Not Your Grandfather’s Windows: New Glazing and Fenestration Technologies to Meet Expanding Energy – And Peak Power-Reduction Goals

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NOT YOUR GRANDFATHER’S WINDOWS:
NEW GLAZING AND FENESTRATION TECHNOLOGIES
TO MEET EXPANDING ENERGY- AND
PEAK POWER-REDUCTION GOALS
R. CHRISTOPHER MATHIS
MATHIS CONSULTING COMPANY
PO Box 18055, Asheville, NC 28814
Phone: 828-678-3500 • Fax: 828-254-5455 • E-mail: chris@mathisconsulting.com
COAUTHOR:
JAMES LARSEN
CARDINAL GLASS
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ABSTRACT
New objectives for building envelope performance, coupled with a serious refocusing on
the peak power demands on our utilities (defined primarily by building energy use and time
of use), require a reevaluation of the importance of fenestration performance. This paper will
review the array of window and glazing systems currently available and summarize the
boundary conditions of their currently available energy ratings, quantify the energy and
power implications of broader marketplace adoption of current technologies, describe several
of the emerging window and glazing technologies, and quantify the HVAC sizing and
operations implications of different fenestration choices.
SPEAKER
R. CHRISTOPHER MATHIS — MATHIS CONSULTING COMPANY
Chris Mathis has served as a scientist in the Insulation Technology Laboratory at the
Owens-Corning Fiberglas Technical Center, was the director of the Thermal Testing
Laboratory for the National Association of Home Builders Research Center, and director of
marketing for Architectural Testing, Inc., a private laboratory specializing in the performance
of buildings and building products. He was a founding member and the first director
of the National Fenestration Rating Council. Today, his business focus is to work with
strategically aligned clients, leveraging that knowledge and understanding to improve buildings,
building products, and the codes and standards that govern them.
ABSTRACT
From Miami to Maine to Malibu, fenestration
technologies have been a focal point
in trying to improve the building energy performance
and building energy codes around
the nation. However, at their best, most
window systems still are the least efficient
aspect of any building envelope. Even
though today’s most energy-efficient window
and glass technologies have evolved to
deliver unprecedented levels of performance,
they still have very little market
penetration, and adoption of these proven
technologies remains slow. Business as
usual often remains the impediment to
achieving specific energy and sustainability
goals.
New objectives for building envelope
performance, coupled with a serious refocusing
on the peak power demands on our
utilities (defined primarily by building energy
use and time of use), require a reevaluation
of the importance of fenestration performance.
From commercial buildings to
residential, from new construction to rehab,
all face the challenge of choosing the right
glazing system that will durably deliver
long-term, predictable, and reliable energy
and power savings.
Architects, engineers, specifiers, and
other professionals in the buildings arena
should update their current knowledge base
about window and glass technology prior to
any new or rehab construction project.
Specific project objectives—with regard to
new energy codes, sustainability, green
building objectives, daylighting objectives,
local utility peak-power concerns, building
operating schedule, and others—can dramatically
affect the decisions made about
window and glazing type.
This paper will briefly review the array of
window and glazing systems currently
available and summarize the boundary conditions
of their currently available energy
ratings for U-factor (thermal transmission)
and solar heat-gain coefficient (SHGC). We
will discuss some of the HVAC implications
of window and glazing choice and several of
the emerging window and glazing technologies
will be described, citing their energy
and power implications and anticipated limitations.
We will also show examples of unanticipated
loopholes and biases in the energy
code that can easily undermine efficiency
and sustainability objectives, depending on
window selection criteria. We hope to provide
architects, engineers, specifiers, and
others interested in building envelope performance
with practical guidance on product
types, performance, selection tools,
code compliance, and specification pitfalls.
BRIEF SUMMARY OF
FUNDAMENTALS
Windows generally remain the weak link
in our building envelope energy performance.
Even with proper attention to key
elements of good building design (such as
attention to orientation, shading, and
appropriate glazing selection), windows still
are net-energy
penalties for most
buildings. We can
lessen this energy
penalty and, in
some cases, even
make windows a
net-energy benefit.
