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 USGNN.com.
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
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
"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
"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."
HERE to take a USGNN.com survey on VLT.
Need more info and analysis about the issues?
HERE to subscribe to USGlass magazine.