As we understand it, the manufacturer actually has that data. If tempered glass is an option for the 400's, Andersen will provide the appropriate DP rating when you chose that option. It should be in their specification.

In general, as we were told by the shop from which we bought our windows, DP rating looks at the whole window, not just the glass. So adding a stronger glass to a window may or may not increase its DP rating.

There is another thread further down (August?) that talks about DP.

skurkul,

As calbay3 pointed out, DP rating is a measure of the entire window unit, not just the glass.

Without discussing specifically the window you mentioned, it is unlikely that changing from annealed to tempered glass would have any effect on the DP rating of any window with a DP rating in that range. Ordinary (annealed) window glass should be able to withstand the force applied to it at that level and I would suggest that that particular rating is based on other than glazing.

But, since I can never leave simple things simple and almost never leave a short answer, I am going to ramble on a bit.

IF you had come to me as a potential window buyer and asked me the same question, I would have replied by asking why you wanted to know the DP rating. Not suggesting that you shouldn't know - it isn't a secret, afterall - but why you felt that you needed to upgrade the DP rating of this particular unit. Perhpas asking why you felt that you needed a higher rating.

The reason I would have asked is because it seems like window and door DP rating is suddenly the new "hot topic" among some winow marketing and salesfolks and that is starting to filter down to homeowners who are looking to purchase windows.

And, again, that isn't a bad thing - it is good for a potential buyer to know all of the numbers in order to make a resonably informed decision about what window to buy. But, with salesfolks on the "DP rating bandwagon" suddenly homeowners are inundated with loads of numbers from salesfolk who often don't have a real indepth knowledge or understanding of what those numbers mean.

Now I am not suggesting that anyone is in that category - and even if a person is - they have a very legitimate right to know what the numbers actually mean in the real world and not just on a salesmans notepad.

All that said, here is a relatively quick explanation of what a DP rating is and what it means. BTW, I may have posted this here before (I don't actually remember), but if I have then please bear with me....

And please feel free (anyone) to ask questions....

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 - while air infiltration is treated separately from both water infiltration and structural, 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.

Obviously, the structural rating of a window is also about the glass as it is about the frame and sash system. In order to get a higher DP rating the window manufacturer may want 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).

The relationship between DP and windspeed is --
"the ratio of the design pressure in psf is the square of the ratio of the wind-speed 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.

Oberon, Thank you for your help. The reason for my concern over DP ratings is that the house to be built is along the coast and subject to high winds and driving rain. For various reason we have decided on the Anderson 400 series windows - although the Marvin Integrity which has a DP rating of 40 was a very close second. My primary concern/question is would adding tempered glass increase the break point performance of a window if it were to be subjected to extreme conditions such as a hurricane?

Under the extreme conditions of a hurricane, tempered glass will have no advantage over the standard Andersen glass, except that it will break like tempered glass should. For maximum protection, Andersen offers impact resistant glass. It will double the cost of the window, but if you want extra protection from windborne debris, it's the best choice for glass.
If you're building on the coast, you may want to check with the local building inspector to see what they require. You should also check with your insurance company, to see what they require. Many insurance companies have higher requirements, or simply will not underwrite coastal policies.

The higher DP rating may be an advantage over driving rain - and possibly over wind as well...

I say "may be" because as a consumer you really don't know why the window or door in question has the rating that it has. Was it because of water infiltration at a certain pressure level? Was it because the locking hardware failed during the higher level testing?

It is entirely possible that a window with a DP30 might outperform a window with a DP40 at keeping rain out of your home simply because the higher DP window had issue at the water infiltration part of the testing while the lower rated window met its limits during the high pressure test.

Now as a homeowner looking at buying new windows you would end up driving yourself bananas worrying about these differences. I would personally suggest that you get references from the window contractor and ask those people what they think of both the window and the installation...that way you can compare apples to apples.

One thing to keep in mind, the installation can be the most important aspect of new windows. If they are installed poorly then it doesn't matter what window you pick - you are going to have problems.

And as MIke said, if you want maximum hurricane protection then you want to look at impact products. These have impact glass, heavier hardware, better adhesives, and usually higher DP ratings to go along with all the other bells and whistles.