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It depends on where you live.

It depends on where you live. Design Pressure (DP) ratings are established by testing the window to pressures equal to 1.5 times the DP requirement. In other words, if a window is rated to DP40, then it is actually tested to 60psf. If a window is rated to DP20, then it is tested to 30psf.
One might assume that there is a direct linear correlation between DP rating and wind speed. One might also assume that a DP of 40 is twice as good as a DP of 20 - or that a DP of 80 must be twice as good as a 40 and four-times better than a 20.
Actually, this isn't the case in the real world.
While a specific DP does equate to a specific wind speed, there isn't a direct linear relationship between increases in wind speed and increases in design pressure. In other words, increasing the DP a certain amount does not result in an equal increase in the window's ability to resist a specific wind speed or pressure.
For example, a window with a lowly DP9 is actually rated for a 60mph wind and it is actually tested at 13.5psf which equates to a wind speed of about 72mph. A rating of DP9 is pretty darn low, but a 60mph wind can be a pretty significant gust.
Imagine that we now look at a window rated to DP15. Sounds darn low and no one in their right mind would want a DP15? Well, 15psf actually equates to about an 80mph wind. Now, as mentioned, the window is actually tested to a level 1.5 times higher than the rating, which for a DP15 equals 22.5psf, and that equals an approximate 95mph wind speed. So while a 15DP doesn't look like much, it really isn't bad at all and it is probably acceptable for any application in the right environment.
And by the way, exceeding a window’s DP rating does not mean that suddenly the window will explode or come crashing into your home; it simply means that the "performance values" of the window are rated to a particular pressure. Can windows "fail" when subjected to wind speeds above the rating? Of course they can, but failure generally means cracked glass or warped frames or something similar and not necessarily catastrophic collapse. However, on the flip side, there are very good reasons why hurricane-prone regions are now mandating minimum DP requirements when building new construction or renovating.
The point being that while DP ratings can be very important, even crucial, in some environments, the idea that a person in an area that rarely or never experiences 100mph wind gusts is worried about the difference between a DP45 versus a DP60 is worrying over something that really isn’t a significant factor. A window or door with a DP15 or DP20 can be every bit as good in the correct environment as a window or door with a DP80.
In our Wisconsin environment, we would rarely or never experience wind occurrences that would be considered the same as in hurricane-prone areas. On a practical side, consider other factors to be significant when choosing windows for new construction or replacement windows for an existing home. Look for U factors below .30 as this value determines its energy performance. Other important considerations are convenience (ease of cleaning/removal) and appearance.
 

Published Tuesday, November 03, 2009