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.

If you’re looking for a window company right now, the best advice we have is to check out our list of the best window companies all over the country.  You can find it right here. 

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10 responses to “DP Rating or Design Pressure”

  1. San Jorge Santiago Avatar
    San Jorge Santiago

    I’m just starting to look at possible companies to replace 10 vinyl double hung window in my house. I’ve gotten a quote from Zen of Boston for their Nirvana Soft Lite Classic of $670 per window. The stats are ok, but their warranty is fantastic. It reads as follows: “ Your Zen Windows will include a LIFETIME WARRANTY with no fine print.
    You’ll never have to spend another dime on your windows. It covers the windows, installation and glass.” Does this quote sound reasonable for this window?

  2. Ken Avatar

    San Jorge:
    Lifetime without fine print begs the question, whose lifetime?
    Your lifetime, the manufacturer’s or the product’s lifetime?

    Price? What is included? Install? Shipping? Taxes? Order fee? Financing? Any or all of the foregoing can be a substancial surprise.

    1. thewindowdog Avatar

      Hi Ken, the warranty will typically be pretty clear about the timeframe. Many times they say they’re good as long as the purchaser owns and lives in the home, sometimes they say they’re good for the life of the window and then the life of the window is never defined. Those definitely seem a little shadier, but they’re not very common. Most of the time a warranty from a reputable window manufacturer will be pretty clear.

  3. mitvel Avatar

    I see that most of the windows manufactures provide DP rating but I found out that Anlin does not do that. They provides the performance class/grade like LC30 or R20 (Catalina sliders), R15 (for Coronado sliders) and HC45 for picture windows in their performance data brochure. I failed to find clear explanation how to interpret these values.
    Can you help us to understand what it means and how we can compare it with DP values for other manufacturers?

    1. thewindowdog Avatar

      Anlin should have the DP ratings even if they’re not shown in the brochure. If you’re trying to compare with DP ratings from other windows it’ll be easiest to just ask them for that info. Anlin knowns what those ratings are so it shouldn’t be a big deal. If the salesperson won’t give you that info I’d be concerned, but I don’t think it would be a problem.

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