The Great Debate: Nonreflective vs Reflective Roofing Membranes

The Great Debate:
Nonreflective vs. Reflective Roofing Membranes
Robert Anderson
Firestone Building Products
250 W. 96th st., Indianapolis, In 46260
Phone: 317-833-5491 • fax: 317-428-5685 • e-mail: andersonrobert@firestonebp.com
3 0 t h RC I I n t e R n a t I o n a l C o n v e n t I o n a n d t R a d e S h ow • M a R C h 5 – 1 0 , 2 0 1 5 a n d e R S o n • 1 1 1
Abstract
The prevalence of reflective roofing membranes has increased in recent years. From an
environmental standpoint, the trend toward reflective roofing membranes can be attributed
to the material’s role in a building’s energy usage, global warming potential (GWP), and the
urban heat island (UHi) effect. recent research shows that the color of a roofing membrane
can also affect rooftop condensation and longevity. Some studies even indicate a “heating
penalty” for using reflective roofs in certain climates, possibly negating the energy savings
achieved from the cool roof.
This presentation will review the impact of color and reflectivity on buildings and the
environment.
Speaker
Robert Anderson — Firestone Building Products
rOBErT anDErSOn is a building envelope solutions manager for Firestone Building
Products. anderson is responsible for promoting the benefits of metal wall claddings, cavity
wall technologies, and premium roofing systems to the architectural community. anderson
has a BS in finance from northern illinois University. He is a member the Construction
Specifications institute and the Building Enclosure Council.
1 1 2 • a n d e R S o n 3 0 t h RC I I n t e R n a t I o n a l C o n v e n t I o n a n d t R a d e S h ow • M a R C h 5 – 1 0 , 2 0 1 5
3 0 t h RC I I n t e R n a t I o n a l C o n v e n t I o n a n d t R a d e S h ow • M a R C h 5 – 1 0 , 2 0 1 5 a n d e R S o n • 1 1 3
INTRODUCTION
The use of reflective roof membranes
has increased steadily in recent years.
Growth can be attributed, in part, to cost
and system preference. The trend toward
reflective roofing membranes is also being
driven by environmental concerns.
a building owner may be partial to reflective
roofing because of the material’s role in
maximizing his or her building’s energy
usage and reducing operating costs—a
more localized benefit. additionally, building
owners may feel that a reflective roof is
providing a societal benefit by reducing the
urban heat island (UHi) effect or the global
warming potential (GWP) of the building.
Voluntary environmentally focused programs,
like the United States Green Building
Council’s (USGBC’s) lEED® program, promote
the use of reflective roof coverings.
These coverings are also mentioned in prescriptive
energy standards like aSHraE
90.1, Energy Standard for Buildings.
Despite the trend towards reflective
options for cost or environmental benefits,
commercial roofing products with a nonreflective
surface continue to be a significant
part of the low-slope roofing market.
In northern climates, for example,
where heating days outnumber cooling,
some believe that a nonreflective black roof
membrane is the logical choice for optimized
energy performance.
recent research has suggested that the
color of a roofing membrane can also affect
rooftop condensation and overall longevity.
Some studies even indicate a “heating
penalty” for using white, reflective roofs in
certain climates, negating the energy savings
achieved from the cool roof. it should
be noted that the effect of a membrane’s
reflectivity is fairly nonexistent when it is
covered with snow.
The topics of cool and reflective roofing
have been studied and reported upon since
at least the late 1980s, and new findings
continue to appear on a regular basis. is a
reflective roof the best choice? Today, there
is still no absolute answer for this question.
a design professional must evaluate
each building to thoughtfully respond and
recommend the best material for peak performance.
In order to separate fact from opinion,
research has been conducted to help guide
designers and members of the building
community. The published literature on
this topic is fairly extensive; some reports
contradict others, but certain conclusions
can be drawn.
First, it is important to recognize that
this is not a “black and white” issue. in
many parts of Canada and Western Europe,
for example, gray-colored roofing is often
used. in fact, a Canadian field monitoring
study of black-, gray-, and white-colored
roofing was included in last year’s rCi
International Convention.1
What about vegetative roofing, aggregate-
ballasted roofing, and dirty white roofing?
These are generally considered “cool”
roofs, but do they provide the same benefits
as reflective roofs? There are many variables
to consider when designing a roof.
From the data, a key conclusion is:
There are several factors that contribute to
the energy performance of a roof system,
and each of these factors—not just membrane
reflectance—should be evaluated to
make a responsible roofing decision.
WHAT IS R EFLECTIVE R OOFING?
WHY D OES IT M ATTER?
“reflective roofing” is not synonymous
with “cool roofing,” nor does it always refer
to white roofs. Additionally, the performance
specifications for low-slope roofs
(a slope of 2:12 or less, based on aSTm
Standard E1918-972) are different from the
expectations of steep-slope options (greater
than 2:12).
To provide clarity and consistency, this
paper focuses on low-slope roofing, which
typically has more stringent requirements
than steep-slope roofing. a 2002 study
showed that a membrane’s reflectivity
diminishes with weathering, but typically
stabilizes after about three years.3 Though
some still consider initial reflectance, many
programs and standards now focus on aged
reflectivity for this reason. To qualify as an
energy star® roof, a low-slope roofing material
must have an initial reflectivity of at
least 0.65 and a three-year aged reflectivity
of 0.50.4 This means that the roof covering,
when new, must reflect 65% of the incident
solar energy.
