What is Design Pressure, and how does it impact the windows or doors you are considering for your home?
Let’s start with the definition of Design Pressure. According to AAMA (American Architectural Manufacturers Association), WDMA (Window and Door Manufacturers Association), and the NAFS (North American Fenestration Standard), Design Pressure (DP) is a rating that identifies the load - induced by wind and/or static snow - that a product is rated to withstand in its end-use application. So basically, 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 specification can fluctuate based on several factors, such as geographic location, the height of the building, the location of the window in relationship to the building, etc. All of these factors are used to calculate what the Design Pressure (DP) is for a specific window or door.
During testing, a window or door is attached to a wooden frame and clamped to a specially designed wall. Air pressure is gradually reduced on the inside of the window, which increases the pressure on the outside until the window fails (breaks beyond repair). The 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. The instant the window fails, the psf is recorded and is used for the calculation. Laboratories will then issue labels that indicate the DP for the window or door – labels used by the Window and Door Manufacturers Association (WDMA), the American Architectural Manufacturers Association (AAMA) and other organizations.
The Design Pressure that you need for the windows in your home will vary. For example, in the State of Florida, the DP required for a window can range anywhere from DP 35 (if you are in the center of the state) to DP 60 (if you are located along the coast). Your local building department can assist you in determining what DP rating you need, based on the location and design of your home. Most municipalities have maps or easy-to-read charts that clearly explain the requirements you will need to meet in order to effectively protect your home with the proper Design Pressure. You can then verify that the replacement windows you are considering meets the required DP rating by checking the sticker on the window and matching it to the engineering drawings for that window.
It’s also important to have a licensed and insured company install your windows and doors. Companies that are reputable will have no problem helping you verify the Design Pressure for your windows and doors. They will assist you with gathering all the information you need to make your decision. If they baulk at your request, you may want to reconsider purchasing from that company.