Air, water, structural is a three part test that determines much about a window’s overall performance. Air infiltration is the first phase, water penetration is next, and structural is the third part of the test.
Windows are tested for air infiltration simulating a 25mph wind or a 1.56PSF pressure load; air infiltration is treated separately from both water infiltration and structural and it is independent of the design pressure of the unit. Said again, the air infiltration rate in a window is not based on the design pressure rating of the unit.
Both water penetration and structural testing, on the other hand, are based on the window DP rating. Water infiltration is tested at 15% of the design pressure and structural is tested at 150% of DP rating.
What this means is that a window with a DP30 is tested for water infiltration at 4.5psf (15% of 30psf) while a window with a DP40 is tested at 6psf (15% of 40).
A window with a DP30 rating should be able to keep out rain when its driven by 42mph winds and a window with a DP40 should be able to keep out rain when driven by 49mph winds…so while water infiltration is DP related, and air infiltration is not, the nature of air and water infiltration is different.
The structural rating of a window is as much about the glass as it is about the frame and sash system. In order to get a higher DP rating the window manufacturer has to consider the thickness and possible heat-strengthening (or tempering) of the glass as well as the use of higher-end hardware and good quality sealants in the frame and sash system. But, interestingly, there is nothing in the structural rating that specifically requires that the unit be air-tight.
A window can leak air like a sieve and still achieve an excellent DP rating. Likewise a window that is sealed tightly can have a lower DP rating but excellent air infiltration numbers. Obviously there are also many units that have both excellent air infiltration numbers and a satisfactory DP rating (relating to both structural strength and water infiltration).
Simply stated, the relationship between DP and windspeed is “the ratios of the design pressures in psf are the square of the ratios of the wind-speeds in mph.”
Our Products for Window Replacement in Houston
Ultra Windows is proud to offer Okna’s Enviro-Star double hung vinyl windows, which have earned a stellar DP60 rating. which means they’re tested to withstand 155mph winds and 60mph wind driven rain.