Are Ballasted Roof Systems Cool?

EXECUTIVE SUMMARY
A combined experimental and analytical
study was initiated to quantify the energy
savings of ballasted roofing systems and to
compare their thermal performance to that
of cool roof membranes. The experimental
design was structured to evaluate how a
mass of three different stone ballast weights
and one paver ballast affected heat flux into
the building and the build-up of the membrane
surface temperature in comparison
to the controls (in this case, both a black
and a white single-ply membrane).
Experimental work included the initial
and subsequent occasional measure of
reflectance and initial estimate of the emittance
of the test samples, weekly organization
of the temperature and heat flux data,
and the comparison of the ballast with the
white and black membrane thermal performance.
This work builds on the earlier work
completed and published in “The Field
Performance of High-reflectance Single-ply
Membranes Exposed to Three Years of
Weathering in Various U.S. Climates,”
which was also prepared by Oak Ridge
National Laboratory for SPRI. This study
investigated the reflectivity and thermal
performance of single-ply membranes when
exposed to the outdoor environment.
Modeling the stone for its thermal performance
is one of the deliverables of the
experimental work. The paver ballast with
weight equal to that of the heavy stone sample
was included to aid in developing the
model. This model will allow the stone to be
entered as a roof component into the
Department of Energy’s (DOE’s) Cool Roof
Calculator, permitting the annual heating
and cooling loads to be calculated for specific
ballast configurations on roofs containing
various insulation levels located in different
regions of the country.
Stone Reflectivity Versus Stone Mass
and the Impact on Heat Flow
After eight months of exposure in east
Tennessee’s climate, the white single-ply
membrane control degraded in reflectivity
by 15%, a similar rate as seen in the earlier
study referenced above that showed the
exposed single-ply membranes degraded 30
to 50% after three years of outdoor exposure.
On the other hand, the paver ballast
increased in reflectivity by 7%, while the
stone ballast reflectivity was assumed to
remain unchanged based on results from
the earlier study.
The study evaluated the effects of mass
on thermal performance by including three
stone ballast weights of #4 stone at 10,
16.75, and 23.5 pounds per square foot; the
10-pound weight being the minimum
allowed for ballasted systems. The paver
weight was 23.5 pounds per square foot,
matching the heaviest stone weight. The
membrane temperature and insulation heat
flux data taken early in the study indicated
that the 10-pound ballast weight produced
thermal values about 30% higher than the
white control. This is substantially better
than what would be expected from the
stone’s reflectivity value of 0.21, which is
73% lower reflectivity than the white membrane
(0.78) and closer to the black reflectivity
of 0.06. As the mass of the stone was
increased with no change in stone reflectivity,
the thermal performance values proportionally
reduced, moving closer to the
exposed white membrane system performance
with the 24-pound ballast (stone or
paver) having only 5% more heat transfer.
Although the paver has a reflectivity of 0.51
compared to the stone value of 0.21, the
paver and 24-pound stone samples had
very similar thermal profiles with nearly
equal high and low values indicating mass
has a greater effect on thermal performance
than reflectivity.
Thermal Performance of Ballast Versus
Reflective Membranes
The white membrane, with a reflectivity
of 0.78, again proved to be an effective tool
for deflecting the sun’s energy from the
building. However, some of this ability deteriorates
over time, as the high reflectivity is
This paper was originally prepared in February 2005 for SPRI (Sheet Membrane & Component Suppliers to the Commercial Roofing Industry)
and presented at the “Cool Roofing… Cutting Through the Glare” symposium sponsored by the Roof Consultants Institute Foundation (RCIF),
Oak Ridge National Laboratory (ORNL), and the National Research Council Canada’s Institute for Research in Construction (NRC/IRC).
White Stone Paver Black
Initial 0.78 0.21 0.52 0.06
8 Month 0.67 0.21 0.55 0.09
% Change (15) 0 7 50
Chart 1: Reflectivity readings.
32 • I N T E R FA C E S E P T E M B E R 2005
lost due to air-borne fallout, biological
growth, and the weathering process. The
study also showed that the ballast samples
were successful in shielding the building
from the sun’s energy even though the ballast
reflectivity was only 0.21. As the ballast
mass increased, the thermal performance
continued to improve. As stated above, the
10-pound ballast was within 30% of the
thermal performance of the new white membrane,
while the 24-pound ballast was within
5%. The mass also delayed the time at
which the maximum membrane surface
temperature and heat flux were reached versus
the white and black controls, a two- to
three-hour delay, dependent on the amount
of the mass. This delay moves more of the
cooling load into the off-peak hours of the
day.
At the eight-month point in the study,
the maximum temperature and heat flux
values for the ballast had moved closer to
the white thermal profile with the 24-pound
ballast closely mimicking the white control.
This is related to the loss of reflectivity of
the white membrane, which is now 0.67,
just above the 0.65 minimum reflectivity
value required for a new product to be listed
as an EnergyStar™ roofing product.