The Solar reflectance index (Sri) is a
similar but more comprehensive rating that
is a measure of a roof’s ability to cast-off
solar heat,5 incorporating not just a material’s
reflectance, but also its emittance (ability
to shed heat). The USGBC employs the Sri
measure in its lEED® guidelines. To contribute
to a lEED® point, a low-slope roof must
have an initial Sri of 82.6 The California
Energy Commission qualifies either a minimum
aged solar reflectance of 0.63 or a
minimum Sri of 75 to qualify as an acceptable
cool roof.7 These various programs
and authorities will be discussed later, but
this serves to point out that there is not an
agreed-upon definition of a cool or reflective
roof. a generally light-colored roof having an
initial reflectance of at least 0.65 is a good
starting point to be considered reflective.
“Cool roofing,” however, is a broader
term. “Cool” roofing materials include vegetative8
and ballasted options. Due largely
to absorptiveness and thermal mass—rather
than reflectance values—vegetative and
ballasted roofing systems have been shown
to perform “in a manner that is equivalent
to (or better than) the white membrane for
a substantial portion of their service life.”9
Generally speaking, there are two motivating
factors to install a reflective roof.
The most obvious is the impression that a
reflective roof will help lower cooling energy
costs in hot temperatures.
The other motivating factor is the perceived
societal benefit of a reflective roof.
many studies have suggested that reflective
roofing can mitigate the UHi effect10,11
or reduce the GWP of a building. This is
related to the first factor pertaining to
energy savings. it is reasonable to assume
that reduced energy use helps to mitigate
carbon emissions.
Urban areas tend to be warmer than
their rural surroundings, resulting in “an
‘island’ of higher temperatures … [both] on
the surface and in the atmosphere.”12 This
The Great Debate:
Nonreflective vs. Reflective Roofing Membranes
means that urban areas take more energy
to cool in the hotter months, but also less
energy to heat in the cold months. although
it seems reasonable to assume that if a
roof can reflect solar energy away from the
building, it could mitigate the UHi effect,
but there is no recorded evidence of this.
This social and environmental advocacy
aspect of reflective roofing has led to a number
of decrees—both voluntary and statutory—
by various organizations and agencies
calling for reflective roofs.
WHO SAYS?
The lEED® and energy star® programs
have already been cited, and it is important
to note what these and other voluntary
programs have to say about reflective roofing
because they impact the materials that
designers and builders use. However, it
is also important to recognize the difference
between elective programs, consensusbased
standards, and building codes. Cities
like Chicago, Houston, new York, and los
angeles all have some requirement for
reflective roofing13 with standards informing
the city codes.
ASHRAE 90.1-2010
aSHraE (formerly the american Society
of Heating, refrigeration and air Conditioning
Engineers) is a “global society advancing
human well-being through sustainable technology
for the built environment.”14 The
organization has produced a number of
standards to further that advancement. For
example, in 1975, aSHraE first published
its Standard 90, which has today evolved into
anSi/aSHraE/iES Standard 90.1-2010,
Energy Standard for Buildings Except Low-
Rise Residential Buildings. This often-cited
energy standard includes one section related
to reflective roofing. Section 5.5.3.1.1, “roof
Solar reflectance and Thermal Emittance,”
outlines the prescriptive measures that must
be taken to have a compliant roof surface.
notably, aSHraE limits the application of
this section to Climate zones 1 to 3 only (see
Figure 1).
in these three climate zones, the standard
mandates that a roof must have a
minimum three-year aged Sri of 64 (or
equivalent reflectance and emittance values)
or increased insulation levels (r-33
to r-35, depending on the roof assembly).
Additionally, this section of the standard
allows for a number of exceptions for roofs
with a sufficient amount of ballast, vegetation,
shading, or other factors.15 This standard
makes no suggestion or requirement
for reflective roofing in climate zones 4 and
above.
aSHraE 90.1 is significant because it
has become the basis for many building
codes worldwide. Specifically focusing on
the U.S., starting with the passage of the
U.S. Energy Policy act of 1992, aSHraE
90.1 has been the benchmark for energyefficient
building. When the act was passed,
all states were required to adopt energy
standards at least as stringent as the most
recent Department of Energy-approved version
of aSHraE 90.1.
Today, aSHraE 90.1 is revised and
republished every three years to align with
the international Energy Conservation
Code’s (iECC’s) three-year revision cycle.
aSHraE 90.1-2010 forms the basis of the
2012 iECC, just as aSHraE 90.1-2007 did
for the 2009 IECC.
as of July 2014 and as shown in Figure
2, a minority of states had yet to enact a
statewide energy code, but more than 40
are currently enforcing some version of
aSHraE 90.1. Twelve have already adopted
the most recent version.16
For those states operating under the
current aSHraE 90.1 in climate zones 1
to 3, a reflective roof (aged Sri of 64) must
be used. aSHraE 90.1 does not include a
requirement for reflective roofing in other
climate zones.
1 1 4 • a n d e R S o n 3 0 t h RC I I n t e R n a t I o n a l C o n v e n t I o n a n d t R a d e S h ow • M a R C h 5 – 1 0 , 2 0 1 5
Figure 1 – Climate zone map from ASHRAE 90.1-2010 Appendix B.
