low- and mid-rise buildings where larger sizes, higher loading requirements, limits on deflection and heavier use are expected
AW (Architectural), commonly used in high-rise and mid-rise buildings to meet increased loading requirements and limits on deflection and in buildings where frequent and extreme use of the fenestration products is expected
Each performance class has a minimum performance grade based on a minimum design pressure for structural capacity, air infiltration resistance or water penetration resistance in order to qualify for that class.
Entry-level (“gateway”) Performance Grades are:
> 15 psf (720 Pa) for R class
> 25 psf (1200 Pa) for LC class
> 30 psf (1,440 Pa) for the new CW class
> 40 psf (1,920 Pa) for the AW class
MI Windows and Doors goes through an extensive testing process with each of it's windows and doors. To see a how MIWD tests its windows and doors, check out the MIWD Testing Video in our Video Library.
More information: http:/aamanet.org/general/2/407/performance-class-overview
*The specific definitions of these terms have been carefully confirmed with the publication of AAMA/WDMA/CSA 101/I.S.2/A440-08, NAFS – North American Fenestration Standard/ Specification for windows, doors, and skylights.
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