When the data are analyzed for only the
daylight hours where cooling of a building is
the major concern, the results at the start of
the study showed the 24-pound stone providing
the same level of performance as the
white membrane, while the paver provided
slightly less savings. If one factors in the
time delay in reaching the maximum temperature/
heat flux (which moves more of
the cooling load into the off-peak hours),
the ballast offers an effective alternative to
white membranes.
However, in the process of acting as a
shield against the exterior conditions, the
ballast mass absorbs some of this energy.
With its mass far greater than that of a single-
ply membrane, the ballasted roof takes
longer to dissipate this energy even though
it has emissivity values equal to those of the
single-ply membranes. This, in turn, keeps
its average temperature and heat flux above
those of the white membrane over a 24-hour
period. In the heat of the summer, this is a
disadvantage to the ballast system.
However, as the outside temperature moderates
both in the spring and fall, this slower
reaction to both high and low temperatures
dampens the heat flow through the roof and
stabilizes the heating and cooling loads in
the building under a ballasted system.
Will Ballast Qualify as an EnergyStar™
Roofing Product?
A roofing product that has a “new” minimum
reflective value of 0.65 and a threeyear
aged value of 0.50 or greater (after
washing) qualifies to be listed as an
EnergyStar™ product for use on low-sloped
roofs. The ballast used in the study does not
meet the current EnergyStar™ criteria. The
study indicates that a ballasted system with
a reflective value of only 0.21 does perform
at the same level of thermal performance as
the rated EnergyStar™ products. There is
also an indication that the damping effect of
the ballast may actually offer equal or better
performance over a full day, month, or
year of operation. More information will be
developed to try to determine whether or
not ballast performs as well as currently
listed EnergyStar™ roofing products.
Essential to this effort is the ability to
model the thermal performance of the bal-
Chart 3: Sunny day average temperature (˚F) and heat flux [BTU/(h•ft2)] values.
White 10# 17# 24# Paver Black
Initial Temp. 81 87 86 86 85 98
Initial Heat Flux 1.9 3.9 3.8 3.2 3.2 5.6
8-Month Temp. 66 73 71 72 70 81
8-Month Heat Flux -1.1 .7 .4 .3 -.4 2.1
Chart 2: Sunny day maximum temperature (˚F) and heat flux [BTU/(h•ft2)] values.
White 10# 17# 24# Paver Black
Initial Temp. 85 104 95 90 90 145
Initial Heat Flux 3 8.5 7 5 5 16.5
8-Month Temp. 100 110 105 95 95 145
8-Month Heat Flux 7 10 8 6 5 17
S E P T E M B E R 2005 I N T E R FA C E • 3 3
34 • I N T E R FA C E S E P T E M B E R 2005
lasted systems with available tools.
Specifically, thermal properties are needed
for use in the transient heat conduction
equation. Preliminary work has been done
with a program that does the inverse: it
uses the transient heat conduction equation
to predict thermal properties to fit the
measurements of heat flux and temperature.
The program had difficulty converging
with the data for the 10-pound and 16.75-
pound ballasts during the summer months
when convection effects in thin layers of
stone could be expected. Early on in the
project and again after nine months, analysis
with the program is showing some hope
of predicting thermal properties consistently
from week to week.
Best estimates, so far, put the thermal
conductivity of the stone at 0.3 to 0.4
BTU/(h•ft•˚F) and volumetric heat capacity
(product of density and specific heat) at 19
to 21 BTU/(ft3•˚F). The corresponding estimates
for the paver are 1.45 to 1.65
BTI/(h•ft•˚F) and 23 to 25 BTU/(ft3•˚F).
With the measured thicknesses of the stone
and paver, these thermal conductivities
yield R-values of 0.3 to 0.4 h•ft2•˚F/BTU for
the 10-pound stone, 0.5 to 0.6 for the
16.75-pound stone, 0.6 to 0.8 for the 23.5-
pound stone, and 0.10 to 0.11 for the 23.5-
pound paver. The ballasts form low R-value,
high thermal mass systems.
Until a consistent picture emerges of the
thermal properties, work cannot be started
with the modeling of the thermal performance
of the systems. Modeling will use the
thermal properties to predict the heat flux
through the fiberboard insulation in each
test section. Comparison to the measured
heat flux will validate the model; or, if agreement
is affected consistently by convection
effects in the thin stones, the comparison
will calibrate the model. A validated or calibrated
model permits prediction of thermal
performance in different locations with
roofs having typical insulation R-value. The
test roofs had minimal insulation R-value in
order to maximize the sensitivity of the
measurements to differences in the ballast
properties.
INTRODUCTION
In warm desert climates, structures
were often made of thick, sand-colored
adobe walls before modern construction
materials were available. These walls had
substantial thermal mass, which helped to
isolate the inside of the building from the
outside environment. In many parts of the
United States, older structures were made
with thick stone walls also providing some
protection from the heat of a summer day
by absorbing the sun’s energy in the wall
mass. On the other hand, light colored
materials protect the building by reflecting
the sun’s energy, reducing the energy load
on a building. Temperature measurements
made at the Buildings Technology Center
(BTC) show that, on a sunny day, a highly
reflective roof surface can be as little as 3˚C
(5˚F) warmer than ambient air temperature,
while a dark, absorptive roof surface can be
upwards of 40˚C (72˚F) warmer. This knowledge
has accelerated the use of roofing
products that offer smooth, highly reflective
surfaces to reduce the energy needs for
cooling the building.