ASHRAE 189.1-2011
Sometimes called “the
green standard,” this is
officially anSi/aSHraE/
USGBC/iES Standard
189.1-2011, Standard
for the Design of High-
Performance Green Buildings.
The “green standard”
is for high-performance
green buildings and is
intended to exceed the energy
standards of aSHraE
90.1. However, this standard’s
section 5.3.2.3 on
the “mitigation of Heat
island Effect – roofs” still
only applies to building
projects in Climate zones 1
to 3.17 Section 5.3.2.3 does
utilize different compliance
criteria than aSHraE 90.1.
However, like aSHraE
90.1, there is still no prescription
or suggestion
to install reflective roofs
except in these southern
climate zones.
aSHraE 189.1 has
also been noticed by the international
Code Council (iCC), and it is a compliance
option in the 2012 International Green
Construction Code™ (igCC). That means
that if one’s building satisfies the aSHraE
189.1 standard, then it is automatically
compliant with the igCC. Sometimes called
an “overlay code,” the igCC can be adopted
by jurisdictions that have interest in a regulatory
framework for green buildings that
goes beyond the scope of the iECC.
LEED®
The USGBC states that its lEED® program
is the most widely used green building
program worldwide, comprising more
than 10.1 billion square feet of construction
space.18 lEED® is a voluntary program that
is motivated by the social factor of reducing
the UHi effect.
The latest version, lEED® v4, offers two
points if a building can be designed with
intent to reduce UHi. Despite this credit’s
goal of mitigating UHi, the credit is not
limited to buildings located in urban areas.
To qualify under credit SSc5, one must
perform a calculation that incorporates “nonroof
measures” (shading over parking areas,
reflective paving, etc.), high-reflectance
roofs, and vegetated roofs.
low-slope roofs with an initial Sri of 82
or an aged Sri of 64 are weighted equally
to vegetative roof areas.19 roof membranes
with a lower Sri value cannot contribute to
this credit.
lEED® does not evaluate other roofing
assembly components (vapor retarders,
insulation, etc.), and it does not consider
climate zones.
THE FUTURE S TANDARD – ANSI/
ASHRAE/IES/USGBC STANDARD
189.1
Though not expected to take effect until
at least 2017, there is a new agreement
among aSHraE, the iCC, USGBC, the
illuminating Engineering Society (iES), and
the american institute of architects (aia) to
create a combined green building standard.
The goal is to simplify the array of standards,
regulations, and voluntary programs
in order to increase the number of buildings
that are designed as high-performance
“green” buildings.
The idea is that the igCC will simply
“become an adoptable, code-enforceable
version of [aSHraE] 189.1,” and igCC compliance
will also “serve as an alternative
system of prerequisites for lEED®.”20
While aSHraE and lEED® will still be
developing their own requirements and
credits, the two bodies are expected to be
aligned. When it happens, this new system
should provide a more cohesive, understandable
framework for code officials,
architects, and building owners.
Energy Star®
energy star® is a voluntary program
within the U.S. Environmental Protection
agency (EPa) that was initially established
in 1992. it has a stated goal of helping
“businesses and individuals save money
and protect our climate.” Today, energy
star® qualifies certain roof products that
can carry the energy star® label; and for
low-slope roofing, the products are roof
coatings and membranes with an initial
solar reflectance of at least 0.65 and a
reflectance of at least 0.50 three years after
installation.21
like lEED®, energy star® takes a blanket
approach to reflectivity. assuming that
reflectivity is typically beneficial, it is not
concerned with whether an energy star®
product is used in an urban or rural area.
To its credit, energy star® does recommend
3 0 t h RC I I n t e R n a t I o n a l C o n v e n t I o n a n d t R a d e S h ow • M a R C h 5 – 1 0 , 2 0 1 5 a n d e R S o n • 1 1 5
Figure 2 – IECC adoptions by state.
residential insulation levels, and it considers
climate zones when doing so.22 Figure
3 clearly employs the map from aSHraE
90.1, though it is a simplified version.
However, energy star® does not currently
approve insulation products, nor
does it mention insulation in any of its
commercial construction recommendations.
ROOFPOINT®
Officially launched in 2012 by the
Center for Environmental Innovation in
roofing (CEir), rOOFPOinT® is a voluntary
program described as a “Guideline for
Environmentally innovative nonresidential
roofing.” This program is by far the most
comprehensive in its consideration of a
roof’s impact on the environment. in addition
to energy management, rOOFPOinT®
evaluates the impact of a roof’s water management,
material management, and overall
durability. Within the Energy management
section, there are six available credits.
Only one of the six credits pertains to
roof surface thermal contribution.23 The
recognition that there are at least five
other factors that contribute to a building’s
energy management is encouraging to many
roofing professionals. rOOFPOinT® does
not take a “one-size-fits-all” approach to
roof design.
