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Understanding Window Design Pressure: How To Deal With Water, Wind, and Air

Wind and Air Infiltration
Hi, I have a new house with Milgard tuscany windows, single hung with grids on top.  I live in a scenic area with extreme wind and weather conditions.  Sustained winds of 45mph with gusts from 80 to 100mph during the winter.  We have blowing snow and freezing rain.  In the summer months the gusts reach 60-65mph with blowing dust and sand.  This winter was our first in this house and we had snow and moisture blowing in where the windows slide up and down as well as between the vinyl around the window and the connection to the actual vinyl frame and loud howling that you could not imagine.  We cannot use casement due to the size of our windows.  What brands of windows would you suggest?

Window sizes are:  Window sizes are 3′ x 6′, 4′ x 6′, and 2′ 6″ x 6′

Thank you for your time~


Hi Julie,
With extreme conditions such as you describe not only do the windows have to work, but the installation has to be perfect. It’s difficult to recommend a specific window brand for you because the windows you need will have to meet higher than average stress ratings in order to give you the performance you want. That’s not resolved with a “brand” so to speak, you will have to know a little bit of technical stuff. So my best approach to your situation was to call the research and development folks at Andersen Windows for information that I will share with you here.

Your problem could be from one of three things:
1. product design
2. product design limitations
3. improper installation

Don’t worry I’ll explain all of this. I’ll start with the technical “science” information that may help you shop for windows. And then allow me to suggest some alternative ways to configure your window style to minimize your problems.

Let’s talk about the product first and the science behind the window.

There are three stress variables to consider with your situation and they all are measured and rated so you can shop smarter:

1) Air infiltration
This is the “acceptable” amount of air a window can let into the home, and there are standards for this number. At some point, the window will allow more air in than it is designed to take.Water and Air Infiltration Window DP Ratings

You can evaluate a window for this standard using the DP rating (design pressure). This measurement needs to be a high number in your case. The higher the number, the greater the window will withstand wind. Mr. Rogers Renewal by Andersen windows generally have a DP rating of 40, which equates to just less than 50 mph sustained winds. This is generally the “target” for residential windows. But you can get this number to 60 with the alternative configuration I’ll describe below.
An even higher performance can be achieved with commercial grade products, (windows that you would see in buildings).

2) Water infiltration
This is the same concept as air only with water. In this case the window is tested by exposing the window to water at the rate of an 8-inch rain per hour for a 15 minute time period. If water comes through the window, it can still pass the test, because there is an “acceptable” range here too. But if water comes through the window and leaks onto the inside wall, then it fails.
The Renewal by Andersen window rates at 25 for a similar style window to yours, but with the alternative configuration you can increase this to 60.

Again, the higher the number, the greater the resistance to water infiltration.

3) Structural Wind Load
This test measures the point at which the sash either jumps out of the frame, or when the glass breaks. These wind stresses will be greater than those of the air or water test, but at the point that the window fails from wind, air and water are absolutely coming into the room!

The Mr. Rogers Renewal by Andersen rating here is 25 for a comparable style window, but can increase to 60 as with the other ratings.

With this information you can see how you simply might have the right house with the wrong window. Most homeowners would never need to consider these stress ratings, but they will be relevant in your situation. You might want to read this thread over on the GardenWeb forums to get a little more information on DP ratings:

No matter how well your window is rated, it all falls apart if the window is not installed properly. And in your home the installation is a MAJOR factor to your satisfaction.

If the window is not installed, squared, insulated, flashed and sealed according to the manufacturer’s instructions, then none of the performance characteristics (the science stuff above) will be achieved. So choose an installer who you trust with your baby, that’s how critical it is.

Usually there is no way for a window to be “modified” to achieve higher performance once it is installed in your home without the risk of voiding all your warranties. So fixing your existing windows is not a realistic option for you.

So What Should You Do?

1) Check out the performance (DP) ratings for the window currently in your home. These numbers may not be readily available, so you will have to contact Milgard directly and be pretty patient about the whole thing. Knowing the current stress numbers is your best starting point.

2) Have someone check the installation to make sure they are installed properly and that is not contributing to your problems.

With this information you can shop to improve the stress numbers in your next selection of windows. And I would suggest that you look at the Renewal by Andersen window as an option because it can provide the increased DP rating with the configuration below and it has made great strides in energy efficiency with Smart Sun glass. Plus, it qualifies for the $1500 energy tax credit.

When you make your next selection, I would suggest that you reconfigure the style of your windows. It will give you the same great look and view, but will create a structural barrier to your wind and weather conditions. Air and Water Infiltration

Instead of the Single Hung style, place a picture window over an awning window. A picture window has the highest ratings for the important science numbers above (Renewal by Andersen rates at 60 for each) and provides no spaces for air to infiltrate or “howl”. By placing an awning window under the picture, you will be able to open the window for a great summer breeze, but when the weather conditions are extreme, the closed awning window will actually close more tightly in the frame protecting you from air and sound. Plus you will enjoy a dramatically warmer room in the cold, windy winter season.

I hope this helps you out. Keep me posted as you work on this project and send more pictures!

Gerry Rogers About Gerry Rogers

Gerry Rogers is the President of Mr. Rogers Windows


  1. Thanks for the in-site on DP info.

    We have installed from brand X supplier DP 30 in our home. Now the inspector will not approve them.

    Looks like we have an yard sale idems as the supplier will in take them in return for the 40DP (or min. 35) needed.
    Not a HAPPY PERSON!!

    Thanks again for the helpfull web site!!

    Larry Olson
    Lady Lake, FL 32159

  2. Larry Olson says:

    We have installed two new windows in our new construction FL room.

    The windows have a wind load rating of +30/-30 (DP) (PSF) as per ASTM E330. The building inspector will not approve this rating. He says it need to be 40+ to meet specs.

    We bought these windows in good faith and now take them out and discard!! NO I don’t think so. $450.00 down the drain!!!

    What should we DO??

    Any comment will help. Thanks,

    Larry Olson

  3. Ted Keller says:

    The final authority regarding window “design pressure” ratings can be found on the AAMA web site under Fenestration Performance Classes Translated
    The testing at AAMA involves design pressure including air and water infiltration. The highest rating is a “AW” of which there is only one residential window rated that highly
    They manufacture casement and awning windows. Their awing window test at AW70 which exceeds a design pressure of 100.

  4. Jerry Hartman says:

    3′ wide by 6′ tall is not too big for a quality casement window. Julie should look into a casement or a dual action (tilt turn) window. If she was consulting us we would recomend a window with a DP of 65 or greater. Tilt-Turns can achieve DP ratings of 85 easily.

  5. Retractable says:

    I love the site layout

  6. Thank you for all of the helpful information, what about other Andersen products and where can I get information on proper installation procedures?

    • Hi Julie,
      You can find a Renewal by Andersen dealer in your area at Renewal dealers handle the process from start to finish with professional installers. With your conditions, I definitely recommend a professional installer. That way you can be sure your guarantee is going to remain in effect if anything does go wrong.

      If you already have a contractor and want to manage the installation yourself you can find out more at Proper installation procedures should always be included with your purchase.

      Let me know how it goes, Gerry.

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