Where do ballasted systems with irregular
earth tone-colored stone surfaces fall in
comparison to new “high-tech” exposed
membrane systems? Some information on
ballasted system thermal performance was
obtained in the original study, “The Field
Performance of High-reflectance Single-ply
Membranes Exposed to Three Years of
Weathering in Various U.S. Climates,” but
was of secondary interest since the primary
focus was on the exposed membranes. The
ballast weight was not measured accurately,
so the data could not be quantified.
Trends indicated that the ballast was
shielding the building from the sun’s heat,
to some extent helping to justify the initiation
of the current study.
Brief History of Ballasted Roofing
Systems
Ballasted systems entered the roofing
market in the early 1970s. The stone used
with these systems is different from the traditional
quarter-inch chip or smaller stone
used with built-up and modified bitumen
roofing. With these last two systems, the
small stones are partially imbedded into the
topcoat of asphalt to protect the asphalt
(same applies to coal tar based systems)
from the harmful rays of the sun. The stone
used as ballast for single-ply systems is
large in size, #4 (.75 to 1.5 inch in diameter)
and larger stone. Ballast comes in other
configurations, such as concrete or rubber
pavers. Ballast is applied in loadings from
10 (the minimum) to over 24 pounds per
square foot.
So with the loose-laid ballasted roof system,
the contractor places all the components
of the roof system, including the thermal
barrier and insulation, unattached on
the roof deck. The membrane is also looselaid
except for attachment around the
perimeter of the building and at roof penetrations.
The ballast is then placed on top of
the membrane, weighing down all the components
to hold them in place. This technique
eliminates the use of copious fasteners
that are used to hold the roofing components
in place, which in turn minimize thermal
bridging. This also eliminates the need
for adhesives to attach the membrane to the
roof deck substrate. Thus, the ballasted
method can greatly reduce the installed cost
of the roof system as well as the time to
install it. In addition, this ballast is basically
fireproof, providing Class A (top rating)
fire protection for the building that is under
the system. EPDM takes great advantage of
this construction with its ability to be factory-
made in large sheets (up to 10,000 sq.
ft.), further reducing the labor and, in turn,
the installed cost of the roof system while
improving overall quality. These benefits
have allowed this system to become a major
factor in the roofing marketplace.
Ballast is also used with the inverted –
protected roof system where the roof system
is built “upside down.” A protective course
may be placed over the deck. The membrane
is then laid down, followed by the
insulation, a filter fabric, and the ballast.
The ballast often used in this application is
pavers because it is often applied where
there will be pedestrian traffic. Plaza decks
and rooftop terraces are a few examples.
The paver offers a trafficable surface with
the insulation acting as a thermal protection
layer and a shock absorber for the
waterproofing system below it. In some
applications, the paver is made from rubber,
yielding a play exercise surface on the
roof. Another form of ballast is mixed soil
media and plants to form a roof garden with
unique aesthetic appeal and performance
characteristics such as stormwater management.
Because of the inherent simplicity of ballast
systems, early proponents focused
mainly on expansion into the market.
During this early period, there was little
technical information available on one
design consideration: namely, how to design
a ballasted roof system to resist the destructive
powers of the wind. This led to a number
of wind performance issues toward the
end of the ’70s and into the early ’80s. This,
in turn, energized the industry to find the
answers for designing a ballasted system for
specific wind zones.
Extensive wind tunnel work was conducted
with thorough verification of the
modeling through field observations, all
leading to the development of the SPRI RP-4
national standard entitled “Wind Design
Standard for Ballasted Single-Ply Roofing
Systems.” This standard outlines design
procedures for ballasted systems for
addressing wind loads on various building
designs in locations across the country. This
standard has proven its merits with these
systems surviving major storm events,
including the hurricane season of 2004. The
development of this standard increased confidence
in the ballast system.
In recent years, new “high-tech” roofing
membranes offering highly reflective surfaces
have become the “new rage” of the
industry. These membranes are used in
fully-adhered and mechanically-fastened
roof systems to take advantage of the reflective
property of the membrane. With these
systems offering aesthetically pleasing roofs
that assist in saving energy for the building
owner, ballast systems now seem a little
old-fashioned and out of step with the
times. Is this truly the case, or are there
hidden attributes to the ballasted system
that have not been identified?
Georgia Tech Infrared Study
The paper titled “Georgia State University
Roof Temperature Study,” written by
Marty Waterfill, CSI, and Patrick Downey,
RRC, CDT, evaluated techniques to measure
roof surface temperatures for buildings
on the campus of Georgia State
University. They compared results from a
hand-held infrared thermometer to a highresolution
multispectral sensor mounted in
an aircraft that did flyovers of the buildings.
