The Case of the Distortion Distress

Question:

Dear Glass Detective,

I want a picture window roughly 8-feet by 8-feet that has to use safety glass for code requirements. Will a tempered glass pane this size have any obvious visual clarity or distortion looking through it?

Thank you,
Kirk M.
Manson, WA

Answer:

Dear Kirk,

Thank you for making contact with the Glass Detective with your questions and concerns relative to the use of tempered glass in a large (8’ x 8’) picture window. This is indeed a good-sized picture window and I certainly understand why you might be concerned. Those of us with a keen eye for glass have all seen buildings that appear to have distortion in the glass panels. The quick assumption is that this is caused by the use of tempered glass. Sometimes it is, but not always.

The “art” of tempering glass (and I refer to as an art because to me it is) has come a long way over the past few decades. Whereas just about every piece of glass that was tempered 40 years ago had some problem with distortion (typically caused by bowing), better equipment and improved processes have allowed modern glass manufacturers to produce a higher-quality of tempered glass than was possible in the past

With this being said, let’s focus in a little more on your specific glass application. You don’t say whether the glass in your picture window is going to be a single pane (monolithic) piece, or a dual-pane (insulating unit of some sort). Either way, you may have a problem. The glass distortion you are concerned about can occur primarily from two causes.

The first is from what is known as “roller wave” distortion which is created at the time of tempering. I won’t go into the technical aspects or reasons for this, but in general, the thinner and larger the glass panel being tempered, the higher the probability for this concern arising. The other concern you will face is the flexing/bowing that takes place when a large piece of glass is glazed into an opening that will be exposed to wind and heat differentials. Your picture window is in such a situation. Because of the large size of your picture window, a good wind may cause the glass to deflect/bow.

Remember that the pane of glass is captured and secured at the perimeter only where it is held tightly in the frame. However, it is not as secure in the center of the pane, 4 feet away from a point of security provided by the framing system. Therefore wind can create a bowing or deflecting as the glass is under load (referred to as wind load). There are guidelines for this such as standards and codes. Roller wave distortion standards will allow for some distortion though I think this is the lesser of your problem.

There are codes and standards for wind loads that will suggest, or even require, certain thicknesses of glass for an opening of this size, depending on where you live. Another issue here is if you decide to use an insulating glass unit in this window, almost all insulating glass manufacturers have concerns about issuing warranties for an insulating unit over 50 square feet. Your unit would be 64 square feet (8’ x 8’) so you will want to discuss this with your supplier if this is the route you are going to take.

With all of this said, If this were my home and code requirements required me to use tempered glass, I would go with a 3/8” piece of tempered glass. If it is to be an insulating glass unit, I would have the outboard lite of the insulating unit made with 3/8” tempered glass and do whatever was needed to adjust my frame to accommodate the thicker panel.

Not everyone will agree with this approach. I strongly encourage you to discuss this in detail with whoever it is that is going to provide this glass to you. You may be absolutely fine with a tempered insulating unit using ¼-inch thick glass and, depending on your landscaping, lighting and perhaps roof overhang, it may all be just fine. Sometimes I get accused of “overbuilding” and this is probably true with glass applications.

So again, get a second opinion from your glass supplier and go from there. Please also feel free to share this response with them. They should be up to speed on the building codes and recommendations for wind load calculations in your area of the country.

I hope this information is of some value to you and I again thank you for contacting the Glass.com Glass Detective with your question. Good luck with your project and I would very much like to know how you proceeded and what the results were if you get the time and chance to send a note over.