Are your windows strong enough?

Depending on the weather, windows sometimes have to hold up under considerable stress. High winds from winter storms or hurricane gales may not be prevalent where you live, but you still want peace of mind that your bedroom window cannot blow in some night during a freak storm, bringing shards of glass and rain with it.

How is window strength determined?

Design pressure (DP) is a performance specification for how strong a window is, expressed in how many pounds per square foot (psf) of pressure the window can withstand. This spec may be targeted for performance in a particular geographic area--for example, one with strong winds--or for a particular type of building design.

When shopping for new windows, look for a design pressure (DP) test label such as those issued by the Window and Door Manufacturers Association (WDMA), the American Architectural Manufacturers Association (AAMA) and others. The label might include results of testing for things like air and water infiltration, as well.

Testing is done in a laboratory: a window is attached to a wooden frame and clamped to a specially-designed test board. Air pressure is gradually reduced on the inside of the window, which increases the pressure on the outside until the window fails - actually breaks apart - and the pressure needed to achieve failure becomes the window's psf.

Test pressure must reach 1.5 times the design pressure for 10 seconds without the window suffering permanent damage. In other words, a window with a DP of 30, would have to withstand 45 psf.

What design pressure (DP) means to you

Using the DP, you can determine the wind speed a window can withstand by taking the square root of the pounds per square foot and multiplying it by 20.01. If you multiply that result by 1.5 (the testing requirement), you will know the maximum wind speed the window has been shown to withstand:

Therefore, if you buy a window with a DP of 30, it should withstand a peak wind speed of 164 mph without being totally destroyed or incurring any permanent damage. Make sure that whatever number the tested speed equals, it is strong enough for your geographic circumstances and building type.