The roof Surface Thermal Contribution
credit is further divided into three separate
parts to address three separate intents:
optimizing net annual building energy efficiency,
optimizing peak energy demand,
and reducing heat island effects. according
to rOOFPOinT®, a ballasted or vegetated
roof can satisfy all three of these goals in
any climate zone. On the subject of reflectivity,
though, the program takes a more
nuanced approach. instead of using the
word “reflective,” rOOFPOinT® refers to
high-, medium- and low-albedo roofs. While
one definition of albedo is “reflective power,”
rOOFPOinT® uses albedo to refer to a material’s
Sri, which incorporates emittance
and reflectivity. The rOOFPOinT® classifications
are as follows:
• a high-albedo roof has an initial Sri
of at least 78 and aged Sri of at least
64
• a medium-albedo roof has a new or
aged Sri between 20 and 64
• a low-albedo roof has a surface with
a new or aged Sri less than or equal
to 20
a high-albedo roof can satisfy the
requirement to reduce heat island effects in
any climate zone, but it is only seen as an
option to optimize peak energy demand in
Climate zones 1 to 5.
1 1 6 • a n d e R S o n 3 0 t h RC I I n t e R n a t I o n a l C o n v e n t I o n a n d t R a d e S h ow • M a R C h 5 – 1 0 , 2 0 1 5
Zone Add Insulation to Attic Floor
Uninsulated Attic Existing 3-4 in. of Insulation
1 R30 to R49 R25 to R30 R13
2 R30 to R660 R25 to R38 R13 to R19
3 R30 to R60 R25 to R38 R19 to R25
4 R38 to R60 R38 R25 to R30
5 to 8 R49 to R60 R38 to R49 R25 to R30
Figure 3 – EnErgy Star®-recommended levels of insulation.
in zones 6 to 8, rOOFPOinT® asserts
that a medium- or low-albedo roof is the
appropriate choice.
The program takes a similar stance on
the topic of optimizing net annual energy
efficiency, where a high-albedo roof qualifies
in Climate zones 1 to 6, a medium albedo
roof qualifies in zones 4 to 8 and a low
albedo roof is seen as optimizing net annual
energy efficiency in Climate zones 5 to 8.24
With rOOFPOinT®, CEIR has inferred
there are regions of the country where a
nonreflective roof has greater energy efficiency
than a reflective roof.
rOOFPOinT® also makes it clear within
its section on “Energy management” that
there are several factors within a roofing
system that affect a building’s energy performance.
in addition to the roof Surface Thermal
Contribution, the rOOFPOinT® program
also provides credits for employing a “High-r
roof System” (referring to a well-insulated
system with a high r value) and for utilizing
best thermal practices, a roof air barrier,
daylighting and rooftop energy systems.25
This voluntary, consensus-based program
is the most thorough environmental evaluation
guideline in the roofing industry.
WHAT D OES THE R ESEARCH S AY?
researchers continue to study reflective
roofing and its impact on the UHi effect, global
climate change, and energy costs. many
studies demonstrate that reflective roofs do
provide energy cost savings in certain areas.
Similarly, several published papers have used
models to illustrate that reflective roof surfaces
can minimize the UHi effect.
However, research does not unanimously
support recommendations to use
highly reflective roofs in all climate zones.
Furthermore, the building community
needs to ask if the models are comprehensive
enough to justify reflective roofing
codes and regulations.
The following review of key research
from the last 10 years may not provide
definitive answers to these questions, but it
should serve to further illuminate the issue.
a note on energy models: all software
models are inherently reliant on inputted
data. assumptive figures are incorporated
in any program’s simulations, and these
figures can have significant impacts on
simulation outcomes. This review notes the
software models in each study, but it cannot
cite every inputted assumption. Some
assumptions are noted, but the reader
is directed to the original publications to
examine their full methodology.
ENERGY U SAGE AND C OSTS
in 2005, James Hoff presented “The
Economics of Cool roofing: a local and
regional approach” at the Cool roofing
Symposium. Using the U.S. Department of
Energy Cool roof Calculator, buildings in
40 cities in the contiguous U.S. were modeled
with both reflective (55% reflectance)
and nonreflective (5% reflectance) roofs.
The model assumed electric cooling and
natural gas heating systems with average
efficiency, using cost data from the Energy
Information Administration. The calculated
annual energy savings of using the reflective
option on a 20,000-sq.-ft. roof varied
from a high of $860 (Phoenix, az) to a net
loss of $100 (Seattle, Wa), and geographic
bands were created to help visualize the
regional impact (see Figure 4).
Overall, this study concluded:
Perhaps the most significant observation
that may be derived from
this analysis is that the energy savings
provided by a reflective roof
may offer little economic incentive
for the average building owner in
many areas of the United States.
…in major mid-west cities, such as
Chicago, Cleveland, and Pittsburgh,
where reflective roofs offer minimal
annual savings of $20 to $60, it is
difficult to see how such small savings
will affect the purchase decision
of an informed building owner.”26
Since at least the late 1980s, Hashem
akbari and his colleagues at lawrence
Berkeley national lab have been publishing
work on the effects of both in-situ reflective
roofing and reflective roofing within predictive
energy models.27 akbari continues to be
a prolific researcher on this topic, both in
terms of energy savings and global environmental
impact.
in 2010, akbari and levinson published
a comprehensive study that used the DOE-
2.1E building energy model to compare
“weathered cool white” roofing with a solar
reflectance of 0.55 to roofing with a solar
reflectance of 0.20. This study simulated
multiple building types (“old” and “new”
building stock, across multiple end uses).
