Design pressure or dp rating is a measure of the strength of a window. It will tell you how well a product will stand up to high winds. The higher the dp rating the stronger the window. It does not relate to energy efficiency, but dp rating is still an important factor to consider.
What’s a good dp rating?
Typically you’ll see residential replacement windows with ratings from 15 to 50. Higher is better. It makes more sense to focus on dp rating when you’re in a situation where wind is of particular concern. For example we if you live on the water or up high on a hill where you’re exposed to the elements this can be important. In those situations I’d suggest going with at least a dp 30 and a dp 50 might be an even better choice. There’s really no need to go any stronger than that unless you’re in a high rise or a hurricane zone.
How can understanding dp ratings help me?
A common sales ploy is to show folks a cutaway view of a thick window next to a thin window and tell them that the thick windows is “strong” and the thin window is “weak”. This is intuitive which makes it an effective way to sell the thicker windows.
We can tell if a potential customer has seen this maneuver when they ask how many chambers are in a specific window or when they ask to see a cutaway.
As you’re considering these options keep in mind that everything in life has tradeoffs and everything in the window business can be measured so you don’t need to use a gut feeling. The thicker window will have a lower vt rating because that thick frame blocks a substantial amount of light, and the thinner window, if well built, could easily have a higher dp rating meaning it’s actually stronger.
So how do you avoid getting sold on something that you don’t necessarily need?
Just like always, avoid taking someone’s word for it and ask about the dp rating. If it’s higher it’s better. The window doesn’t need to be thick to have a higher rating and a thick window without a higher rating is blocking light for no purpose other than to say it’s “better”.
Remember that the cutaway sample really doesn’t show you anything. It gives you an idea, but the rating tells you for sure. There’s no need to guess when it’s so easy to be sure.
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