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Building Envelope Design Under the 2009 Codes: Glazing Ratio Requirements

May 15, 2012

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10900 N.E. 8th Street, Suite 810, Bellevue, WA 98004
Phone: 425-451-1301 • Fax: 425-289-5958
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Every three years, a new energy code is adopted by many states. The 2009 and 2012
codes set the trend for very significant improvements to the thermal performance of the
building envelope. These new codes are challenging building owners who desire large
expanses of vision glass without giving up valuable floor space for the thicker walls needed
to accommodate more insulation. Through a case study of a high-rise residential tower in
the Pacific Northwest, this seminar will present solutions that satisfy the desires of owners
and the need to comply with one of the strictest energy codes in the nation.
MEDGAR MARCEAU is a licensed professional engineer and building science consultant
at Morrison Hershfield. He has over twelve years’ experience in building envelope systems,
whole-building energy simulation, and life cycle assessment. Medgar has authored and
coauthored dozens of publications on the subjects of energy use in buildings and life cycle
assessment. He received his BS in engineering from the University of New Brunswick and
master’s degrees in applied science from Concordia University and applied mathematics
from DePaul University.
STÉPHANE HOFFMAN is a building science specialist with Morrison Hershfield and is
the managing principal for its Seattle office. Hoffman specializes in the design of building
envelope systems to control water penetration, air leakage, vapor diffusion, and thermal efficiency.
His work experience includes everything from low-rise wood-frame residential construction
to high-rise commercial developments and a mix of new construction, rehabilitation,
and historic restoration. Hoffman holds a master of engineering degree from McGill
University and a master of architecture degree from the University of Montreal. He is a
licensed professional engineer in British Columbia and Delaware.
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This paper examines how the maximum
allowable glazing ratio (also called fenestration
area)—mandated by the latest energy
codes—affects the design and construction
of buildings. The scope of this paper is commercial
buildings, which includes multiunit
residential buildings over three stories. The
case study is based on an analysis undertaken
during the design of a high-rise residential
building in western Washington.
Energy codes stipulate minimum requirements
for energy-efficient design and construction
of commercial and residential
buildings. In the United States, there is no
nation-wide energy code,1 so code adoption
and enforcement occur at the state and
local level. Some states have adopted
ANSI/ASHRAE/IESNA Standard 90.1-
2007, Energy Standard for Buildings Except
Low-Rise Residential Buildings (ASHRAE
90.1-2007). However, most states have
adopted the 2009 International Energy
Conservation Code (2009 IECC).2 A few
states—including California, Oregon, and
Washington—have their own “home-grown”
energy codes. Every three years, energy
codes are revised to continuously improve
the energy efficiency of new and renovated
buildings. Consequently, stricter energy
conservation measures are adopted with
each code cycle. Since this paper’s case
study building is in the state of Washington,
the paper will also discuss some of the
requirements of the 2009 Washington State
Energy Code (2009 WSEC).
Energy codes typically have three
options for demonstrating compliance: prescriptive,
performance, and trade-off.
Prescriptive requirements are specified minimum
performance requirements in the
code. For example, in western Washington
(which is in climate zone Marine 4), steel
stud walls enclosing a residential occupancy
(other than single-family) are required to
have a minimum of R-19 batt insulation
plus R-8.5 continuous insulation without
thermal bridges other than fasteners. Using
the performance option, compliance is
demonstrated through whole-building computer
simulation to show that a proposed
building has an annual energy performance
that is less than or equal to the annual
energy performance of the standard reference
design over a typical meteorological
year. For example, if the proposed heating,
ventilation, and air conditioning (HVAC)
system is much more energy-efficient than
the code-mandated minimums, energy simulation
could be used to show that the
expected energy savings from the HVAC can
make up for the poorer thermal performance
of a less-than-code-compliant wall.
In ASHRAE 90.1 and 2009 IECC, annual
energy performance is based on predicted
annual energy cost. Hence, this method is
also called the energy cost budget method.
In WSEC 2009, annual energy performance
is based on annual energy consumption.