Using local energy generation, emission,
and cost data for 236 cities, akbari and
levinson created an estimate of energy cost
savings by switching to reflective roofing.
The research demonstrates that every state
in the U.S. would experience annual energy
cost savings on a “per-unit conditioned roof
area” basis if it switched its roofing from a
0.20 reflectivity to 0.55.
3 0 t h RC I I n t e R n a t I o n a l C o n v e n t I o n a n d t R a d e S h ow • M a R C h 5 – 1 0 , 2 0 1 5 a n d e R S o n • 1 1 7
Figure 4 – Regional energy savings for 20,000 sq.-ft. roof area (Hoff, 2005).
Furthermore, it demonstrated that “retrofitting
80 percent of the 2.58 billion
square meters of commercial building conditioned
roof area in the USa would yield
…an annual energy cost saving of $735
million.” The study acknowledges a heating
penalty for using reflective roofs in some
northern climates, but it is very nearly
always offset by the savings from less cool
energy. The model shows energy cost savings
in all regions with “old” building stock,
and in nearly every region with “new” building
stock (Figure 5).28
In a paper from the Proceedings of the
2014 RCI International Convention, energy
use was also modeled on both black and
white roofs. This study did not take the step
of converting usage to actual costs, but the
findings are still relevant.
Using the U.S. Department of Energy’s
EnergyPlus modeling program and Building
Energy Codes Program stand-alone retail
building model, researchers examined one
representative city from each climate zone.
roof assemblies with three alternative insulation
systems were modeled with both
reflective white roofing (Sri of 70) and black
roofing (Sri of -4). The multifaceted study
draws several conclusions, but regarding
the energy usage of buildings with reflective
and nonreflective roof coverings, the paper’s
authors state:
Overall energy use is lower for white
roofs in cooling-dominated climate
zones (zones 1 and 2), and lower for
black roofs in heating-dominated
climate zones (zones 5 to 8). in
mixed heating and cooling climate
zones (zones 3 and 4), the differences
are very small between light- and
dark-colored roof membranes.”29
note: This study also has findings
regarding reduction of greenhouse gasses,
to be covered in a later section.
THE U HI E FFECT AND GWP
When looking at the UHi and GWP
studies, it is important to note that the
results are based on complicated predictive
modeling software. Though the market
shift toward increased
reflective roofing has
been underway for two
decades, there are no
published studies that
evince a reduction of
a regional heat island.
individual buildings
have been analyzed,
but regional or global
impacts on the use of
reflective roofing have
only been modeled.
The rooftops of
Athens, Greece, were
the subject of a paper
published in 2008. The
research looked at the
potential heat island
mitigation of converting
all roofs in athens
to a higher reflectivity.
They judged the
base albedo of the
area as 0.18 and ran
simulations on both
a modest increase (to
0.45) and an extreme
rise (to 0.85) using
“the ‘urbanized’ version
of the nonhydrostatic
fifth-generation
Pennsylvania State University-nCar
mesoscale model (mm5, version 3-6-1).”
measured at a 2-meter height, the reduction
in athens’ temperature was as high as
1.5°C given the 0.45 albedo scenario, and
as high as 2.2°C for the extreme scenario.
The publication provided no justification
on the basis for the 2-meter height, but
this measurement dimension is often used.
The researchers suggest that increasing the
rooftop reflectivity would reduce the UHi
effect, stating:
This analysis shows that adopting
large-scale high albedo measures by
using building materials with high
solar reflectance can significantly
reduce ambient temperatures. Cityscale
application of cool materials
will result in a reduction in energy
consumption by reducing both
direct radiative heating of buildings
and ambient temperatures.30
in 2010, there were at least three major
studies that were published on the effects
1 1 8 • a n d e R S o n 3 0 t h RC I I n t e R n a t I o n a l C o n v e n t I o n a n d t R a d e S h ow • M a R C h 5 – 1 0 , 2 0 1 5
Figure 5 – Annual energy cost savings for new office prototype (Akbari and Levinson, 2010).
of reflective roofing
on climate, and they
each relied on different
energy models. The
previously mentioned
akbari and levinson
paper used the DOE-
2.1E building energy
model to compare roofs
with a solar reflectance
of 0.20 to “weathered,”
reflective roofing with
a reflectance of 0.55
nationwide. its findings
related to CO2
emissions were similar
to its discoveries on
energy cost savings:
There were reductions
with the reflective roofing
model.
