OK, so we have discussed the causes of thermal stress in glass and we have reviewed the basic types of glass (annealed, heat strengthened and tempered) used in building projects. Your question might be: How do these two discussions tie together?Well, not to beat around the bush, the most common reason that thermal stress breakage in glass continues to be experienced in buildings is that the wrong type of glass is used! Now, many of you are probably saying (or at least thinking): What the *&!@# is this guy talking about? Fair enough. Let me explain.

Remember our discussion concerning temperature gradients and the fact that … ah nuts, let me just repeat it here: “A general ‘rule of thumb’ is that a gradient of 40° F is sufficient to result in a probability of glass breakage of 8 lites per thousand. With the popular high performance tinted and coated glass products in use today, temperature gradients of 60° or more are easily possible.”

We all know about the popularity of today’s high performance low-E coatings and high performance tinted glass. And, of course, they are often used in combination to create even better performance. They are great products that save energy, improve occupant comfort and offer pleasing aesthetics. They also absorb and/or reflect a high percentage of the incident solar energy. When they absorb the energy, they get hot; and when they get hot the scene is set for very high thermal stress to be created in the glass and for breakage to occur. In order to minimize thermal stress breakage, the specified glass type must be sufficiently strong.

Allow me to use an analogy based on cars. If you want a sedate family sedan with a gas efficient 4 or 6 cylinder engine, then typical “S” rated tires are sufficient. If you step up to a sport sedan with a super-charged V-6, or performance V-8, then you will need a “U” or “H” rated tire. If you really have a “mid-life crisis” and decide you need a real “barn-burner” – say a Dodge Challenger, or my favorite – the Chevy Camaro with the 426 HP engine – then you’ll be looking at “V” or “W” rated tires. The ratings relate to the ability of the tire to provide stability and durability to be able to safely run at higher speeds.

Now, back to glass: If clear glass or a lightly tinted glass (the family sedan) is used, then annealed glass is likely to be sufficient to resist thermal stress in typical design conditions; but if high performance tinted glass, or coated glass or a combination of both (the “barn-burner”) is used, then it is very likely that strengthened glass will be required to resist the thermal stress.

I have been and continue to be amazed at how often expensive, high performing glass products are specified and used, but fabricated with annealed glass. Believe me, I know the reasons why it’s done, but this is truly a case of “pay now, or pay a lot more later.” The additional cost of strengthening the glass is but a fraction of the total cost of the wall system and is cheap insurance when compared to the cost of replacing glass after the building is completed and occupied.

We all know the old saying: “The right tool for the right job.” Well, use the right glass for the right application!

A final word: Recall that in my previous blog I pointed out that heat strengthened glass is approximately twice as strong as annealed – plenty strong enough to resist the thermal stress generated in any practical application. And, if it does break, for whatever reason, properly heat strengthened glass is much more likely to remain in the opening than tempered glass. Also, properly heat strengthened glass is not susceptible to spontaneous breakage. It is, in my opinion, a better option than tempered glass for all but safety glazing applications, or where tempered glass is specifically required by code.

Enough said. We’ll go on to something new next time.