Commercial properties are much more susceptible to a range of unique environmental factors, like high-speed winds, hurricane debris, and steady rains, than your home. And that means commercial windows must be much stronger and more durable.

But how is the strength of a window determined?

All commercial windows on the market are required to pass a battery of tests to determine their strength and dependability. The most common tests are in three areas: Wind pressure (or load) testing, hurricane impact testing, and water and air penetration testing.

Windows are then assigned a performance rating. Building codes in different regions of the U.S. require windows of a specific performance class, dependent on the type of weather that region experiences. For example, in the Mid-Atlantic, where hurricanes wreak havoc on commercial properties, building codes typically require impact-resistant windows that can withstand high-pressure loads.

Here, we talk about a few of the most common types of tests windows must pass, as well as how each of these tests is conducted.

Each of these strength tests determines how much wind (which puts significant pressure on a window’s glazing) the window can sustain, how well the window defends against wind-blown rain, and the strength of impact a window can take.  

Design Pressure Testing

Commercial windows tend to be exposed to direct wind – especially in mid-rise and high-rise buildings – which put a lot of stress on the window. And some tests measure the window’s “Design Pressure,” which the maximum wind load a window can withstand.

The necessary DP rating for your property’s windows is dependent on a range of factors including Your region, the building’s height, and the windows’ orientation on the structure.

Pressure testing is conducted in laboratories using a pressure-generating machine. Primarily, the window is attached to a wall that reduces pressure on the interior of the window (this is referred to as negative pressure). As the harmful pressure increases, the pressure on the exterior face of the window increases, as more negative pressure is added.  

During testing, the test pressure must withstand 1.5 times the DP rating for at least 10 seconds. For example, a window with a 50 p.s.f. rating must reach 75 p.s.f. for at 10 seconds. You’ll find this rating on many commercial window labels. So how does DP relate to actual weather? Here’s an example: Windows with a 50 DP rating can withstand winds at about 170 mph; that’s equivalent to winds of a Category 5 hurricane.

Rain and Air Penetration Testing

Another common type of strength testing is called an air and water penetration test. This test measures the window’s ability to prevent air leakage and water leakage. Most commercial windows leak a tiny bit of air, so some air is permitted in the test. But the air leakage is so slight that it has a minimal effect on thermal performance.

The more important test is water resistance testing. In this test, pressurized water is sprayed directly at the entire window to determine the amount of rainfall a window could withstand per hour, and both measures related to the DP rating. For example, a higher DP means the window is more resistant to water and wind penetration. A DP rating of 50, would say the window could resist 8” of wind-driven rain at 50 m.p.h. over a one-hour period.

Hurricane Impact Testing

Another test of strength looks at the window’s resistance to high-impact debris that is flung in high-speed hurricane winds.

The test is one of the most interesting to watch. Mostly, a standard 2×4 is shot directly at the glass at about 35 mph. Hurricane windows act as a barrier, bouncing the piece of wood back. To pass the test, the glazing can’t shatter and must prevent the projectile from moving through the window.

When to Use High-Strength Windows

Ultimately, the DP rating for your windows will be determined by the installer. A window installation company will decipher the types of windows you will need based on your building’s location and unique environmental requirements.

In addition to choosing the right DP rated windows, an experienced window installation firm will ensure the windows are correctly installed. Proper installation is essential for providing the DP rating because a faulty installation can decrease the windows’ load or increase its susceptibility to wind-driven rain.