ASHRAE 90.1-2007, 2009 IECC, and 2009
WSEC have a third option for demonstrating
compliance of the building envelope
called, respectively, “building envelope
trade-off option,” “total UA3 alternative,”
and “component performance building
envelope option.” All of these are “trade-off”
options, which can be thought of as an
intermediate path between the prescriptive
and performance paths. Using the trade-off
option, buildings whose design heat loss
rate is less than or equal to the target heat
loss rate will be considered in compliance.4
With most states having implemented
the 2009 IECC during the 2007-2012 global
financial crisis, relatively few buildings
were permitted under this new code, and
hence few designers have had to deal with
the challenges of complying with the new
thermal performance requirements of the
building envelope. One of these new
requirements is the 40% limit on fenestration
area. For those buildings that have
been permitted under the new code (and
where the new code is actually enforced),
this new limit has had a significant impact
on the design of so-called “glass” buildings.
Under the prescriptive building envelope
requirements of ASHRAE 90.1-2007,
Section, Fenestration Area, states,
“the total vertical fenestration areas shall be
less than 40% of the gross wall area,” and
“the total skylight area shall be less than
5% of the gross roof area.” Fenestration is
defined as “all areas (including the frames)
in the building envelope that let in light,
including windows, plastic panels, clerestories,
skylights, doors that are more than
one-half glass, and glass block walls.”
Similar requirements exist in the 2009
IECC. Under the prescriptive building envelope
requirements, Section 502.3.1,
Maximum Area, states that the vertical fenestration
area shall not exceed 40% of the
gross wall area, and that skylights shall not
exceed 3% of the gross roof area.
Fenestration is defined as “skylights, roof
windows, vertical windows (fixed or moveable),
opaque doors, glazed doors, glazed
block, and combination opaque/glazed
And there are similar requirements in
the 2009 WSEC, although there is a subtle
but important difference. Under the prescriptive
building envelope requirements,
Section 1323.1 states that the percentage of
total glazing relative to the gross exterior
wall areas shall not be greater than 40% for
the vertical glazing and overhead glazing.
Although the requirement is specified in
terms of glazing and not fenestration, the
definition of glazing in the WSEC is similar
to the definition of fenestration in ASHRAE
90.1-2007.5 However, the requirement is in
terms of total glazing, which includes skylights.
Skylights are included in the glazing
area, even though they are not counted as
part of the gross exterior wall area. This has
the effect of further limiting vertical glazing
areas on buildings when there is also a significant
skylight area. One important exception
to Section 1323 is that glazing on “the
display side of street level of retail” can be
excluded from the glazing area calculation.
In all three of these codes, the limitation
on fenestration area—or glazing area as it is
called in the WSEC—essentially comes
down to a limit on the area of vision glass.
One can still design an all-glass building
using the prescriptive path as long as no
more than 40% of the gross wall area is
vision glass and at least 60% is the cladding
for the opaque portions of the envelope,
such as spandrel glazing. The spandrel
glass need not be opaque as long as it is the
cladding on an opaque wall (such as in a
curtain wall shadow box).
For example, Figure 1 shows an all-glass
building that has a combination of reflective
and transparent vision glass (50% of the
gross wall area) and reflective and opaque
spandrel glass (also 50% of the gross wall
area). Under the 2007/2009 prescriptive
requirements, if this building envelope were
designed today, it would not comply with
current energy codes. Yet, this building is
closer to current typical building designs,
which often have vision glazing areas in
excess of 60%. With the new codes, buildings
taking the performance or trade-off
paths to compliance with the aim of
increasing the area of vision glass more
than 40% must now demonstrate that the
“effective U-value” of the opaque wall area—
be it spandrel glass or some other opaque
cladding system—meets or exceeds the
overall U-value of the prescriptive approach.
This is made even more challenging with
the additional requirement to also account
for heat loss due to thermal bridging. The
amount of thermal bridging that must be
accounted for varies by code. For example,
the 2009 IECC states, “the UA calculation
shall be done using a method consistent
with the ASHRAE Handbook of Fundamentals
and shall include the thermal
bridging effects of framing materials.” The
2009 WSEC takes it even further when
defining continuous insulation as “insulation
that is continuous across all structural
members without thermal bridges other
than fasteners and service openings.” As we
will demonstrate in this case study, it can
be difficult to significantly exceed the 40%
limit on the vision glazing area using glazing
systems that are commonly available on the
market today.
