No Straight Answer  
The Complicated Relationship Between  
Larger Glass and Thermal Performance  
By Nick St. Denis  
esthetics and performance—architects demand the  
Glass is getting bigger and bigger, a trend that exists in  
best of both worlds.It’s an ongoing challenge that the perpetuity thanks to the architectural mandate of massive  
glass and glazing industry takes on time and time views, maximum daylight and sleek facades. Does the use  
again. Designers continue to ask for more glass, and just as of larger glass come at a cost from an energy standpoint?  
A
importantly, more from glass.  
According to two industry experts, “yes and no” is a short  
answer.“It depends” is a shorter one.  
Measuring Thermal Transmittance  
The center of  
A glazing system’s thermal transmittance is measured  
by its overall U-factor, which is an area-weighted average  
of the frame, edge-of-glass and center-of-glass U-factors.  
glass in a unit  
traditionally  
performs better  
thermally  
than the  
edge of glass  
and metal  
Therefore, the larger the glass area when compared  
to the edge of glass and frame, the more the center of  
glass will dominate,” says Dr. Helen Sanders of Tech-  
noform. “Generally, the center-of-glass U-factor will be  
lower (better) than the edge-of-glass and frame U-fac-  
tors, and so as a fenestration unit gets larger and the  
glass-to-frame ratio increases, the average U-factors  
will become lower.”  
Tom Culp of Birch Point Consulting adds that with a  
larger overall glazing area, this difference will have to  
be included in the building models to determine the  
overall impact on heating, cooling and daylighting. But  
as far as the specific performance properties such as  
U-factor, “the thermal performance of the glazing and  
the framing will generally be different,” he says, “so  
changing the size and the ratio of these will affect the  
real overall thermal performance.”  
framing…  
He, too, points out that the framing often has a higher  
U-factor than the glazing, so as the glass-to-frame ratio  
changes with larger sizes, the overall unit’s U-factor  
may decrease.“But there has been significant recent ad-  
vancement in framing systems and new thermal barrier  
technology,” Culp says. “So this is not always the case  
anymore, and the frame can sometimes be the same or  
better than the glazing.”  
Sanders says that another thing to consider is that  
larger glass may also need more sealant if it is structur-  
ally glazed in order to withstand windloads. “Increas-  
ing sealant height will likely increase the edge-of-glass  
U-factor, negatively impacting the overall system U-fac-  
tor,” she says.  
but improvements in framing systems  
and thermal barrier technology has  
minimized this difference, and in  
some cases eliminated it.  
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USGlass, Metal & Glazing | October 2017  
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More Metal  
Less Glass  
Less Metal  
Larger Lites  
Going from a glazing area that uses more metal to one that uses less metal and larger lites of glass should, in general,  
improve thermal performance—since the center of glass has a lower U-factor than the edge of glass and framing.  
Digging Deeper  
If this is not communicated correctly, Culp says it can  
While an increase in the glass-to-frame ratio likely will lead to significant HVAC sizing errors by the mechanical  
reduce the overall thermal transmittance of a glazing sys- engineer.“For this reason, several manufacturers provide  
tem,this doesn’t necessarily translate to energy savings for additional information such as AAMA 507 performance  
the building itself.  
If the fenestration area gets larger at the expense of  
continued on page 64  
opaque wall areas, then the energy trade-off is now rel-  
ative to an opaque wall, not to the same glazed area with  
more framing [and] edge of glass,” says Sanders.“In this  
case, the trade-off is less good in terms of thermal flow,  
as the opaque wall has a significantly lower U-factor, and  
higher R-value, than glass. In this case, larger glass areas  
will still result in higher overall heat transmission through  
the envelope.”  
Another interesting point to consider, Culp notes, is  
that codes use the U-factor and solar heat gain coefficient  
SHGC) calculations at one standard size, as defined in  
NFRC 100. This is regardless of the real product size, so  
a curtainwall with large or small glass sizes will get the  
exact same rating as long as the same glazing make-up  
and framing system is used.  
U-Factor Versus SHGC  
ince the presence of opaque framing elements  
generally reduces solar heat gain coefficient  
S
(SHGC), there is a trade-off between SHGC and  
thermal transmittance when increasing or decreasing  
frame-to-glass area.  
Dr. Helen Sanders of Technoform says the extent  
to which the thermal energy loss matters more than  
SHGC depends on the type of building and the cli-  
mate zone.  
“For large, core-dominated buildings that are  
cooled even in winter months in cold climates, a  
decrease in U-factor may not be as noticeable as a  
significant increase in solar heat gain coefficient in  
the glass. In these cases, the solar heat gain coef-  
ficient may dominate the energy performance, and  
increasing glass size and reducing frame area may  
reduce energy performance.”  
(
This is not real, and can greatly understate the perfor-  
mance of the actual product,so this has been a contentious  
debate within industry and the National Fenestration Rat-  
ing Council (NFRC) for years,”says Culp.“Using a standard  
size is simple and good for apples-to-apples comparison in  
product specs or for code compliance, but can be 20-per-  
cent off from the real performance at the real size.”  
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October 2017 | USGlass, Metal & Glazing  
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