The roof types that were measured were
built-up and modified bitumen with different
surfacing, as well as ballasted EPDM.
The data in the paper were very limited, so
items such as surface reflectivity were not
supplied, except for the three modified bitumen
roofs that had their roof surface color
identified. Even so, Table 1 shows that the
ballasted systems had the lowest surface
temperature readings of the group of roofs.
This information added additional support
for the current study on ballast thermal
performance.
Membrane Reflectivity Versus Ballast
Thermal Mass
The river-washed stone used with the
ballasted system, when laid over a substrate,
produces a rather irregular surface
that scatters any reflected light in many
directions. Some light will reflect off one
stone only to strike other stones, leading to
multiple absorptions and low reflectivity.
Stone comes in many colors, from dark
browns and reds to bright white. These
stone types will produce reflectivity values
from below 0.20 to over 0.40; however, none
will qualify as an EnergyStar™ roof for lowslope
roof applications.
Pavers have flat surfaces that can be
finished to any surface smoothness and
color. Hence, there is an opportunity to produce
products with reflective values from
below 0.2 to well above 0.65, the value at
which a roofing product qualifies with
EnergyStar™. However, there is a penalty to
achieve this higher reflectivity, for it takes
additional manufacturing procedures to
produce the smooth or glazed surfaces,
greatly increasing the cost of the pavers.
The paver that was used in this study had
an initial reflectivity of 0.52, which is below
the EnergyStar™ threshold.
Ballast mass is a factor independent
from either surface color or finish. Ballast
with high thermal mass requires considerable
energy to raise its temperature, therefore
absorbing much of the sun’s energy
and shielding the building from it. The
unknown is just how effective ballast is in
shielding this energy when its mass is in
the 10- to 24-pounds-per-square-foot range
and comes in different forms, both stone
and paver. The stone, with its open structure,
has air cavities, while the paver is a
dense material. How do they affect thermal
performance in comparison to Energy-
Star™-listed reflective products?
FIELD TEST FACILITY
Roof Thermal Research Apparatus
The Roof Thermal Research Apparatus
(RTRA) at the Oak Ridge National Laboratory
located in Oak Ridge, Tennessee,
was constructed in the late 1980s for documenting
the effects of long-term exposure of
small, low-slope roof test sections to the
East Tennessee climate. The RTRA has four
4-ft. by 8-ft. openings in its roof to receive
different instrumented low-slope roof test
sections. Each 4-ft. by 8-ft. test section may
be divided into multiple areas.
The original use of the RTRA showed inservice
aging effects with CFC and alternative
blowing agents for polyisocyanurate
foam insulation boards in roofs covered by
black and white membranes. Each test section
was divided into two 4-ft. by 4-ft. areas,
one with a black membrane and the other
with a white membrane.
In the late 1990s, the RTRA was used to
document the thermal performance of lowslope
roofs coated with reflective coatings.
Each test section was divided into 2-ft. by 2-
ft. areas with as many as eight different
surfaces on a test section.
Currently, three of the four test sections
are being used for the ballast systems project.
Each test section is divided into two 4-
ft. by 4-ft. areas. One contains the ballast
systems for the 10-pound and 16.75-pound
tests. The second contains the 23.5-pound
tests, both stone and paver. The third contains
the control systems, one with a black
membrane and the other with a white membrane.
The fourth test section continues to
be used to show the in-service aging effects
for polyisocyanurate foam insulation
36 • I N T E R FA C E S E P T E M B E R 2005
Table 1. Roof surface temperatures from the
Georgia State University Roof Temperature
Study.
Roof Type Ave.Temp
CTP BUR 113
CTP BUR 115
CTP BUR 146
MB – white granules 138
ASP BUR 112
CTP BUR 136
CTP BUR 146
CTP BUR 137
CTP BUR 151
CTP BUR 123
MB – white granules 150
CTP BUR 132
EPDM Ballasted 117
EPDM Ballasted 111
EPDM Ballasted 124
MB – white granules 141
ASP BUR 118
EPDM black surface 152
ASP BUR 150
Roof Family Ave.Temp
CTP BUR 133
ASP BUR 126
MB Granules 143
EPDM Ballast 117
EPDM Black Surface 150
Note:
CTP = Coal Tar Pitch; ASP = Asphalt;
MB = Modified Bitumen multi ply
boards, now with third generation blowing
agents. Figure 1 is a photograph of the
RTRA that shows the entire building,
including the weather station.
A dedicated data acquisition system is
housed inside the RTRA. It acquires the
outside temperature and relative humidity,
wind speed, and direction ten feet above the
roof of the RTRA. The total horizontal solar
insolation and the total horizontal infrared
radiation are measured at the top of the
railing in Figure 1. There are also many dedicated
input channels for thermocouples
and for millivolt signals, such as those produced
by heat flux transducers. Jack panels
are conveniently located under the test
sections on the inside of the RTRA walls to
make for short lead wires from the test sections
to the jack panels. Data are acquired
under control of a database that is specific
to each experiment. The database instructs
the data acquisition program as to what
data to acquire and how often. Most channels
are polled every minute. Data are
stored in a compressed historical record.
