The Debate About VLT Heats Up
by Megan Headley
“I’ve had a concern for years 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 USGlass.
Obviously the industry is available to respond to designers’ demands,
but, Carney points out, it’s the glass industry’s job to educate those
architects and designers as to when and where high VLT is most appropriate.
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.” He continued, “The week
before that, I was involved in discussions on another project being reglazed.
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
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 solar heat gain coefficient, U-value, VLT
or light to solar gain.” Yet he notes that the promotion of high VLT is
a response to designers’ demands for certain types of glass.
There are lots of glasses out
there that are really clear, and some architects really want that, but
I’ve always seen it as more of a niche product.
—Arlene Z. Stewart, AZS Consulting
“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
“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 continues, though, that, “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 run-off
and staining. Solar shades are a natural landing spot for birds. You then
have the issue of cleaning 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 as more of 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,”
It also can depend on the building type.
“I prefer not 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 causes 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.”
Megan Headley is the editor of USGlass.
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