Is the Glass Industry Overselling VLT?
July 16, 2010

"A concern I've had for years [is] that the industry may be overselling the very high levels of visible light transmittance (VLT)," Greg Carney of C. G. Carney Associates, Inc. recently wrote to™.

Case in point, he explained, "I visited a facility on the West Coast that had a wall of office area windows facing west and virtually all of the window shades were pulled down. The amazing part was that only 8 percent visible light was coming through the windows." Carney continued, "The week before that, I was involved in discussions on another project being reglazed and the replacement glass is specified to be a 1 1/16-inch insulating glass unit with 68 percent visible light transmittance and 0.38 solar heat gain coefficient. This makes me think that the occupants of that building will have even more complaints about glare and heat gain."

Mike Krasula, senior manager of commercial products for Pilkington North America, is more than familiar with the problem.

"I think most architects and people in general cannot appreciate how much light is coming into a building and the other ramifications that have to be addressed because of that decision. The Pilkington headquarters building in Toledo, Ohio, has between 8 to 12 percent daylight transmissions, which are similar to a pair of sunglasses. My office faces east. I have my blinds partially closed because of the glare and to a certain extent heat from the sun. Situations like this exist all over the country," Krasula says.

But is it overselling if that's what the architects demand?

PPG is among many glass manufacturers and fabricators that promote a high VLT in its glass products. As Rob Struble, business communications manager for the company's Performance Glazings division, points out, "I think the glass industry, as a whole, promotes the best performance virtues of its products - whether it's SHGC, U-value, VLT or LSG." Yet he notes that the promotion of high VLT is a response to designers' demands for certain types of glass.

"PPG has sought (and invested heavily) in understanding and answering the needs of designers in all regions and climate zones of North America. It would be myopic for us to think that architects, who are very smart and talented people, would use whatever we produced and promoted," Struble says.

"New product development for us is driven by direct work with the architects to identify what they need and want, and then products are produced," agrees Glenn Miner, director of construction markets for PPG.

Part of that interest in higher VLT may be because architects are finding ways to balance bright daylight with issues such as glare and heat gain, "and increasingly using sophisticated models and energy analysis tools to do so," Struble adds.

"Surprisingly I think more and more we're seeing architects question the idea of glare," comments Arlene Z. Stewart, also a consultant, with AZS Consulting. "I think there's got to be a balance - but I don't know that they're finding it through glass, they're doing it through other devices."

"Window treatments or some type of exterior solar shade can be purchased to reduce glare," Krasula agrees. He counters, though, by adding, "This adds cost to the project and is an issue for the environment. It is interesting that the aluminum or steel solar shades are produced by a process that adds more carbon dioxide into the atmosphere, which is one of the things we are trying to eliminate. You also have the additional issues of runoff and staining. Solar shades are a natural landing spot for birds. You then have the issue of how do you clean the windows with all of the solar devices that are hanging off the exterior of the building."

Stewart says designers of large commercial projects do seem to be thinking more "along the lines of shading and angle of incident light and how it's going to hit things inside." Still, she adds, "There are lots of glasses out there that are really clear, and some architects really want that, but I've always seen it more of as a niche product."

A niche, perhaps, depending on the building's location.

"In parts of the country, there has been an unmistakable trend toward higher performance transparent glass and the glass industry has sought to satisfy this. Yet in other parts of the country, you will see very few transparent buildings, I believe due to how architects design for the client base and environmental conditions of a particular region," Struble says.

It also can depend on the building type.

"I don't prefer to specify VLT more than 40 percent for libraries or offices, but for hospitals, malls, schools and houses I prefer to select glass with high VLT and low solar control," says Mohammad Bitar, an area sales manager at INTRACO in Jordan. "From my experience the high VLT is healthy, but in the same time it cause glare inside the building," he adds.

In those regions where glass allows more light transmission, some of that incoming daylight can be helping to reduce the need for electric lighting.

"Approximately half of the energy consumed by a commercial building is due to interior lighting. Anything our industry can do to reduce this level of consumption, while managing the solar energy, is a good thing," says Mike Rupert, director of technical services for PPG.

"Studies have shown the benefits of natural lighting in productivity and occupant wellness-and these oftentimes do not show up in the number crunching that some analysts perform," Miner adds.

And, Krasula is quick to point out, there are plenty of other glass properties that can be selected to balance that desire for visible light.

"With all of the coatings and frits that can be put on the glass to address balancing the glare, daylight and solar control issue, [glass manufacturers have] products that fit the bill." He adds, "Glare is definitely an issue that is overlooked in the overall equation. We always want to address light and heat, but fail to address the issue of glare."

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