For ongoing experiments, averages every 15
minutes of all variables are written weekly
to a spreadsheet. Special reports can be
generated for further detail on time dependency
down to the frequency in the historical
record.
Ballast Project
Test Sections
Figure 2 is a
photograph taken
on top of the RTRA
that shows the
three test sections
being used for the
ballast systems
project. The controls
are in the foreground
and the ballast
systems are in
the background
beyond an uninstrumented
area for
unmonitored exposure of materials.
To begin construction of the ballast systems,
pavers 2 inches thick and 2 feet
square were weighed on a scale to determine
their weight per unit area. It was 23.5
pounds per square foot. Three of the four
pavers required for a 4-ft.-square test section
were sawed in half in order not to have
any seams at the center of the paver test
section. A whole paver occupies the center,
and halves complete it. The required weight
of stone in the test area to achieve the same
loading as the pavers was determined for
the heaviest stone. The lightest stone ballast
loading was set at 10 pounds per
square foot, which is the minimum allowed
for a ballast system, and it did supply 100
percent coverage of the membrane. The
third paver was set at the average of the
heaviest and lightest. Buckets were used to
carry the #4 stone from the scale to the roof
of the RTRA, where it was distributed inside
frames to confine the ballast to its assigned
area. Exactly enough stone was used to
achieve the 10-, 16.75-, and 23.5-poundper-
square-foot loadings. Separate determinations
were made of the weight of stone to
S E P T E M B E R 2005 I N T E R FA C E • 3 7
Figure 1: Roof Thermal Research Apparatus (RTRA) with weather station.
Figure 2: Test sections configured for the ballast test.
exactly fill a bucket and the volume of the
bucket. This yielded a density of 92.4 lb/ft3
for the stone. Dividing each loading by the
density of the stone yielded average thicknesses
of 1.30, 2.18, and 3.05 in., respectively,
for the three stone ballast systems.
Due to the nature of the stone, the thicknesses
vary over the area of each stone test
section.
Instrumentation of the Test Sections
The instrumentation for each 4-ft. by 4-
ft. test section is shown in Figure 3. The
metal decks are exposed to the conditions
inside the RTRA, which is maintained year
round between 70°F and 75°F by an electric
resistance heater and a small, through-thewall
air conditioner. The membranes (in the
case of the unballasted controls), or the top
surfaces of the ballast, for the other test
sections, are exposed to climatic conditions.
Thermocouples on the decks and at the top
of the test sections monitor the direct
response to the imposed conditions.
Additional thermocouples are at the internal
interfaces. Wood fiberboard insulation
1.5 in.-thick is used to maximize sensitivity
to differences among the test sections. At
the interface between 1 in.-thick and 0.5
in.-thick pieces of insulation, a heat flux
transducer (HFT) is embedded in the top of
the thicker insulation board. Each HFT is
especially calibrated in the same configuration.
Thermocouples are deployed at the
level of each HFT, 6 in. and 12 in. from its
center to monitor any significant heat flow
in the horizontal direction. Thermocouples
at the other levels are 6 in. from the center
of the test section.
The irregular
upper surface of
the stone-ballasted
test sections
presents a
special challenge
for monitoring
surface temperature.
Figure 4
shows the scheme
that was adopted.
Aluminum
wire is strung
across the middle
of the frame
from side to
side in both
d i r e c t i o n s .
Thermocouples
are attached to
the wires with plastic wire ties. The lead
wire between each measuring junction and
its nearest wire tie is bent to hold the measuring
junction against a stone at the top of
each test section. At the top of the paverballasted
test section, a shallow hole was
drilled into the top of the central paver,
about 6 inches from its center, and the thermocouple
was epoxied in place with its
measuring junction touching the bottom of
the hole.
A property of primary interest for modeling
thermal performance of roofs is the
solar reflectance of the roof surface. It was
measured for the surfaces of the test sections
with two different techniques. For the
smooth-surfaced controls and the relatively
smooth-surfaced pavers, a Devices &
Services solar spectrum reflectometer was
taken onto the RTRA and used according to
ASTM C 1549-02, “Standard Test Method
for Determination of Solar Reflectance Near
Ambient Temperature Using a Portable
Solar Reflectometer.”
Solar reflectance for the white TPO
membrane at five locations on its surface
averaged 0.779 for measurements on
3/12/2004 and decreased to 0.666 on
9/27/2004. For the black EPDM membrane,
the average solar reflectance at five
locations was 0.060 on 3/12/2004 and
0.090 on 9/27/2004. Measurements on the
central full-sized paver yielded 0.516 on
3/12/2004 and 0.553 on 9/27/2004.
Seven locations were measured in March
and five in September on the paver.