Measured as an
average per conditioned
roof area, the
study claimed that
every state except
alaska would realize
annual reductions in
CO2 emissions if all
of their rooftops had
a reflectivity of 0.55
instead of 0.20 (Figure
6). This paper also included an attentiongrabbing
statistic: “retrofitting 80 percent
of the …commercial building conditioned
roof areas in the U.S. [to a reflectance of
0.55] would yield…an annual CO2 reduction
of 6.23 mt, offsetting the annual CO2
emissions of 1.20 million cars or 25.4 peak
power plants.”31
along with three other colleagues, the
authors of that paper were also involved in
publishing another study in 2010 related to
the issue. This time, the Catchment land
Surface model within the naSa GEOS-
5 atmospheric General Circulation model
was used to look at the outgoing radiation
from the earth to assess the climate
effects of a 0.10 increase in the reflectivity
of all “roofs and pavements in the urban
areas in temperate and tropical regions of
the globe.”32 The inclusion of pavement is
notable, as the study does not differentiate
the roofing impact versus the paving
impact. nonetheless, the researchers found
that for every 0.03 increase in surface
albedo, global temperature would decrease
by ~0.008 K during the months of June,
July, and august. Though the study notes
that “a more meaningful evaluation …would
require simulations which better characterize
urban surfaces and represent the full
annual cycle,” it still does indicate that
a higher surface reflectivity can decrease
global temperatures.33
“Effects of White roofs on Urban
Temperature in a Global Climate model,”
also published in 2010, contains relevant
findings on reflectivity and UHi. This study
used the urban canyon model (ClmU) coupled
with the Community Climate System
model (CCSm) to compare the effects of
urban roof surfaces with a 0.90 roof albedo
to a “control” albedo of 0.32 using climate
simulations from 1941 to 1999. across
all urban areas that were modeled, their
control UHi was 1.2°C warmer than the
2-meter air temperature of rural surfaces,
while the high albedo heat island was only
0.8°C warmer, a 33% reduction in the UHi
effect. Before getting too excited about this
reduction, the authors point out that the
extremely high 0.90 albedo that was modeled
“could not practically be achieved.”
Additionally, they found that the use of
white roofing will significantly increase the
use of space heaters. in fact, space heating
increased by a greater amount than air conditioning
use decreased in this model, so
“end-use energy costs must be considered
in evaluating the benefits of white roofs.”
The paper also notes that in higher latitudes,
the heat island reduction is smaller,
and “any benefits gained from a reduction
in the summertime heat island need to be
considered in the context of increased heating
costs in winter.” Despite the study’s
limitations, it does clearly conclude that
increased reflectivity “is an effective way of
reducing the urban heat island.”34
Despite the different methodologies
employed in those three studies of 2010,
they all imply that more reflective roofing
leads to decreased ambient temperatures.
In 2011, a new study showed a different
result. “Effects of Urban Surfaces and White
roofs on Global and regional Climate”
included a simulation that converted all
roofs to white, with a reflectance of 0.65
(from a base assumption of 0.12), using
3 0 t h RC I I n t e R n a t I o n a l C o n v e n t I o n a n d t R a d e S h ow • M a R C h 5 – 1 0 , 2 0 1 5 a n d e R S o n • 1 1 9
Figure 6 – Annual CO2 emission reduction for new office prototype (Akbari and Levinson, 2010).
the GaTOr-GCmOm computer model, “a
global gas, aerosol, transport, radiation,
general circulation, mesoscale, and ocean
model.” This model did show that white
roofs caused cooling in “local” population-
weighted ground and ambient temperatures.
“However, feedbacks of the local
changes to the large scale resulted in a
gross global warming. …Whereas, the population-
weighted air temperature decrease
due to white roofs was ~0.02 K, the global
temperature increase was ~0.07 K.” This
study, too, has limitations; the authors
note white roofing’s effect on local energy
demand and emissions is not accounted
for in the study. The comprehensive model
used in this study does include an atmospheric
model, though, and it implies that
high-albedo roofs change atmospheric stability
and clouds. it also incorporates “feedbacks
to the larger scale,” where “higher
reflection also increased air heating by
black and brown carbon in soot.”35 The
study found that a worldwide conversion to
white roofs would warm the Earth.
REFLECTIVE R OOF S URFACES AND
CONDENSATION
Commonly, condensation is considered
to be what happens when warm, moist air
contacts a cold surface, resulting in moisture
collection. in roofing assemblies, this
can happen when air from the interior of
a conditioned/heated building infiltrates
through a roof deck and insulation joints
before finding a condensing surface on the
underside of a roofing membrane, typically
in a cool climate. Within the last decade,
some published papers have considered
the potential for a cooler white membrane
to increase the amount of moisture within
a roof assembly, as compared to a dark
membrane.
WUFi®, a hygrothermic modeling program,
was used to model moisture in
roofing assemblies for the 2008 study,
“Condensation Problems in Cool roofs.”
Here, researchers simulated both black and
white roofing membranes in three representative
U.S. cities—Phoenix (warm), Chicago
(temperate), and anchorage (cold)—on two
different multilayer roof assemblies over a
period of five years. The first type of roof
assembly modeled was a “self-drying roof.”
This is a common type of roof construction
that has no vapor barrier other than
the roof membrane, allowing any moisture
within the roof assembly to “dry out to
the interior of the building.” Though selfdrying
roof assemblies are not often used
in alaska, when the roof has the ability to
self-dry, they found accumulation of moisture
within the roofing assembly only in the
extreme temperature of anchorage with the
white roof. In each of the three cities, the
model showed white rooftops would generate
more moisture during the course of the
year than the black roofs; but except for the
white roof simulation in anchorage, all were
able to dry out during the summer. The
study determines that self-drying roofs perform
well in most locations “independent of
the applied surface color,” except “locations
with low average temperatures” (Figure 7).
When analyzing an unventilated roof
deck assembly with the membrane applied
1 2 0 • a n d e R S o n 3 0 t h RC I I n t e R n a t I o n a l C o n v e n t I o n a n d t R a d e S h ow • M a R C h 5 – 1 0 , 2 0 1 5
Figure 7 – Water content of flat self-drying roof (Bludau et al., 2008).