But in today’s production
home environment
and with
our cookie-cutter
commercial building
mentality, we
rarely give these
critical design and
selection variables
the full attention
they need or
deserve.
The problems
associated with
selecting the right
windows and glazing
are amplified by the many changes that
have occurred to window and glazing technology
over the last decade. No longer can
we merely specify “aluminum windows” or
“wood windows.” Most windows today are
complex engineered systems composed of
dozens of different materials designed to
give functionality, finish, color, security,
and a variety of other performance attributes.
Energy efficiency is just one of many
critical performance attributes that will not
fit behind one of those simple, outdated
generic window descriptors.
Historically, we improved window performance
by adding layers of glass. Each
layer gave us added resistance to heat flow
and different solar heat-gain properties,
shown graphically in Figure 1. With callous
disregard for the job site crew, efficiency
advocates in the 1970s were envisioning
quad glazing systems, trying to make a thin,
visually transparent system compete thermally
with a fairly well insulated wall.
Innovations in low-e coatings, frame
NOT YOUR GRANDFATHER’S WINDOWS:
NEW GLAZING AND FENESTRATION TECHNOLOGIES
TO MEET EXPANDING ENERGY AND
PEAK POWER-REDUCTION GOALS
Figure 1 — U-factor and SHGC potential from multiple layers
of clear glass. (Chart generated using ASHRAE Handbook
values and equations for determining center-of-glass U-factor
for multiple glazing layers).
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design, thermal breaks in aluminum, low
conductance spacers, and composite materials
resulted in double glazed systems that
could deliver U-factors approaching the
quad glazed performance objective. Figure 2
shows how glazing U-factors and SHGCs
evolved with the first generation of low-e
coatings. These early low-e coatings were
designed to be highly transmissive of a large
portion of the solar spectrum while fairly
reflective to long-wave radiation (heat),
making them much better insulators than
their previous clear or tinted glass counterparts.
Today, these low-e coatings are generally
referred to as High Solar Gain Low-e
glazings (HSLE).
As cooling-load management and reduction
became increasingly important in
building energy performance, the glazing
industry responded with an increasing
array of coating technologies. These new
coatings could essentially be fine tuned to
address specific solar transmission and
reflection objectives while maintaining high
degrees of visual clarity. While almost all of
the low-e coatings provided low U-factors
(less than 0.35 BTU/hr x sq ft x ºF), these
newer coatings began to focus on ever lower
SHGCs, targeting cooling load reductions
and addressing the growing peak power and
air-conditioning demand.
Figure 3 shows how glass performance
has dramatically changed from these early
low-e coatings, now with coatings finetuned
for various levels of additional solar
control. These coatings enabled heretofore
unachievable reductions in solar heat gain
while maintaining highly visible transmittance.
These new mid- and low-SHGC glazings
provided dramatic cooling energy savings,
reduced peak cooling loads (enabling
smaller HVAC systems to be installed), and
provided improved occupant comfort year
round. (Note the dramatic downward shift
in the SHGC values for double, triple, and
quad glazing incorporating these new solar
control coatings.
When we put these new glazings into a
variety of different window-framing systems,
we get a variety of whole-product performance
values. For example, the same
glazing in a low-conductivity frame (such as
wood, vinyl, fiberglass, and others) will perform
similarly (in terms of U-factor and
SHGC) to the glass-only values. Putting this
same glazing into a high-conductivity frame
(such as aluminum or steel) can result in
dramatically higher U-factors and SHGC
values than those of the glass alone. The
window industry has recognized this fact
and has developed an array of improved
thermal break technologies for highly conductive
frames that are designed to get
whole-window performance values much
nearer to the glass values.
Figure 4 shows how whole-product performance
(i.e., window performance)—even
with a low-conductivity material such as
vinyl—still shifts away from the glass-only
values. The values shown here essentially
represent the performance limits of passive
(nonswitchable) glazing technologies in
improved frames that are available today.
To protect against misleading performance
claims describing glass-only or
frame-only performance, the industry
worked with the federal government, state
agencies, and specifiers in the late 1980s to
develop a reliable window energy-performance
rating system. Today, most window
Figure 2 – First generation low-e coating impacts on glass energy performance.