For the stone-covered test sections, a
Davis Energy Group roof surface albedometer
was taken onto the RTRA and used with
guidance from ASTM E 1918-97, Standard
Test Method for Measuring Solar Reflectance
of Horizontal and Low-Sloped Surfaces
in the Field. The albedometer measures
the solar reflectance of a surface as the ratio
of the output of a solar spectrum pyranometer
when inverted (facing downward
toward the surface) and facing upward during
an interval of constant solar irradiance.
The area of the ballasted test sections is
only 4 ft. by 4 ft., not the 4 m. by 4 m. (13
Figure 3: Thermocouple and heat flux transducer placement relative to
the center of each 4-ft. by 4-ft. test section.
Figure 4: Thermocouple measuring junctions placed against pieces of stone at the top of the
stone-ballasted test sections.
38 • I N T E R FA C E S E P T E M B E R 2005
ft. by 13 ft.) recommended in E-1918 for use
of the instrument. In order to minimize the
effect of shadows from the assembly on the
test section during use of the albedometer,
a standard 50 cm. (20 in.) height of the sensor
above test sections is specified. It is
achieved by the support stand that is part
of the assembly.
Because of the relatively small size of
the ballasted test sections, the standard
height was relaxed. A special guide was
made to achieve heights of 10 in., 15 in.,
and 20 in. above the surfaces while manually
holding and leveling the pyranometer
and its support arm long enough for a
steady response from the millivolt meter
that monitors the output of the pyranometer.
Apparent solar reflectance was measured
at these three heights. Shape factor
algebra yielded the fraction of the pyranometer’s
view taken up by the stone. The
remainder is surroundings at some constant
but unknown reflectance. The reflectance
of the surroundings was varied by
trial and error until the reflectance of the
stone was constant with the height of the
pyranometer above the surface. It was concluded
that the solar reflectance for the
stone ballast is 0.21 ± 0.01. Within the precision,
it is the same value obtained during
the SPRI study and indicates that the
reflectance of the stone is constant.
Precision better than ±0.01 would require
an improved apparatus for measuring
reflectance at small heights.
A property of secondary interest for
modeling thermal performance of roofs is
the infrared emittance of the roof surface. It
is difficult to measure for thermally massive
systems, especially the irregular surfaces of
the stone-ballasted systems. In general,
non-metallic surfaces have infrared emittance
near 0.9. This value is assumed to
apply to all the test sections in the ballast
system study and has been verified often in
the SPRI study for single-ply white and
black membranes.
To model the thermal performance of
the ballasted systems with available tools,
thermal conductivity and volumetric heat
capacity (product of density and specific
heat) of the ballast are needed for use in the
transient heat conduction equation.
Preliminary work has been done with a program
that does the inverse: it uses the transient
heat conduction equation to predict
thermal properties to fit the measurements
of heat flux and temperature. The program
had difficulty converging with the data for
the 10-pound and 16.75-pound ballasts
during the summer months when convection
effects in the stone could be expected.
Early on in the project and now again after
nine months, analysis with the program is
showing some hope of predicting thermal
properties consistently from week to week.
Best estimates so far put the thermal
conductivity of the stone at 0.3 to 0.4
BTU/(h•ft•˚F) and volumetric heat capacity
at 19 to 21 BTU /(ft3•˚F). The corresponding
estimates for the paver are 1.45 to 1.65
BTU /(h•ft•˚F) and 23 to 25 BTU /(ft3•˚F).
With the measured thicknesses of the stone
and paver, these thermal conductivities
yield R-values of 0.3 to 0.4 h•ft2•˚F/ BTU for
the 10-pound ballast, 0.5 to 0.6 for the
16.75-pound ballast, 0.6 to 0.8 for the 23.5-
pound ballast, and 0.10 to 0.11 for the
23.5-pound paver. The ballasts form low Rvalue,
high thermal mass systems.
Until a consistent picture emerges of the
thermal properties, no work can be done
with modeling the thermal performance of
the systems. Modeling will use the thermal
properties to predict the heat flux through
the fiberboard insulation in each test section.
Comparison to the measured heat flux
will validate the model or, if agreement is
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40 • I N T E R FA C E S E P T E M B E R 2005
affected consistently by convection effects
in the thin stones, calibrate the model. A
validated or calibrated model permits prediction
of thermal performance in different
locations with roofs having typical insulation
R-value. The test roofs had minimal
insulation R-value in order to maximize the
sensitivity of the measurements to differences
in the ballast properties.
EXPERIMENT RESULTS
The ballast study went live on March 12,
2004 with the start of the data collection
that continued through 36 weeks at the
point of this writing. Figure 5 shows the
week results for the average heat flux either
into the building (positive) or out of the
building (negative) for a 24-hour period. The
three distinct assemblies: black-surfaced
membrane, ballast, and the white-surfaced
membrane are visible in the figure. As the
study moved into the summer period, the
ballasted configurations began to show
some separation as the heavier systems
provided better shielding of the building
from the heat. As the study moved into the
fall, the white assembly began to move closer
to the ballasted systems because of the
deterioration of its reflectivity due to aging;
however, its reflective value of 0.67 is still
above the EnergyStar™-minimum requirement
of 0.65. As the assemblies move into
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Figure 5:
Average
weekly
heat flux
through the
insulation
under the
ballast and
the control
membranes
through
week 36 of
the project.