Why does the energy code care about
thermal performance of glazing? Glazing
performance is directly related to both heating
and cooling loads in buildings. The thermal
performance of glazing units and their
framing system drive the majority of heat
loss through the building enclosure.
Furthermore, typical glazing assemblies significantly
underperform with respect to
heat loss when compared to typical opaque
wall assemblies, especially when considering
the implementation of the codes’
requirement for continuous insulation.
Solar gain through vision areas is one of the
three main sources of heat gains (the other
two are lighting and people). Solar gains are
a major component of cooling loads.
Unmanaged heat gains can have a significant
impact on the design of commercial
buildings; therefore, unlike in residential
building, the code specifies maximum solar
heat gain coefficients (SHGC). For example,
in the prescriptive residential requirements
of 2009 IECC Section 402.1.1, there is no
SHGC requirement in climate zones 4 to 8,
and it is 0.3 in climate zones 1 to 3; whereas,
in the prescriptive commercial requirements,
the SHGC requirements are more
stringent in climate zones 1 to 3, and there
are requirements in all climate zones.
Although commonly available technologies—
such has low-emissivity coatings—
can mitigate solar heat gains, the impact on
cooling loads is still significant when large
amounts of glazing are used.
There is an optimum fenestration area
that minimizes the energy consumption in
buildings, but it is building- and envelopespecific.
And depending on occupancy, it
may or may not be climate-specific.
Determining the optimum area requires
whole-building energy simulation (commonly
called energy modeling). The key to finding
the optimum is to realize that there are
trade-offs to be made between synergistic
but competing design objectives. For example,
6 in buildings with high internal heat
loads, such as in typical commercial buildings,
the relationships are
1. Decrease window area or its solar
transmission, and cooling energy
use is decreased; and
2. Increase window area or its daylight
transmission, and lighting energy
use and associated heat gains are
The trade-offs apply to the building’s
perimeter and are actually independent of
climate. In both Chicago and Houston, the
optimum fenestration area is 15% of the
gross wall area for double-pane clear windows
and daylighting controls (U = 0.60,
SHGC = 0.6, and VT = 0.63).7 In the same
cities, the optimum fenestration area jumps
to 45% for triple-pane, clear, low-e windows
with overhang and daylighting controls (U=
0.20, SHGC = 0.22, VT = 0.37). However, it
should be noted that these percentages
were determined from an energy simulation
that ignored heat loss at thermal bridges.
Therefore, we caution designers in applying
Figure 1 – This building has a 100% glazed wall, but 50% of the wall area is
vision glass (reflective and transparent), and 50% of the area is spandrel glass
(reflective and opaque).
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these results blindly to all buildings. It would be instructive to
reproduce the referenced study while also taking heat loss at thermal
bridges into account.
The objectives of placing a limit on the maximum fenestration
area are to minimize heat loss in winter and heat gains in summer.
On building projects that are targeting a larger fenestration area, the
limit will encourage designers to use new technologies to minimize
heat loss and solar heat gain to trade off the additional heat
exchange through large fenestration areas.
As our case study will demonstrate, buildings designed with fenestration
areas greater than 40% will require higher-performing
glazing in better thermally designed framing systems to achieve
glazing U-values significantly below the code-mandated value of
0.40 Btu/h·ft2·°F (per the 2009 WSEC). And designers will need to
consider opaque wall assemblies that have much more thermal
resistance than what is common today. However, the real challenge
will be in achieving higher U-values for spandrel glazing in all-glass
buildings. As we will demonstrate in this case study, the inherent
thermal bridging of the framing system can significantly impact the
performance of the opaque wall assembly in these buildings, especially
when the three-dimensional (3-D) heat loss between the vision
glazing and spandrel glazing is considered.