Figure 3 — Energy-performance spectrum of low-e coatings available today. HSLE
= High-Solar Gain Low-E; MSLE = Mid-Solar Gain Low-E; LSLE = Low-Solar Gain
Low-E.
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systems carry the NFRC rating and energyperformance
certification. (Figure 5).
As building efficiency demands grew,
the demands on ever-improved window
energy performance grew. Comparative Ufactors
and solar heat-gain coefficients
(SHGCs) evolved from marketplace chaos to
codes and incentive programs that specifically
reference and reward improved window
system performance.1 Programs like
EnergyStar® and others embraced these
comparative ratings and have established
marketplace incentives for selecting and
specifying better windows.
As can be seen from Figures 1 through
4, there are technological limits to passive
technologies. U-factor is constrained by
coatings, wavelengths, layers, and the thermal
conductance of the assembled materials.
Similarly, there is a technological limit
to how low one can go with solar heat-gain
coefficients while still retaining the ability to
see through the window. And while even
lower SHGCs can be achieved, they usually
come by compromising visual clarity and
reducing daylighting potential.
Emerging technologies offer the promise
of even better performance levels. For
example, switchable glazings offer the hope
of window systems that can be fine tuned to
climate, occupant, and building needs,
changing from highly transmissive to
opaque, depending on the type and level of
switching control. However, the cost of
these switchable technologies is far beyond
typical design budgets, and the durability of
these systems over time is still being
assessed.
Providing visual connection to the outdoors
while also limiting unwanted solar
gain are the two most critical elements of
most window selection criteria. Solar control
is so important that some of the emerging
technologies focus on exterior coatings,
frits, and shading systems to further assist
in building-load management.
WINDOWS AND THE ENERGY CODE
Our nation’s first commercial energy
code was published by the American
Society of Heating Refrigerating and Air
Conditioning Engineers (ASHRAE) in 1975.2
As a response to the oil embargo, this code
focused on quantifying and establishing
reasonable performance limits for building
components and systems. Soon, a residential
energy code (the Model Energy Code)
followed, defining minimum energy-performance
attributes for homes. Today, the residential
and commercial energy codes are Figure 5 — NFRC label.
Figure 4 — Whole window U-factor and SHGC range available today. HSLE = High-
Solar Gain Low-E; MSLE = Mid-Solar Gain Low-E; LSLE = Low-Solar Gain Low-E.
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embodied in the International Energy
Conservation Code (IECC). Many state
codes have adapted the IECC for their specific
use.
The IECC focuses on minimum levels of
energy efficiency for buildings. Our early
building-energy simulation programs initially
focused on heating-energy use. As our
recognition of the importance of air conditioning
evolved, so did the computer programs.
We soon realized that managing the
loads from windows and glazing was one of
the most critical energy-management practices
we could employ. Today, those programs
have evolved, enabling a fairly robust
assessment of anticipated building-energy
performance for both heating and cooling.
Both the residential and commercial
energy codes evolved with two paths of possible
code compliance. The first was a simple,
prescriptive set of requirements that,
when met, demonstrated compliance. The
second path, called the performance path,
allowed for computer
simulations to be conducted
allowing designers
and engineers to try
different combinations
of variables that could
meet the energy-performance
targets. Prescriptive
compliance
offered the simplicity of
a single checklist and
easy enforcement. The
performance path
offered the flexibility of
trying multiple options to achieve compliance.
Both compliance paths have fallen
victim to the law of unintended consequences.
UNANTICIPATED CONSEQUENCES
The early energy codes sought to establish
different performance requirements for
each building envelope element. By
attempting to be all-inclusive, being certain
to address all building materials, the codes
established a structure of unintended bias
that remains in place today. This bias
evolved over time from trying to use economic
criteria to help inform and set minimum
efficiency requirements. Each building
technology was evaluated separately as
to its costs. As a result, steel walls have different
minimum efficiency requirements
than do wood walls or concrete walls.
Similarly, aluminum windows have different
energy performance requirements than
nonmetal windows.
While this economics-based structure
seems reasonable, it results in buildings
with differing energy budgets based on the
materials used. It assumes (at least at a
mathematical level) that different building
products do not compete for the same
building element.