S E P T E M B E R 2005 I N T E R FA C E • 4 1
Average Weekly Heat Flux, BTU (h•ft2)
Right: Figure
6: Average
weekly heat
flux (daytime
only) through
the insulation
under the
ballast and
the control
membranes
through week
36 of the
project.
NOTE: Preliminary results can show the observed differences among the test
sections with some preliminary conclusions. In addition to the behavior on
sunny days already presented to SPRI by André Desjarlais, which shows peak
shaving and peak shifting due to ballast, the weekly behavior of heat flux may
be of interest. Figures 5 and 6 show this so far in the project.
Average Weekly Daytime Heat Flux, BTU (h•ft2)
the first cold weather of the study, the thermal
curves all collapse together.
In Figure 6, the weekly heat flux averages
are shown for just the daylight hours,
the period when the white membrane is
reflecting the sun’s energy, reducing the air
conditioning load. As with Figure 5, this figure
shows the same distinct curves for the
assemblies with the black surface having
the greatest heat flux, the white the least,
and the ballast in the middle. There is
greater separation between the four ballast
assemblies in this scenario as the mass factor
has a great effect as the heat develops in
this part of the day. However, as the assemblies
move into summer, the 24-pound ballast
assemblies begin to match the white
membrane for heat flux and by fall has
equaled or bettered the white membrane. As
the first of the cold weather hits, the data
duplicate Figure 5 with all but the black
membrane collapsing together.
Figures 7 through 12 show the thermal
data collected for each assembly for a 24-
hour period. There is a set of two charts –
one membrane temperature and one for
heat flux – for a specific day in the spring,
summer, and fall. Figure 7 and 10 are
spring readings taken on April 5 when
everything is new. Starting at sunrise, the
membrane temperature climbs with the
white peaking first (85˚F) followed closely by
the 24-pound paver and stone assemblies,
which peak slightly higher at 90˚F. Next to
peak is the 17-pound ballast followed by the
10-pound ballast and then, at a considerably
higher temperature, the black membrane
at 145˚F. The chart shows the ballast
variables are close to the white membrane
in peak temperature reached, but offer one
unique property in that as the weight
increases, the time the peak temperature is
reached is delayed. This delay can be in the
range of three hours, pushing more of the
cooling load into the off-peak hours of the
day, saving both energy and dollars.
Figures 8 and 11 show the readings
taken during the summer period where the
24-pound paver and stone are now performing
basically equal to the white membrane
for peak temperature with the 17 and
10 pound ballast peaking just over it.
The fall readings shown in Figures 10
and 12 now show the 24-pound assemblies
42 • I N T E R FA C E S E P T E M B E R 2005
Figure 7: Membrane temperatures for a clear spring day in east
Tennessee.
Figure 8: Membrane temperatures for a clear summer day in east
Tennessee.
Figure 9: Membrane temperaturees for a clear fall day in east
Tennessee.
Figure 10: Heat fluxes through the insulation for a clear spring day
in east Tennessee.
Membrane Temperature, Degrees F Membrane Temperature, Degrees F
Heat flux through insulation, BTU/(h•ft2) Membrane Temperature, Degrees F
Test section Covering or loading Thickness (in.) Solar reflectivity
3/12/2004 9/27/2004
Black control Bare EPDM 0.045 0.06 0.09
White control Bare TPO 0.050 0.78 0.67
10# stone 10.0 lb/ft_ on EPDM 1.3 0.22 Not done
17# stone 16.75 lb/ft_ on EPDM 2.2 0.22 Not done
24# stone 23.5 lb/ft_ on EPDM 3.1 0.22 Not done
Paver 23.5 lb/ft_ on EPDM 2.0 0.52 0.55
peaking in temperature
first with the
white membrane
peaking at a higher
temperature. The
17-pound assembly
is peaking at a temperature
that is
basically the same
as the white. At the
fall reading, the
white membrane is
still above the
EnergyStar™ minimum
reflective value
of 0.65, indicating
that the ballast
systems do perform
as a cool roof.
An additional item
to note is the
reflectivity for the
24-pound paver is
0.51, while the 24-
pound of stone is
0.21, yet the thermal
curves fall pretty
much on top of
each other during
the daylight hours.
This indicates that
after a certain
weight, mass becomes
the controlling
factor instead of
reflectivity for
shielding the building.
Yet at other
times of the day, the
paver and ballast
thermal curves separate,
showing they are not the same and
making it more difficult to model the ballast
for use in the energy model calculators.
There is some indication that the ballast
may have two R-values, depending on
whether heat is moving into or out of the
building.
Figure 12. Heat fluxes through the insulation for a clear fall day in
east Tennessee.
Figure 11: Heat fluxes through the insulation for a clear summer
day in east Tennessee.
Table 2. Membrane reflectivity changes during the first six months of the project.
