Choosing Impact-Rated Glass

Q:

We are planning to replace all of the original single-pane windows (1997 construction) in our home with windows that are more energy efficient and that will meet storm codes for South Florida. Although we’ve found a lot of valuable information online, we’ve yet to come across anything that explains the difference between 5⁄16-in.-thick glass and 7⁄16-in.-thick glass. Some of the window estimators we’ve met with are pushing the 7⁄16-in. option as better, while others are telling us that 5⁄16-in. glass will work just fine. Is there a difference?

Luis Rosario, None

A:

Greg Smith, a 20-year veteran of the glass industry, replies: Because you live in South Florida, I’m assuming that you are talking about impact-rated laminated glass in your windows rather than standard, nonimpact-rated glass. Impact-rated glass is made by laminating a 0.090in. thick, clear plastic interlayer between two sheets of standard glass. In North America, that interlayer is typically either polyvinyl butyral (PVB) or SentryGlas Plus, both of which meet the same debris impact standards (Miami-Dade County or ASTM E1996).

The 5⁄16-in.-thick laminated glass you mention is probably going to be two layers of 1⁄8-in.-thick glass sandwiching a 0.090in. thick interlayer, and the 7⁄16-in.-thick laminated glass will probably be made from two sheets of 3⁄16 in. thick glass sandwiching the same 0.090in. interlayer. The interlayer is what stops debris from penetrating window glass and entering your home during a major storm. From an impact standpoint, therefore, there is very little difference between the thinner glass and the thicker glass on each side of that interlayer. In fact, laminated glass can be as thin as 1⁄4 in. and still meet impact standards.

I suppose a window manufacturer could argue that there is a very slight advantage in terms of impact resistance between the 7⁄16-in.-thick and 5⁄16-in.-thick laminated glass, but if it were my call, I would not base my decision on that difference.

There are potential structural, wind-load, or pressure-load advantages to using the thicker glass in the laminated sandwich, however. For instance, you can meet wind- or pressure-load requirements (mandatory in certain areas) by increasing the thickness of the glass used in the laminate. But even these requirements don’t mean thicker glass is a must. Tempering or heat-strengthening the glass can accomplish the same results. Be sure to clarify with the window representatives that they are talking about the thickness of the impact-rated glass and not the nonimpact-rated glass of the dual-pane insulating-glass unit. The nonimpact glass is simply there to contain the gas between it and the laminated glass, completing the insulating glass unit. The sacrificial glass won’t provide any additional impact protection.