The case study building is a high-rise residential building located
in western Washington. It has an all-glass, custom-designed curtain
wall, five levels of below-grade parking, two levels of retail
space, and 19 residential floors (see Figure 2). The gross wall area is
83,809 ft2. As Figure 2 shows, the design goal was to maximize
vision-glazing areas (upwards of 80%) to take advantage of abundant
views of the region’s natural beauty and to appeal to consumer
Starting with a code-matching building that has 40% vision glazing,
opaque walls with R-19 batt insulation, and R-8.5 continuous
insulation, the target heat-loss rate (UA) is 14,991 Btu/h·°F. (Of
this, 2,841 Btu/h·°F is for the opaque envelope, and 12,150
Btu/h·°F is for fenestration.)8 The relative UA of the fenestration
compared to the opaque portion of the envelope shows that 81% of
the heat loss is through the fenestration
(12,150/14,991 × 100% = 81%). That leaves
a very small percentage of the opaque wall
heat loss that can be traded off. And even a
better fenestration U-factor will not change
the fact that most of the heat loss is
through the fenestration.
At this point, the project design team
was confident that a fenestration U-factor of
0.35 Btu/h·ft2·°F would be attainable with
the custom-designed curtain wall. This Ufactor
is a weighted average that includes
the vision glazing and the framing. Note
that this glazing system already has a much
better U-factor than the code-maximum Ufactor
of 0.40. By incrementally increasing
the thermal performance of the glazed
spandrel assembly (expressed as 1/U, or
the “overall effective R-value”), we determined
a range of maximum allowable fenestration
areas as shown in Figure 3. For
Figure 2 – The case study building is an all-glass high-rise
residential building.
Figure 3 – For a given fenestration U-factor of 0.35 Btu/h·ft2·°F, the maximum
allowable fenestration area increases as the overall effective R-values of the
opaque wall (that is, the glazed spandrel assembly) increases, but it approaches a
limit of approximately 52% of the gross wall area.
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example, to get to a 50% fenestration area, the glazed spandrel assembly
would have to have an overall effective R-value of about R-30. With 50% fenestration,
the proposed UA is 14,964 Btu/h·°F. (Of this, 1,714 Btu/h·°F is
for the opaque envelope, and 13,250 Btu/h·°F is for fenestration.) Thus, the
relative UA of the fenestration compared to the opaque portion of the envelope
shows that now 88% of the heat loss is through the fenestration (compared
to 81% for a code-matching building). However, the architect felt that
50% glazing would not meet the design goal for large expanses of floor-toceiling
vision glass, so we explored other options for increasing the fenestration
The next step was to look at how improving the fenestration U-factor
could help increase the amount of allowable vision glass. Using the same
incremental procedure described above, we calculated the maximum allowable
fenestration area for a range of fenestration U-factors ranging from 0.31
to 0.35 Btu/h·ft2·°F in 0.01 increments. Figure 4 shows the resulting required
thermal performance of the glazed spandrel assembly for a given fenestration
U-factor. The results show that very significant improvements to thermal performance
of the fenestration system would be required to increase the maximum
allowable fenestration area. But even with a U-factor of 0.31 and an
overall effective R-value of R-30, the maximum allowable fenestration area is
only about 57%. But based on the project’s budget and technical constraints,
the design team determined that a fenestration U-factor of 0.35 and overall
effective R-value of the glazed spandrel assembly of R-33 was the most realistic
choice. This allowed the project to have a fenestration area of 51%—a far
cry from the architect’s original vision of upwards of 80% vision glazing. The
glazed spandrel assembly is shown in Figure 5. Two-dimensional (2D) thermal
modeling9 was used to determine the U-factor of the curtain wall. The
curtain wall is thermally broken. The thermal model accounts for 2D thermal
bridging through mullions and steel studs in the wall. In order to maximize
the amount of floor-to-ceiling vision glass, the designer chose to orient
the spandrel panels vertically instead of horizontally as shown in Figure 6.
The above case study was based on overall effective R-values and
accounted for thermal bridging as required by energy code. Standard practice
in North America to account for thermal bridging within the building
envelope is to consider thermal bridging within an assembly (for example, a
steel stud wall), but to ignore thermal bridging at architectural and structural
details—including interfaces—where walls, windows, floors, and roofs
come together. Whole-building energy modeling procedures for performance-
based compliance in energy codes and standards are either silent on
thermal bridges relating to details and transitions (such as slab edges, shelf
Figure 6 – The final design will consist of a
combination of vertical vision and opaque glazing to
meet the calculated maximum fenestration area of
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Figure 5 – The glazed spandrel
assembly has a 2D overall effective Rvalue
(1/U) of 33 h·ft2·°F/Btu (the figure
shows a horizontal cross section
through a vertical mullion).
