DP Rating Confusion

skurkulAugust 13, 2006

We are considering Anderson 400 (dp30) or Marvin Integrity (dp 40) windows for a house to be built along the ocean with lots of wind and driving rain. Can anyone tell me if there is a significant difference between these two dp ratings?

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A Design Pressure rating of 30 is teated for structual integrity for 10 seconds in both directions with a wind speed up to 132 MPH. DP 40 is tested up to 155 MPH. DP 50 is tested up to 173 MPH. The higher the DP rating the higher the water resistance as well.

A typical hurricane has sustained winds of 100 - 150 mph. Winds in some stronger storms may exceed 200 mph.

Hurricane classifications:
Level 1, 74-95 mph
Level 2, 96-110 mph
Level 3, 111-130 mph
Level 4, 131-155 mph
Level 5, 156+ mph

To over simplify it (ignoring gusts and projectiles), theoretically a DP30 rating should survive a Level 3 hurricane, a DP40 a Level 4 hurricane and DP 50 a Level 5 hurricane.

Your building code should have information about design wind speed in your area.

    Bookmark   August 13, 2006 at 10:53AM
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Mightyanvil provided really good technical info. We are on a hillside with winds up to 40-MPH in heavy winter storm and stronger gusts regularly whip pea gravels around impacting our French doors. Driving rain used to force water through the bottom of our old Aluminum windows & doors.

After switching to windows and doors rated DP40, we have had no leakage problems. Regardless of brands, the higher the DP in severe weather area, the better. You may also want to consider tempered glass of certain thickness. Most shops, unless you ask, will order 1/8 inch glass versus 1/4 inch thick glass.

1/4 inch thick tempered glass facing the area most vulnerable to damage is worth considering.

    Bookmark   August 13, 2006 at 5:45PM
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I would first ask where exactly are you building this home? As mightyanvil pointed out, your local building code may have something to say about the required DP rating of your windows and doors - or it may not...

The DP rating of a window or door is based on laboratory pressure testing in pounds per square foot or psf.

Air, water, structural is a three part test that determines much about a window's overall performance. Air infiltration is the first phase, water penetration is next, and structural is the third part of the test.

Windows are tested for air infiltration simulating a 25mph wind or a 1.56PSF pressure load - air infiltration is treated separately from both water infiltration and structural and it is independent of the design pressure of the unit. Said again - the air infiltration rate in a window is not based on the design pressure rating of the unit.

Both water penetration and structural testing, on the other hand, are based on the window DP rating. Water infiltration is tested at 15% of the design pressure and structural is tested at 150% of DP rating.

What this means is that a window with a DP30 is tested for water infiltration at 4.5psf (15% of 30psf) while a window with a DP40 is tested at 6psf (15% of 40).

A window with a DP30 rating should be able to keep out rain when its driven by 42mph winds and a window with a DP40 should be able to keep out rain when driven by 49mph winds...so while water infiltration is DP related - and air infiltration is not - the nature of air and water infiltration is different.

The structural rating of a window is as much about the glass as it is about the frame and sash system. In order to get a higher DP rating the window manufacturer has to consider the thickness and possible heat-strengthening (or tempering) of the glass as well as the use of higher-end hardware and good quality sealants in the frame and sash system. But, interestingly, there is nothing in the structural rating that specifically requires that the unit be air-tight.

A window can leak air like a sieve and still achieve an excellent DP rating. Likewise a window that is sealed tightly can have a lower DP rating but excellent air infiltration numbers. Obviously there are also many units that have both excellent air infiltration numbers and a satisfactory DP rating (relating to both structural strength and water infiltration).

Simply stated, the relationship between DP and windspeed is -- "the ratios of the design pressures in psf are the square of the ratios of the wind-speeds in mph".

A window with a DP30 is rated to a pressure level equivalent to a 110mph windspeed, but it is tested (for structural) at a pressure equivalent to 164mph.

A window with a DP40 is rated to a pressure level equivalent to a 127mph windspeed, but it is tested (for structural) at a pressure equivalent to 190mph.

If you are curious about calculating the numbers yourself, a while back (at the request of a specific window company as a matter of fact) I wrote a couple of simple formulas that will allow you to do so...

If wind-speed is known, then:
W/25 * 0.0624 * W = psf
Where W = wind-speed

Or, if Design Pressure is known, then:
SQRT(psf) * 20.01 = wind-speed

    Bookmark   August 13, 2006 at 7:29PM
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Thanks for the dynamite information, oberon. We had just had new doors installed when TD Ernesto blew up this Labor Day. We had 60-80 mph winds driving 10 inches of rain for 8 hours right against the DP35 Certainteed sliding glass doors. They leaked like a seive. We filed a warranty claim and learned that we could have ordered DP 50 (I extrapolate from you numbers that that would have been good for 56 mph wind-driven rains). The warranty was denied, but they did 'add extra weather protection to bring the door up to almost DP50 standard.' I'm not sure whether to believe this or not. I wondered how French doors and sliding glass doors might compare for rainworthiness--if they both have the same DP rating, do they both keep water out with similar effectiveness?

