Elastomeric Roof Coatings
The Use of Elastomeric Acrylic Roof Coatings to Reduce
the Air Conditioning Load of Low-Slope-Roof Buildings
Abstract
In 1991, the Committee on Science, Engineering and
Public Policy of the National Academy of Sciences,
National Academy of Engineering and the Institute of
Medicine published a report titled Policy Implications
of Greenhouse Warming. One mitigation option in their
findings was the use of “white surfaces” “… to reduce
air conditioning use and the urban heat island effect by
25% through planting vegetation and painting roofs white
at 50% of U.S. residences.”
Dow Construction Chemicals has been investigating
the use of white acrylic roof coatings to reduce air
conditioning demand since 1981. Early “bird house”
experiments conducted by Rohm and Haas Company
(now a wholly owned subsidiary of The Dow Chemical
Company) clearly demonstrated that white elastomeric
acrylic coatings could reduce the internal temperature of
uninsulated and poorly ventilated buildings significantly.
Moreover, these coatings could prolong the life of a roof
by protecting the asphalt roofing material below from
degradation by heat, sunlight, water and thermal shock.
It became readily apparent that the degree of dirt pickup
resistance had a dramatic effect on the solar reflective
and, ultimately, the air conditioning energy demand.
Simply stated: The longer the roof coating retained its
white appearance, the better its effect in reducing the
building heat load.
To quantify this, a novel laboratory technique was
developed to predict the relative dirt pickup of typical
reflective coatings.
In cooperation with Mississippi Power Company and the
University of Southern Mississippi, we conducted a series
of comprehensive full-scale studies on actual buildings
to quantify the effect of these elastomeric coatings on
reducing air conditioning demand.
A secondary objective was to determine the “energy
penalty” associated with heating a thermally reflective
coated building versus a similar building covered with a
conventional black asphalt roof. The study showed that
the coating reduced the peak air conditioning energy
demand by 25%. The cost of coating and labor could be
amortized in approximately two years. The energy penalty
was minimal in this study.
The then-current (1994) laboratory specifications for
acrylic roof coatings and a technique for assessing
surface reflectivity and dirt pickup resistance are also
documented here.
TECHNICAL PAPER: FIELD STUDY
Excerpts from a presentation by William A. Kirn RRC
At the Cool Building and Paving Materials Conference/Workshop
July 11-12, 1994
Gaithersburg, MD
Early Reflectivity Experiments
In 1981, our researchers began investigating the potential
benefits for solar reflective coatings to increase the albedo
of the roofing membrane composite. Early “primitive”
experiments included the use of bird houses roofed with
asphalt shingles and coated with reflective elastomeric
acrylic roof coatings. Common meat thermometers
were inserted into the closed interior, and the inside
air temperature was measured as a function of solar
radiation. Similarly, although slightly more scientific, an
infrared thermometer was used to measure the surface
temperature of light and dark surfaced roofing materials.
As predicted, white reflective acrylic roof coatings greatly
reduced the surface temperatures of roofing membranes and
subsequently reduced the air temperature inside the bird
houses.
University of Southern Mississippi Study1
Based on the encouraging results of these rather simplistic
experiments, we participated in a cooperative research
project with the University of Southern Mississippi and
Mississippi Power Company to quantify the effects of
white reflective acrylic roof coatings when applied to actual
full-scale roofs. Three similar buildings were constructed
in the Hattiesburg campus of the University of Southern
Mississippi. Two buildings were of similar design using
construction techniques and insulation guidelines prevalent
in the 1970s. The third building was constructed using
revised and upgraded insulation guidelines from the 1980s,
consistent with the “Good Cents” program espoused by
the Mississippi Power Company. This program encouraged
the use of increased insulation in the foundation, walls
and ceiling, and installation of glass windows and airtight
weatherstripping.
The “Bird House Test”
Inside Thermometer
White model buildings were made with a
black roof and a white roof, then put out
in the sun to compare heat development.
Outside Conditions: 87ºF and Sunny
Inside Thermometer
Black Roof
White Roof
Outside Roof Temperature
135ºF
110ºF
Inside Temperature
128ºF
97ºF
Each building was individually heated and cooled with a heat
pump. Telemetry data were acquired for outside and inside
air temperature, solar radiation, time, inside and outside
relative humidity, wind velocity and watt-hours consumed.
Data were recorded every 15 minutes for one year – the
duration of the study. Inside air temperature was kept
constant at 24ºC. Our building (“RH-ERC”) and the “Control”
building were similarly constructed using minimal insulation.
The RH-ERC building was coated with a Rhoplex™ EC
acrylic-based elastomeric roof coating over the smooth
surface black asphalt built-up roof. The Mississippi Power
Company “Good Cents” building was heavily insulated, but
also was covered with a smooth surface black asphalt builtup roof.
continued on page 3
TECHNICAL PAPER: FIELD STUDY I
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continued from page 2
After one year of continuous monitoring, the Rhoplex™ EC elastomeric acrylic coated building had 21.9% lower energy
consumption in the summer compared to the control building. The white coating also reduced the energy demand by
3.99% in the winter.