Figure 4 – Maximum allowable
fenestration area for a given thermal
performance of the opaque wall (that
is, the glazed spandrel assembly)
assuming fenestration U-factors in the
range of 0.35 to 0.31 Btu/h·ft2·°F.
angles, and sheet metal flashings), they
allow these thermal bridges to be ignored
through partial or full exemptions, or the
procedures reduce the apparent significance
of thermal bridges through oversimplification.
The reasons for these omissions
appear to be based on the following:
• The belief that details do not have a
significant impact on the overall
building envelope performance and
on whole building energy use
because they comprise a small area
compared to the total envelope area.
• Past experience that shows it would
take too much effort to quantify all
thermal bridges, which often have
complex three-dimensional (3-D)
heat flow paths.
• The lack of comprehensive thermal
transmittance data for standard
However, recent work10 accounting for 3-
D heat flow through details has shown that
the overall performance of many common
wall assemblies is much less than what is
currently assumed by many practitioners.
Irrespective of the small areas of highly conductive
materials that bypass thermal insulation,
the effect on overall energy consumption
is significant, and simple changes to
assembly design may be more effective at
reducing energy use than adding more insulation.
In addition, accounting for these
details is now easier because straightforward
procedures to quantify the impact of
common details have been developed, and
thermal transmittance data for standard
details are now readily available in a catalogue
published by ASHRAE.11 Realistic
expectations of building envelope performance
are necessary to make informed decisions
related to building energy efficiency.
3-D heat loss through curtain wall systems
is very significant and should not be
ignored. For example, using the results of 3-
D thermal modeling such as that shown in
Figure 7, installed insulation with a nominal
R-value of R-33 results in an assembly with
an overall effective R-value of about R-9. In
Figure 7 – On the left, a typical opaque spandrel area of curtain wall with insulation in the backpan and spray foam
insulation applied to the inside face of the backpan (through the steel-framed stud wall). On the right, the same detail
showing the typical pattern of temperature distribution (normalized temperature index) as a result of three-dimensional heat
Figure 8 – Overall effective “R-value” (1/U) of spandrel areas of curtain wall with a
range of insulation in the backpan and either no insulation in the stud wall
cavity or 2 in. of spray foam insulation in the stud wall cavity applied to the
inside surface of the backpan.
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fact, due to heat loss through exposed vertical
and horizontal mullions in vision areas
down to the opaque spandrel areas, at the
intersection of vertical and horizontal mullions,
and at curtain wall anchors, there is
a diminishing return on the effectiveness of
installed insulation as shown by the results
in Figure 8. With conventional materials
and a typical “good” thermally broken curtain
wall system, the overall effective Rvalue
will be in the range of R-5 to R-9.
However, with new materials such as vacuum-
insulated panels (about R-40 per in.)
and nonmetal curtain wall framing, we are
starting to see this limitation be exceeded.
As challenging as the maximum fenestration
area is, it will be even more challenging
once the new 2010/2012 codes are
adopted. Both the 2010 version of ASHRAE
90.1 and the 2012 version of the IECC have
decreased the maximum fenestration area
to 30% of the gross wall area.12 As states
adopt these codes, getting beyond 30% will
be even more challenging than getting past
40% with the current code. Therefore, it will
be even harder to meet code if one wants to
also exceed maximum fenestration areas
using commonly available systems that are
on the market today. We expect that these
stricter requirements in the energy code will
eventually trickle down to LEED. Presently,
to earn more than three points under
Energy and Atmosphere Credit 1,13 the
design team must demonstrate through
whole-building energy simulation that the
energy cost performance of its proposed
design exceeds the baseline design. The
baseline design is a building conforming to
ASHRAE 90.1-2007. One of the proposed
changes for the next version of LEED (to be
balloted June 1, 2013)14 is to reference
ASHRAE 90.1-2010 as the baseline. With a
maximum fenestration area of 30% in the
baseline design, the total energy budget
with which the design team has to work
becomes even less. Whether it is to meet
code or to earn points under Energy and
Atmosphere Credit 1, exceeding 30% fenestration
area will require even higherperforming
envelope systems or greater
energy savings in other areas to trade off.