    Bookmark   October 5, 2006 at 1:53PM
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Well, we had a nother northeaster last weekend. Winds at 35 mph for 48 hours, gusts to 48 mph. The DP35 Certainteed sliding glass doors, supposedly upgraded to almost DP50 standard, leaked again. DP50 should have held firm for both, and in fact I think DP 35 should not have leaked.

Stay away from Certainteed!!

    Bookmark   October 16, 2006 at 5:31PM
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The flip side of the coin is this. You could have the highest DP unit available and still have a leaker. Installation becomes even more critical when you are dealing with DP ratings. The DP rating is based on "proper installation". This means proper flashing, counter flashing, installation method, sealants, etc etc.
You don't mention where the water is leaking in from, but I would make sure the top of the unit is flashed, has a drip cap, and that the sides, head, and sill are heavily caulked. It may help minimize/eliminate the water. Most water problems I've encountered are solved this way.

    Bookmark   October 21, 2006 at 11:57PM
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Actually, it can become even a bit more confusing.

I didn't mention this in the earlier post because I felt that it was already complicated enough, but with your observations, slacktide, I thought I would add to the thread...

Window DP rating allows for a different DP for testing water infiltration and structural. In other words, your window could have been tested at a DP50 for water infiltration and a DP25 for structural - conversely, you could have a window that was tested at DP25 for water and DP50 for structural.

The actual rating that the window receives would be the lower of the two test pressures. But, what it all comes down to is that two windows with an identical DP25 rating could have very different water infiltration rates despite having the same design pressure.

    Bookmark   October 22, 2006 at 8:38PM
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you might want to check building codes. Marvin's Impact glass maybe what you are looking for.

    Bookmark   October 23, 2006 at 12:51PM
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Impact glass will not improve DP performance.

    Bookmark   October 23, 2006 at 9:35PM
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I moved into a new house Feb '05 in Blacksburg Virginia with 37 Andersen 200 Series double hung low E windows. After the house was landscaped near the end of March we had one of the infamous yankee clippers fly through with 25 - 35 mph winds and 45 - 60 mph gusts. The next morning if found the top of the lower sash, sill, and floor covered with mulch that had blown through the window. It looked as if someone had thrown a handfull of mulch at the inside of the window. We have these fronts on the average of at least once per week throught out the cold months November - April. The same thing happened each time. The house is drafty near the windows and the perimeter is noticably cooler than the interior. I watched the house being built and am fairly confident that overall the house is fairly tight (no air through the electrical outlets). A blower door test confirmed an overall fairly tight house. However, with the house slighly pressurized a smoke test revealed smoke rapidly exiting through the windows at the same points where the mulch and the wind enter... mainly through the crack where the sashes meet between the sash and the jamb. It is very noticeable around where the counter balance string penetrates the top of the bottom sash and also where it penetrates at the top of the window. I have also done smoke tests at these points during windy conditions and found the smoke swirls vigorously whenever there is a gust. The smoke pen also glows very brightly because of the air penetration during gusts. During the test the local weather station was reporting 25 mph winds and 35-45 mph gusts. And of course after cleaning off the window sash and sill, mulch was again found peppered all over the place the next morning (as always under these conditions). The windows in question were Andersen 200 series double hung. Many of the windows are tall 72 - 78 inches I believe. They are not tilt out. The builder discouraged because of air infiltration, but did not warn that the non-tilt might give me these problems. Incidently I had the same problem with my Thermatru doors.

I have have many meetings with the builder and the window distributor, and the regional Andersen representative (not just a local person). After admittedly feeling a draft initially, both the builder and the Andersen rep toughened their position and blamed me for being overly sensitive to cold air, the location of the house with no wind breaks (they knew this when recommending these windows), the shape of the house has two juts on the northwest exposure that help trap the wind (this they knew also). Andersen sent a crew to "install "tune-up" kits on all the windows. That may have helped minimally at best, but far from satisfactorily. They then wanted to put storm windows on claiming that they would sell me $500 storm windows for $50 each... must think I'm pretty stupid. Although I found this ludicrous I let them place storm windows on the two windows near the mulch bed. The mulch infiltration was considerably less but still far from acceptable (actually 80-90% became trapped between the storm and the window), but the air infiltration continued almost unabated, as demonstrated by smoke test. It doesn't take an astrophysicist to figure out why mulch falls to the bottom when it gets between the two windows.