Consider: In the commercial code, if one
chooses an aluminum window for a project,
then the U-factor and SHGC have to meet
certain specific requirements. However, if
one chooses a wood, vinyl, or fiberglass window
product, then the maximum U-factor
and maximum SHGC values are lower—
again, resulting in different minimum energy
budgets for a code-compliant building,
depending on which product is being used!
Table 1 shows this current code structure
and the performance requirements for
fenestration in the commercial code. The
values shown are the values proposed for
the 2010 version of ASHRAE Standard
90.1—the national model code for commer-
U-Factor
Climate Zone 1 2 3 4 5 6 7 8
Nonmetal framing 0.51 (0.32)a 0.40 0.35 0.32 0.30 0.30 0.26 0.25
Metal framing, fixed 0.73 (0.50)a 0.50 0.46 0.38 0.38 0.35 0.29 0.29
Metal framing, operable 0.81 (0.65)a 0.65 0.60 0.45 0.45 0.43 0.37 0.37
SHGC — All Frame Types
Max. SHGC (assembly) 0.25 0.25 0.25 0.30 0.30 0.35 0.40 0.40
Min. VT/SHGC (assembly)
Vertical fenestration 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1
Table 1 — Prescriptive fenestration requirements anticipated in ASHRAE 90.1-2010.
Values shown in parentheses are for extremely hot climates where the cooling design temperature exceeds 95ºF.
Note: Just a subset of the entire table is shown here.
VERTICAL FENESTRATION
(30% maximum of walls associated with the building envelope)
Climate Source Energy (Mbtu) for
Zone Heating and Cooling
Metal Operable Nonmetal Difference
Phoenix 2 Dry 1,751 1,642 7%
Houston 2 Humid 2,137 2,050 4%
Baltimore 4 1,807 1,709 6%
Minneapolis 6 1,748 1,616 8%
Table 2 — Source-energy summation for four climate zones, comparing metal and nonmetal
windows.
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cial buildings, slated for publication in
September.
The energy code shows different maximum
U-factors for fixed metal-framed windows,
operable metal-framed windows, and
nonmetal windows. U-factors are a measure
of thermal transmission—a performance
measure. Why do we care about what material
the window is made out of?
Each designer will select the window
design or frames that he or she wants for
his or her project to meet (or exceed) the
requirements of the code. (Note: This same
structure exists for insulated roofs [depending
on how they are insulated] and for insulated
walls [are they steel, concrete, or
wood?] and for floors, slabs, and basements.
The implication is that these different
materials are not competing for the
same wall, roof, or floor area.) The consequences
of this structure are shown below.
We considered just one example of the
unintended consequences of this code
structure, analyzing a medium-sized commercial
building in four climate zones. The
building analyzed is one of the DOE commercial
benchmark buildings.3 The building
analyzed is a 3-story, 53,630-sq-ft office
building with a 32% glazing area (as a percentage
of the wall area) and a 1.5 aspect
ratio.
Holding every other variable constant,
what are the implications of merely changing
from one code-compliant window and
framing system to another (i.e., from metal
to nonmetal)? Table 2 shows the resultant
summation of heating and cooling energy
for the four options.
Several observations can be made. First,
code-compliant does not mean equal energy.
While the mix of heating to cooling may
change, the medium office building’s energy
budget, peak-power demands, and carbon
implications are dramatically different —
just by selecting a different code-compliant
window. From Houston to Minneapolis,
there is a 4% to 8% difference in energy use.
So one unintended consequence of this
regulatory approach results in an energyperformance
loophole. In this example, a
building that consumes 8% more energy is
just as code-compliant as one that consumes
less merely by selecting a metal
product over a nonmetal one.
Another unintended consequence is
that makers of nonmetal windows have a
more stringent U-factor requirement to
meet just because their base material is less
conductive. Again, different energy budgets
for different materials? Isn’t the code about
performance?
While the code marketplace sorts out
these material biases, the implications of
glazing performance on building energy use
continue to take center stage. At the most
recent ASHRAE meetings, one of the most
controversial and significant energy-savings
elements debated was the prescriptive value
for the 2010 version of Standard 90.1,
shown in Table 2.