Figure 8 – Moisture content in OSB layer (Bludau et al., 2008).
to oriented strand board (OSB, wood) over
wooden rafters infilled with fiber insulation
and an interior vapor retarder, the results
were different. In these simulations, the
moisture content of the OSB was closely
monitored to ensure that its water content
did not exceed the critical 20%, a point
where the material faces potential degradation.
The 20% threshold was exceeded in
both Chicago and anchorage when the white
roof was used. When an interior vapor barrier
is used in these climates, the authors
recommend that “construction should be
built up with a dark surface” (Figure 8).
This study cautions that “if a cool roof
is designed for a temperate or cold climate,
its moisture behavior should be analyzed
. . .to avoid critical water content.”36
When looking at a traditional metal
deck with 2- to 3-in. insulation boards and
a white membrane roof, Ennis and Kehrer
seemed to confirm those results for a selfdrying
roof in their 2011 paper. again using
the WUFi tool, their model concluded that
white roofs in Climate zone 5 produced
more than double the amount of condensation
as black roofing. “However, within
the parameters used in this study, both
roofs returned to a dry condition during
the course of the year.”37 Another facet of
this paper was the inclusion of actual field
research. Test cuts were performed on ten
light-colored roof systems in climate zone
5. The test cuts were done in the winter
(February and march, 2010) in the morning
(before 10:30 a.m.) on roof systems with
vapor retarders that had been in service no
less than five years. Three of the ten investigated
roofing systems were damp below the
membrane, and “minimal damage” to the
insulation was observed. The conclusion on
this moisture, though, was that “minimal
effect had occurred to the roofing assembly
that would affect its integrity, insulating
value, or performance. no detrimental effect
to the roof system was noted.”38 A third portion
of this paper focused on a two-year field
evaluation of a re-covered roofing assembly,
which resulted in a now-familiar finding:
the black roofing membrane had a greater
“drying rate” than the white membrane, but
the difference was negligible—it “did not
affect the performance of the fasteners or
insulation over the two-year period of the
study.”39 This study contains several overall
conclusions, and the first two are noteworthy
in the context of this review:
1. Situations where moisture accumulation
occurs are design issues.
2. When designing a roof system membrane
color, in addition to other
variables such as building conditions,
insulation levels and local
weather conditions must be considered
in order to prevent moisture
condensation and subsequent accumulation
within the assembly.40
This is consistent with the comments of
Hutchinson in his 2009 publication, though
his comments about cool roofing and the
design community are even more candid.
He urges designers to take a comprehensive
approach to evaluating roof system performance
and warns, “Cool roofing and its
single-component mentality are resulting in
roof-system failures and impending litigation.”
The paper documents existing roof
systems with condensation and mold growth
on the surfaces below reflective roofing, as
well as condensation leading to the presence
of ice within the lap seams of reflective
membranes. it also relates another concern
with reflective roofing—the detrimental effect
of roof-reflected radiation on materials adjacent
to the reflective roof. Excessive heating
from reflected solar energy is shown to have
deteriorated masonry joints and other cladding
systems. Concerned with condensation
and other consequences of reflective roofing,
the paper concludes:
It is imperative that everyone in
the design and construction industry
realize the benefits of designing
quality roof systems regardless of
the type of roof cover and move away
from suggesting a single-component
solution.41
CONCLUSION
Though not included in the literature
review herein, a thoughtful analysis of the
energy use of black and white roofs was presented
for the 2009 rCi Building Envelope
Symposium. after pages of roof energy-use
modeling graphs (using the DOE Cool roof
Calculator), the author concludes:
The issue of reflectivity has been overstated
and can lead to undesirable
outcomes. …Conserving energy when
designing a roofing assembly can be
accomplished by a responsible selection
of the different components and
the proper level of insulation.42
after reviewing these recent research
updates on reflective roofing, it is hard not
to agree with that statement. There is little
or no debate on the notion that reflective
roofing will lower heating and cooling energy
usage on minimally insulated buildings
in the southern U.S. On the topic of reflective
roofing, the absence of debate ends
there. The color of a roof surface will have
an impact—sometimes positive and sometimes
negative—on a building’s interior
conditions such as occupant comfort, the
service life of the roof itself, adjacent building
materials, local air temperatures, and,
potentially, even global climate. However,
it is only one factor that a responsible
designer should consider as he or she takes
a holistic approach to designing a roof. if
a roof membrane color is chosen without
consideration of the attachment method,
the amount of insulation, and the air and
vapor barriers within the assembly, then
the opportunity to optimize the building’s
energy use will be missed. Furthermore,
this entire roof system must be designed
with continuity to the walls to truly serve its
role within the building envelope. Only after
this multicomponent roof system analysis
and weighing of the needs of the building,
its owner, and its occupants can the “correct”
roof surface be determined.
REFERENCES
1. G. Finch, m. Dell, B. Hanam, and
l. rickets. “Conventional roofing
assemblies: measured Thermal
Benefits of light to Dark roof membranes
and alternate insulation
Strategies.” RCI International Convention
Proceedings (2014).
2. a STm Standard E1918-97.