So after I asked them to remove the storms and refund my money for the windows, Andersen wrote me that there was nothing wrong with their windows and that they had nothing more to offer than their sympathy. The builder took the same position. So I'm left very bitterly disappointed in both the builder and Andersen. I have frankly been too busy to fight this during the summer months, but with winter returning, I plan to continue to seek full and satisfactory resolution. I have had three builder friends from other states who have been rather appalled at both builder's and Andersen's stance on this and have encouraged me to persist.

Has anyone encounter something like this before, and do you have any recommendations for coming to a satisfactory resolution? I would be happy to show anyone photos of the mulch peppered windows and video clips of smoke test.

One aside... I spoke with a Pella rep who previously had over 20 years with Andersen (was very professional and careful not to trash his competitor). He did tell me yesterday that Andersen did in the past have an issue with air infiltration through the balance box. That would fit with where I find the most severe air penetration... where the balance strings penetrate the sash frame.
Any tips and advice would be greatly appreciated. I spent a lot of money on this home only to have it cold and drafty. I am not the least bit happy about the situation. My previous house was a 40 year old split foyer in which my son's and I put replacement windows. I bought the best that Certainteed had at the time and installed sealed and insulated them myself. There was no air infiltration even on bitterly cold and very windy nights. I would often run my hand all around the windows and hardly believing how tight they were. I run my hands around these Andersen 200 series windows and can feel the breeze coming through the cracks that I mentioned previously.

Thanks for listening and I hope someone may have a suggestion as to how to resolve this.


    Bookmark   November 17, 2006 at 2:04AM
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Hi Don,
Very sorry to hear of what is happening. I have threthreee suggestions:

1. Start a new thread with a new title that speaks to your challenges and then copy and paste this post to that new thread. A new thread with a catchy title will catch more people's eyes faster. It is currently buried in this thread and may not be seens by those who should be seeing this.

2. I saw a post recently of someone who had a leakage problem with a window company, do not remember the brand. He started a website, posted pictures, video and all sorts of information to show his problem. The window company did threaten to sue and the site has since shut down. I have not seen any post regarding a settlement. This is definitely a somewhat aggressive approach and can have some rather irritating consequences.

3. Have you tried BBB or Consumer Complaints organizations in your state? Search out local or state or national organizations, public or private (independent non-profit) and contact them with this problem. They may be able to rattle the tree and make something happen.

The last resort is to retain an attorney. If the windows are rated to perform at a certain level and they are not, the warranty should kick in and Andersen or the builder need to take care of the problems.

Finally, if the windows are not rated to perform in such wind condition and you agreed to their installation, it will be hard to get anything from the builder and Andersen. They can turn the table and blame it on you but you know this already.

Good luck.

    Bookmark   November 17, 2006 at 5:20PM
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That's an interesting problem, don. Not one I've heard of before. It sounds like you are describing the Andersen Narrowline double hung. It's the only non-tilt in their portfolio. Historically, this has been a good performing window. I have Narrowlines that are 30+ years old in my area, and have not run into the problems you describe.
I'm very curious about the mulch getting in. That would take a significant gap somewhere high on the unit. I would inspect the checkrail weatherstrip, to insure both pieces are coming together properly. Then I would look at the top of the upper sash. There are 2 pieces of weatherstrip up there, one on the sash and one on the frame. Finally, I'd look under the bottom sash, to insure that bulb weatherstrip is contacting the sill. Many times, painters remove these pieces, and forget to put them back on.
With the design of the balances & stops, coming around the sash sides would be very hard. That would account for the really good air infiltration ratings these windows receive in testing.
If all of those contacts points are good, then I would look under the interior casing and around the exterior. If air is leaking around the window frame, it would "blow" across the window and feel like it's coming from the sash/jamb area. Make sure the exterior was properly wrapped, taped, and caulked when the original install took place. I've found a lot of "leakers" resulted from poor flashing. Be sure the area around the frame is insulated.
Hopefully, this will lead to correcting your problems.
If any of the weatherstrip areas I've mentioned are not contacting the proper area, it's possible someone over-torqued the screws when installing the windows, and bowed the frames. Check for plumb, level, and square frames. Then look at how the sash sits in that frame. Is the reveal even?
The balance boxes are in the head jamb, but sealed behind the head jamb liner. In my years of using this product, I've never had a leakage issue there.
The Narrowline carries a DP20, which isn't all that great. But they can be upgraded to DP50 according to the website. The DP20 unit carries a .10 air leakage rating, which is a very good rating. The DP50 upgrade includes a higher sill stop, which indicates that under 75psf of pressure, the sill stop needed to be bigger to handle the load. And even then, the air leakage rating went to .19. That's still a good rating, as codes allow units to have a .3 air leakage rating.
So I guess the short version would say: these windows should have no trouble handling the exposure and conditions you described. Check them thoroughly, maybe even have them checked out by another contractor/installer who might be able to look at them with a fresh set of eyes.
I would keep after the reps, and ditch the storm units.
Good luck.

