Size Does Matter

The development of spectrally selective glass tints and coatings has enhanced the solar and thermal properties of glass tremendously. When used in insulating glass units, in combination with argon or krypton gas fill, r-values greater than 4 and solar heat gain coefficients less than 0.3 are possible. Combined with relatively high light transmittance that can be used to supplement artificial lighting, the energy conservation of new glass products is significant. Add to this the fact that, as a building material, glass is very economical, and that the generous use of glass makes it easier to lease office space. All together, it is not surprising that glass continues to be a favorite.

As someone who has spent the better part of half a century in the glass business, I am personally delighted to see lots of glass being used. After all, I am a PPG retiree who looks forward to his pension check and lots of glass being used is a good thing! Keep on using it, but use it with good judgment.

During the last several years of my time with PPG, I had several customers ask my support in cautioning against the use of excessively large insulating glass units. They were concerned about the long-term performance of such units. Their concerns had merit and, in fact, many insulating glass fabricators have long had restrictions on the size of units that they would warrant. There are a number of factors that impact the long-term performance of excessively large insulating glass units. Some occur during manufacture and some in the field. As a result of my customer’s concerns, as well as my own experience, I authored a technical document for the PPG web-based technical library addressing the subject. The title is “Large Insulating Glass Units – Design Considerations.” My intent is not to recreate that document here, as it can be accessed by CLICKING HERE. However, I will offer the following for your consideration.

Among issues to consider:

· Glass damage due to difficulty in handling large, heavy insulating glass units;

· Ruptures in seals and spacer welds due to the flexibility of large spacers;

· Uniformity and press-out of the primary seal;

· Excessive deflection under wind-load (remember that just because heat strengthened and tempered glass are strong enough to resist the wind-load, they will deflect the same as annealed glass); and

· Increased risk of thermal stress breakage.

In addition, large units have economic consequences. Metal glazing members must be stronger, i.e. thicker, to eliminate excessive deflection due to the weight of the unit and avoid potential damaging contact to adjacent units. While setting blocks can be moved closer to the ends of the horizontals to reduce deflection, they should never be closer than 6 inches to the ends of the horizontal. And, horizontal sill member deflection can result in water not draining properly, which then leads to premature failure of the insulating glass units.

Like I said, I love to see lots of glass used and it’s not my intent to discourage anyone from using it. However, it is important to think things through and consider the “unintended consequences” before simply specifying excessively large units.


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