3. W.a. miller, meng-Dawn Cheng, S.
Pfiffner, and n. Byars. “The Field
Performance of High-reflectance
Single-Ply membranes Exposed
to Three Years of Weathering in
Various U.S. Climates” (2002).
4. energy star. <www.energystar.gov/
index.cfm?c=roof_prods.pr_crit_
roof_products>.
5. l awrence Berkeley national laboratory.
<energy.lbl.gov/coolroof/
ref_01.htm>.
6. U.S. Green Building Council, LEED
Reference Guide for Building Design
and Construction, V4 (2013).
7. California Energy Commission,
2013 Nonresidential Compliance
3 0 t h RC I I n t e R n a t I o n a l C o n v e n t I o n a n d t R a d e S h ow • M a R C h 5 – 1 0 , 2 0 1 5 a n d e R S o n • 1 2 1
Manual for the 2013 Building Energy
Efficiency Standards (2013).
8. S.r. Gaffin et al. “a Temperature
and Seasonal Energy analysis of
Green, White, and Black Roofs.”
Columbia University, Center for
Climate Systems research (2010).
9. a . Desjarlais et al. “Evaluating the
Energy Performance of Ballasted
roof Systems.” Single Ply roofing
industry (2007).
10. H. akbari and S. Konopacki. “Energy
Impacts of Heat Island Reduction
Strategies in the Greater Toronto
area, Canada” (2001).
11. H. akbari and S. Konopacki. “Calculating
Energy-Saving Potentials of
Heat-island reduction Strategies.”
Energy Policy 33 (2005).
12. U.S. Environmental Protection
agency. <www.epa.gov/heatisland/
about/index.htm>.
13. Cool roof rating Council. <coolroofs.
org/resources/rebates-andcodes>.
14. aSHraE. <www.ashrae.org/aboutashrae>.
15. aSHraE. anSi/aSHraE/iES Standard
90.1-2010, Energy Standard
for Buildings Except low-rise
residential Buildings (2010).
16. International Code Council. <www.
i c c s a f e . o r g / g r / D o c u m e n t s /
stateadoptions.pdf>.
17. aSHraE. anSi/aSHraE/USGBC/
iES Standard 189.1-2011, Standard
for High-Performance Green
Buildings Except low-rise residential
Buildings (2011).
18. U.S. Green Building Council. <www.
usgbc.org/about>.
19. U.S. Green Building Council. lEED
Reference Guide for Building Design
and Construction, V4 (2013).
20. P. melton. “One Standard to rule
Them all: lEED, igCC, 189.1
to Be Parts of a Single System.”
Environmental Building News 23
(2014).
21. energy star. “Program requirements
Product Specification for roof
Products, Version 2.3” (2012).
22. energy star. <www.energystar.gov/
index.cfm?c=home_sealing.hm_
improvement_insulation_table>.
23. Center for Environmental innovation
in roofing. “rOOFPOinT Guideline
for Environmentally innovative nonresidentail
roofing” (2012).
24. ibid.
25. ibid.
26. J. Hoff. “The Economics of Cool
roofing: a local and regional
approach.” rCi Foundation Cool
Roof Symposium Proceedings (2005).
27. Berkeley lab. <heatisland.lbl.gov/
publications/author/2>.
28. H. akbari and r. levinson. “Potential
Benefits of Cool roofs on Commercial
Buildings: Conserving Energy,
Saving money, and reducing
Emission of Greenhouse Gases and
air Pollutants.” Energy Efficiency 3
(2010).
29. Finch et al. (2014).
30. a. Synnefa, a. Dandou, m. Santamouris,
m. Tombrou, and n. Soulakellis.
“On the Use of Cool materials
as a Heat island mitigation Strategy.”
Journal of Applied Meteorology and
Climatology 47 (2008).
31. H. akbari and r. levinson. (2010).
32. S. menon, H. akbari, S. mahanama, i.
Sednev, and r. levinson. “radiative
Forcing and Temperature response
to Changes in Urban albedos and
Associated Offsets.” Environmental
Research Letters, 5 (2010).
33. ibid.
34. K.W. Oleson, G.B. Bonan, and J.
Feddema. “Effects of White roofs
on Urban Temperature in a Global
Climate Model.” Geophysical
Research Letters 37 (2010).
35. mz. Jacobsen and J.E.T. Hoeve.
“Effects of Urban Surfaces and
White roofs on Global and regional
Climate.” Stanford University (2011).
36. C. Bludau, D. zirkelbach, and H.m.
Kunkel. “Condensation Problems in
Cool Roofs.” International Conference
on Durability of Building materials
and Components. istanbul, Turkey
(2008).
37. m. Ennis and m. Kehrer. “The
Effects of roof membrane Color on
moisture accumulation in low-
Slope Commercial roof Systems.”
international roofing Symposium
proceedings (2011).
38. ibid.
39. ibid.
40. ibid.
41. T. Hutchinson. “Challenging What’s
Cool.” eco-structure January/February
(2009).
42. S. ibrahim. “Sustainable roof
Design – more Than a Black and
White Issue.” RCI Building Envelope
Symposium Proceedings (2009).
1 2 2 • a n d e R S o n 3 0 t h RC I I n t e R n a t I o n a l C o n v e n t I o n a n d t R a d e S h ow • M a R C h 5 – 1 0 , 2 0 1 5