WINDOW PERFORMANCE
IMPLICATIONS ON LOADS AND
POWER
While we focus on improving our energy
codes, we also have to be aware of the implications
of window selection beyond the
boundaries of the building. Consider what
happens each day as we “turn on” our
buildings. As the day progresses, the
demand for more electricity—especially for
air conditioning—steadily increases.
Managing and even reducing cooling loads
become critical components of managing
utility peak demand, as well as addressing
carbon emission-reduction goals. Figure 6
shows the impact of varying window SHGC
on peak power demand and carbon emissions
for this same medium office building
in four climates.
So as we pursue green building, Energy
Star® buildings, and other beyond-code
objectives, the importance of cooling-load
reduction cannot be overstated.
SPECIFICATION GUIDANCE FOR
BUILDING PROFESSIONALS
There are many criteria that go into the
selection of windows and glazing—from the
strength to withstand tornadoes and hurricanes,
to resistance to salt air on the coast,
to providing ease of use and access to view,
to low maintenance and long-term performance
reliability, to energy attributes like
U-factor and SHGC, to building façade aesthetics.
And there are always trade-offs to
be made. Does one want proven durability
and long-term performance? Or are aesthetics
more important? Is one measuring
and trading carbon? Or should one be
focused more on points for a rating system?
Following is a list of attributes that
should be considered when selecting any
window or glazing system:
1. Meet the design loads required by
code for the building (based on wind
speed, exposure class, location in
wall, unit size, importance factors,
and mean roof height).
2. Know the basic code requirements
(air leakage, water penetration).
3. Know what your project requires
(forced-entry resistance, impact
resistance, operating force, safety
glazing).
4. Know the energy requirements (certified
U-factor and SHGC values).
5. Know the other window-related properties
that will contribute to meeting
overall design objectives (visible
transmittance, daylighting potential,
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Figure 6 — Window SHGC impacts on peak power and carbon emissions.
glare management, views).
6. Clearly define any beyond-code
objectives and the role that windows
play in achieving those objectives
(peak-power and demand-charge
implications, carbon-emission implications).
CONCLUSION
This paper was meant to provide an
overview of the primary energy-related
issues associated with window and glazing
selection. The array of available products
and technologies has dramatically changed
in just the past few years. The lessons
learned about windows a decade ago may
be totally inappropriate to today’s energycode
requirements. Yesterday’s technologies
may be insufficient to address our beyondcode
and green-building design objectives.
Architects, engineers, and specifiers must
consider the expanding array of technologies
as opportunities but not forget to fully
address all of the fundamental specification
requirements that ensure proper window
and glazing selection for each particular
project.
FOOTNOTES
1 National Fenestration Rating
Council.
2. ASHRAE Standard 90-1975, American
Society of Heating Refrigerating
and Air Conditioning Engineers,
Atlanta, GA.
3. P. Torcellini, M. Deru, B. Griffith, K.
Benne, M. Halverson, D. Winiarski,
and D.B. Crawley, ACEEE Summer
Study on Energy Efficiency in Buildings,
DOE Commercial Building
Benchmark Models, 2008.
REFERENCES
ASHRAE Standard 90-1975, American
Society of Heating Refrigerating and
Air Conditioning Engineers, Atlanta,
GA.
ASHRAE Advanced Energy Design
Guides, ASHRAE, Atlanta, GA.
John Carmody et al., Residential
Windows: A Guide to New
Technology and Energy Performance,
3rd Edition, Norton Professional
Books, 2007.
John Carmody et al., Window Systems
for High-Performance Buildings, Norton
Professional Books, 2004.
National Fenestration Rating Council.
P. Torcellini, M. Deru, B. Griffith, K.
Benne, M. Halverson, D. Winiarski,
and D.B. Crawley, ACEEE Summer
Study on Energy Efficiency in
Buildings, DOE Commercial
Building Benchmark Models, 2008.
Alex Wilson, “Rethinking the All-Glass
Building,” Environmental Building
News, Volume 19, Number 7, July
2010.
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