S E P T E M B E R 2005 I N T E R FA C E • 4 3
Heat flux through insulation, BTU/(h•ft2) Heat flux through insulation, BTU/(h•ft2)
Items to be Completed in the Study
The following items in the study are to
be completed:
1. Complete the data collection:
a. For one full year.
b. Through the second summer.
2. Model the stone characteristics for
use in the energy calculators:
a. Thermal conductivity.
b. Volumetric heat capacity (product
of density and specific heat).
3. Quantify the ballast performance
against the EnergyStar™ requirements.
4. Determine the value of the ballast
time delay for energy cost savings.
REFERENCES
Childs, P.W., T.W. Petrie, and J.A.
Atchley. “Comparison of Techniques
for in situ, Non-damaging Measurement
of Infrared Emittances of Lowslope
Roof Membranes.” Submitted
for review and publication in
Proceedings, Thermal Performance of
the Exterior Envelopes of Buildings
VIII Conference, Clearwater Beach,
Florida, December 2-6, 2001.
Incropera, F.P., D.P. DeWitt. Fundamentals
of Heat and Mass Treansfer.
3rd ed. John Wiley & Sons, New
York. 1990.
Miller, W.A., M.D. Chen, A. Pfiffner, and
N. Byars. “The Field Performance of
High-reflectance Single-ply Membranes
Exposed to Three Years of
Weathering in Various U.S. Climates.”
ORNL/TM-2002. Oak Ridge,
TN, Oak Ridge National Laboratory.
The Single-Ply Roofing Institute.
2002.
Petrie, T.W., A.O. Desjarlais, R.H. Robertson,
and D.S. Parker. “Comparison
of Techniques for in-situ, Nondamaging
Measurement of Solar
Reflectance of Low-slope Roof Membranes.”
Presented at the 14th Symposium
on Thermophysical Properties
and under review for publication
in International Journal of
Thermophysics, Boulder, CO: National
Institute of Standards and
Technology. 2000.
Reagab, J.A., D.M. Acklam. “Solar
Reflectivity of Common Building
Materials and its Influence on the
Roof Heat Gain of Typical
Southwestern US Residences.”
Energy Building, Vol. 2, 237. 1979.
Taha, H., D. Sailor, H. Akbari. High
Albedo Materials for Reducing
Building Cooling Energy Use, LBL-
31721, Lawrence Berkeley National
Laboratory, Berkeley, CA, 1992.
Waterfill, M. and P. Downey. “Georgia
State University Roof Temperature
Study.” Interface, March 1999. 10-13.
Wilkes, K.E. Model for Roof Thermal
Performance. ORNL/CON-274. Oak
Ridge, TN, Oak Ridge National
Laboratory. 1989.
44 • I N T E R FA C E S E P T E M B E R 2005
Richard Gillenwater, Carlisle SynTec Inc.’s manager of
advanced projects, began his career with Carlisle in 1979.
Gillenwater has held such positions as manager of technical
services, manager of systems engineering, manager of new
product development, and most recently, director of research
and development. Additionally, Gillenwater is active in the
Single Ply Roofing Industry (SPRI), EPDM Roofing Association
(ERA), and the Cool Roof Rating Council (CRRC).
Dick Gillenwater
Tom Petrie is a research engineer in the Buildings Envelope
Program in the Buildings Technology Center (BTC) at the Oak
Ridge National Laboratory (ORNL). His special expertise is
design, execution, and analysis of short-term experiments in
the BTC’s Large-Scale Climate Simulator on a variety of residential
and commercial roof assemblies. He carries out
longer-term experiments on the BTC’s outdoor test facilities
or by remote data acquisition from field sites. Petrie joined the
BTC staff in 1995, after research activities since 1988 with
the BTC while on the mechanical engineering faculty at Marquette University. He
earned a BSME from Marquette University in 1964, a MSME from the University of
Minnesota in 1967, and a Ph.D. from the University of Minnesota in 1969.
Dr.Tom Petrie
Dr. William A. Miller has 20 years of experience as a research
engineer for Oak Ridge National Laboratory. He has designed
and directed the setup of numerous refrigeration systems and
served as principal investigator of a contract awarded ORNL
by the Gas Research Institute. He earned his doctorate of philosophy
in mechanical engineering and is presently using his
expertise in finite difference heat conduction to quantify the
long-term energy and durability benefits of highly reflective
roofing materials in support to the Department of Energy’s
Roof, Wall, and Foundation program supported by the Office
of Building Technology, State and Community Programs.
Dr.William A. Miller
André Desjarlais is the Group Leader of the Building Envelope
and Materials Research programs at Oak Ridge National
Laboratory. Desjarlais has been involved in materials
research for over 30 years – first as a consultant, and for the
last 15 years with ORNL. He participates in ASHRAE, ASTM,
SPRI, RICOWI, the National Fenestration Rating Council, the
Building Environment and Thermal Envelope Committee, and
the RCI Foundation Board. His areas of expertise include
building envelope and material energy efficiency, moisture
control, and durability.
André Desjarlais