The focus on continuously improving
the energy efficiency of new building
through energy codes will drive a demand
for higher-performance glazing and better
thermal-performing frames. It will also provide
an incentive for emerging technologies
such as vacuum-insulated glazing, vacuuminsulated
panels, electrochromic glass, and
other innovative applications that can help
improve the thermal performance of glazing
systems. At the same time, it will also bring
a closer examination of the justifications for
increased applications of vision glass on
projects. Increasingly, designers will be
required to demonstrate that increasing the
fenestration area will add value to a project.
The 2012 IECC will have an exception to the
glazing ratio for buildings where 50% or
more of the floor space benefits from daylighting.
In turn, this will drive a need to
address the benefits and impacts of
increased glazing early in the conceptual
design phase of such projects. Early collaboration
among the design architects,
mechanical engineers, and building envelope
consultants will be even more crucial
on these projects.
1. Except for federal buildings.
2. From the status of state energy
codes and adoption maps on the
U.S. DOE’s Building Energy Codes
Program website:
3. UA is the overall rate of heat transfer
through the building envelope
per unit of time, induced by a unit
temperature difference between the
environments on each side of the
envelope. It is equal to the sum of
the products of thermal transmittances
(that is, the U-factor) multiplied
by their respective areas.
4. The 2009 IECC and the 2009 WSEC
require accounting for conductive
heat loss and solar heat gain (refer
to 2009 IECC, Section 402.1.4,
“Total UA Alternative”; and 2009
WSEC, Section 1330, “Component
Performance Building Envelope
Option”). However, the procedure in
ASHRAE 90.1-2007 is more complex
because it requires accounting for
climate, lighting, thermal mass, and
building orientation (refer to
ASHRAE 90.1, Section 5.6, “Building
Envelope Trade-Off Option” and
“Normative Appendix C”).
5. Except that the WSEC does not
specifically mention plastic panels,
and the threshold of glass in doors is
not specified.
6. Example quoted from High Performance
Building Façade Solutions,
LawrenceBerkeleyNational Laboratories,
/concepts.html. Original source is
R. Johnson, R. Sullivan, S. Nozaki,
S, Selkowitz, C. Conner, and D.
Arasteh, 1983, “Building Envelope
Thermal and Daylighting Analysis in
Support of Recommendations to
Update ASHRAE/IES Standard
90.1,” Battelle Pacific Northwest
Laboratories, Richland, WA.
7. U is U-factor (thermal transmittance),
and it includes the effects of
framing but not the interface
between framing and adjacent constructions;
SHGC is solar heat gain
coefficient; and VT is visible transmittance.
8. Calculations were made using the
2009 Nonresidential Energy Code
Compliance Forms from the Northwest
Energy Efficiency Council,
9. THERM Finite Element Simulator,
version 6.3.45, 2012. Regents of the
University of California. Developed
and maintained by U.S. Department
of Energy, Lawrence Berkeley National
10. Morrison Hershfield Ltd., 2011,
ASHRAE RP-1365, Thermal Performance
of Building Envelope Construction
Details for Mid- and High-
Rise Buildings, American Society of
Heating, Refrigeration and Air-conditioning
Engineers, Inc., Atlanta,
11. Ibid.
12. 2012 IECC, Section C402.3.1, Maximum
area. However, the maximum
allowable area is increased to 40% if
certain exceptions are taken, such
as using automatic daylighting controls.
13. LEED 2009 for New Construction and
Major Renovations, 2012, U.S. Green
Building Council, Washington, DC.
14. From a letter by the USGBG president,
“Important News About LEED
2012: A Message from Rick Fedr
i z z i , ” w w w . u s g b c b l o g .
ews – a b o u t – l e e d – 2 0 1 2 -me s –