    Bookmark   November 18, 2006 at 1:51AM
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I can really feel for Don. We bought a home in coastal North Carolina almost five years ago and have slowly discovered a number of water infiltration issues. As we are prone to have lots of hurricanes in the area, I have spent lots of time trying to decide what new windows to install so as to get the best value for our money. My contractor is a big fan of Marvin products and they seem to be of good quality, but I am still confused by what to make of their DP ratings. One of the Marvin sales reps told me that a Marvin DP rating of 40 cannot really be compared to a DP rating of 60 on a builder grade window because it depends on what laboratory or group is doing the tests that are used to calculate the ratings. If this is true, it seems the DP rating really isn't much of a standard at all. Can anyone comment? I'm about to throw in the towel on trying to evaluate these products. Thanks

    Bookmark   July 30, 2007 at 11:57PM
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Don't give up yet, there are ways to address this.

First, if you read back up this thread, you will find a post by Oberon that details what the DP rating is all about. It of course also describes how DP is good for one area but does not talk about air infiltration.

I am guessing that the Marvin sales rep may be refering to this difference. If a window claims DP-60, is it completely DP-60 air tight? If you can find out the brand of your builder-grade windows, you can then research specifically what those windows are claiming.

Our own experience in the SF Bay Area is that "builder grade" is not necessarilly good windows. It depends on the builder and who makes the windows. We used to foolishly assume builder grade must be "good", we were wrong.

We have Marvin windows and doors for our house. So far, they have held up really well. We have no hurricanes and only gusts that can reach 60-MPH during very strong winter storms. So far, no air or water intrusion at all. The old aluminum "builder grade" windows of unknown DP rating were horrible. Wind whistled through and blew bubbles of water through the sash seams.

If your local code specifies DP-40 windows, Marvin's will do really well. If you dislike Marvin, Andersen should be a good choice too.

Good luck!

    Bookmark   August 1, 2007 at 11:05PM
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Has anyone ever been told by Andersen that their gliding Narroline glass doors are SUPPOSED to collect water in the inside sill track during NORMAL (NOT driving) rain?
Cheers, AMH

    Bookmark   December 29, 2007 at 1:44PM
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Mike, glass will impact the structural rating of a unit.

Test a 6060 fixed unit with single pane 2.2mm glass and then test the same unit with a laminated impact resistant glass. The first may give you a DP20 and the latter will give you a DP80 structurally. Glass will not impact water resistance or air resistance.

    Bookmark   April 6, 2011 at 12:21PM
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All this talk about DP doesn�t matter, The fact is no manufacture will stand behind there product when it comes to water infiltration. they will say the water came in at the time that surpassed there DP rating and there is nothing you can do to prove them different. These statements come from 11 years of supplying doors to contractors on the Oregon coast. Before your purchase call the manufacture and ask "WILL YOU GUARANTEE NO WATER WILL COME THRU YOUR DOOR". Not that folks that can afford a beach front property will want vinyl sliders; it is the only system that has welded bottom corners with a weeping system built in, combined with a sill pan and butyl membrane and a experienced contactor is your best bet. If anyone knows anything different in a manufacture, please HELP!

    Bookmark   April 16, 2011 at 12:51AM
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Windows aren't made to go in a submarine. I would suggest that if your experience is that a product will leak water unless the manufacturer warrants against it says more about the quality of the windows you used to sell than it does about the reality of a high DP rated window.

    Bookmark   April 16, 2011 at 2:45PM
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It's actually a widely accepted misconception that DP has anything to do with keeping out the elements. As explained quite well in the attached article, 'Design Pressure describes a product's ability to withstand uniform loads caused by either wind, or in the case of skylights, snow. [...] This means that 'DP' refers only to the structural loading performance of a product and does not indicate any tested performance levels relative to air infiltration resistance, water penetration resistance or any other performance criteria specified in the 101/I.S. 2/A440 standard.'

Here is a link that might be useful: Fenestration Standards

    Bookmark   April 27, 2011 at 7:09AM
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Hey folks, I just ran across this thread after doing a search for DP rating information. I know it is an old thread, but on this thread ahooke asked if anyone had ever heard of a sliding glass door that would leak water into the inside sill track, and that Andersen claimed it was normal for it to do this. Wish I had seen this earlier. The answer is YES. Hurd's sliding glass doors has the weep holes to the inside track and if the tracks are free of debris, the water should flow to the outer edges of the sill and then to the outside. I was suprised to learn this myself, because I think I might have a problem if I saw my high end windows with standing water on the inside of the sill track.

    Bookmark   September 21, 2012 at 1:48PM
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