This is REALLY LONG, but what the heck...
Impact rated windows are tested to specific air, water, and structural requirements, and they are required to successfully stop an 8', 9lb, 2x4 that is fired at the window at 50fps or 34mph either once or twice, or more times, depending on the test.
If the impact window is able to stop the 2x4 within specific guidelines, then the unit is subjected to 9000 high and low pressure cycles at up to 100% of the DP rating.
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. The window must complete air, water, structural testing and then complete impact requirements as well in order to be considered an “impact” product.
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, for example, should be able to keep out rain when it’s 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).
And if anyone is interested, 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. I can show the actual (I am avoiding formulas, unless someone is actually interested in them).
However for comparison, 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 and 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.
Impact (or “hurricane”) windows are made with laminated glass, upgraded hardware, upgraded frames and other components, and all sealed in place with some of the strongest silicone (or other) adhesives on the market.
Virtually all the major window manufacturers have impact products available. Some perform extremely well, others barely get by.
Laminated glass is simply two (or more) lites of glass bonded to a plastic interlayer for strength. Laminated glass is no stronger against breakage than is the glass it is made of. Let me say that again - laminated glass breaks as easily as the glass it is made of. I mention that because many folks have the mistaken impression that laminated glass / impact windows won’t easily break. They will break, but the glass adheres to the plastic interlayer and keeps the envelope of the home closed.
There are four major food groups in the impact glass world (a few other minor ones as well) relating to the interlayers or types of laminated glass used to manufacture impact windows.
First we have PVB or Polyvinyl Butyral. This is the stuff that is in the windshield of your car. It is relatively soft and very flexible, yet it is also tough and doesn’t tear easily. This is probably the most widely used product in the impact glass market since it does great when impacted. It stops whatever hits it and stretches to absorb the impact (such as a persons head in a car accident). PVB is a good product and is the choice for most manufacturers.
The second interlayer type is a hybrid of PVB with a layer of PET film between the PVB layers. This is a very tough product and it performs very nicely. This product is available from less manufacturers than standards PVB – although some window companies may offer both options.
The third type of interlayer is called SGP or Sentry Glass Plus. This is quite a bit different from PVB in that it is very stiff and very tough. It is becoming something of the product of choice in some of the toughest applications (including some bullet and bomb resistance applications). It is also a bit more expensive than PVB and may be overkill for some residential applications.
The fourth product line is the resin laminates – where a liquid resin is poured between two lites of glass and allowed to cure. I personally see liquid resin laminate as the “mom and pop shop” of laminated glass – although some larger manufacturers do use it. For the manufacturer it is cheap, it is easy, and it is an acceptable “mom and pop shop” product. Personally, I am not impressed with performance or longevity…my opinion only, but I wouldn’t use it in my house.
Most window manufacturers buy their laminated glass from a laminated glass manufacturer (which does make sense!), but a few laminate their own glass such as PGT (who was mentioned in an earlier post).
Windows used for impact applications are generally going to be rated to either Dade County (also known as TAS 201, TAS 202, and TAS 203), or to ASTM E1886 and E1996 protocols. Generally, if you live outside of Dade, Broward, or Palm Beach counties you will be seeing ASTM – which is referenced in the Florida building code.
ALWAYS get LowE glass. In your part of the country you want to look for a LowE with a low number SHGC (solar heat gain coefficient). Tinting is a good bit less effective at keeping your home cool than is a LowE coating. LowE coatings are available in impact products.