It was originally theorized that black roofs would absorb heat in the winter, and an “energy penalty” in the winter would
be incurred if the roof was coated with a reflective material. The data from this study demonstrate this is not true.
The working hypothesis for this result was, since black bodies are more ideal energy radiators, heat absorbed during
the winter daylight would be more easily emitted during the nighttime. The financial payback for coating and labor
investment provides a payback period of 2.07 years and a return on investment of 48.3%. One additional benefit for
coating is the flexibility of application as the coating can be applied at any time during the life of the building.
The more heavily insulated “Good Cents” house had 29.8% lower energy demand in the summer and 42.1% lower
demand in the winter. The payback period for this investment was 3.05 years with a return on investment of 32.8%.
One serious limitation of the insulation option is it can only be incorporated during construction or major renovation,
unlike the more versatile reflective coating.
Building Specifications
Construction Material
Building 1 “Control”
Building 2 “RH-ERC”
Building 3 “Good Cents”
R11 4" Batt Fiberglass
R11 4" Batt Fiberglass
R30 10" Batt
Fiberglass
No Insulation
No Insulation
Perlite R2.85 1.5
Urethane R11.7
Windows
Single Glass Standard
Single Glass Standard
Double Glass Tight Fit
Doors
Wood Door Standard
Wood Door Standard
Metal Insulation
Weatherstrip
Foundation
No Insulation
No Insulation
1.5" Ins. R11.7
Perimeter
Roof System
Built-up
Built-up w/RHOPLEX™ EC
Coating
Built-up
Ceiling
Walls (Concrete Block)
TECHNICAL PAPER: FIELD STUDY
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Performance Criteria for
Acrylic Reflective Roof Coatings
High albedo roof coatings have demonstrated effectiveness
in reducing air conditioning demand and in lowering energy
costs. However, in order to function properly, these coatings
must demonstrate field performance in severe exposure
conditions. Most notably, roofs are structurally dynamic;
i.e., they expand and contract as a result of thermally
and seismically induced movement. Thus, roof coatings
must tolerate these movement dynamics over a wide
range of temperature extremes to function satisfactorily.
Additionally, they must adhere to the roofing substrate and
tolerate possible long-term contact with standing or ponded
water. This water may contain microorganisms, which can
adversely affect the coating, so mildew and algae resistance
are also required.
Most notable from a reflectivity standpoint is the need to
maintain high infrared and near infrared reflectivity, and
dirt pickup and mildew resistance. A reflective roof coating
may possess excellent initial albedo properties, but rapidly
become dirty and lose its ability to reflect infrared and near
infrared radiation. At the time of this study, two coating
specifications existed for elastomeric acrylic roof coatings:
the Roof Coatings Manufacturers Association #6 and the
Southern Florida Building Code Chapter 34 for acrylic
maintenance coatings. These specifications listed minimum
laboratory requirements necessary to achieve successful
field performance. Unfortunately, neither specification
included a test method for dirt pickup resistance, the key
property required for high albedo coatings. In 1994, a task
group drafted a specification for acrylic roof coatings in
D.08 (Roofing). But it did not include a test for dirt pickup
resistance. Also, no test method existed for measuring this
property (at that time).
In the absence of a specification, our researchers employed a
simplistic method for determining the dirt pickup resistance
of roof coatings. Simply stated: It involved the application
of a brown iron oxide pigment to the surface of the coating
(or other reflective membrane), wiping lightly with a cloth
and measuring the percent whiteness retained versus the
unsoiled area using a Gardner Colorgard II Reflectometer.
Details of this methodology are found in the Appendix.
A second missing requirement for reflective acrylic roof
coatings at the time of this study was a measure of the
coating’s solar reflectance by ASTM E903 or similar test.
At the time, properly formulated acrylic coatings using
titanium dioxide and zinc oxide pigments could achieve
83% reflectance and no standard existed for this property.
Specialty pigments had been identified, such as barium
sulfate, ceramic, borosilicate and polytetrafluoroethylene
spheres, which were touted as useful for increasing solar
reflectance of conventional coatings.2,3
TECHNICAL PAPER: FIELD STUDY
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Cost/Benefit of Acrylic Roof Coatings
Several comments made during the Proceedings
of the Workshop of Cool Building Materials in
February 1994 questioned the investment return for
coating a roof with a high albedo coating. This study
conclusively demonstrated the investment benefit
of reflective roof coatings over modestly insulated
structures in warm climates. More recent studies
have shown the benefit of these coatings decreases
as the ceiling or attic insulation is increased,
increasing the payback period. However, these
findings do not diminish the value of the coatings
as they perform their most important function: to
protect and prolong the life of the roof. The coatings’
ability to protect the roofing membrane from attack
by the harmful effects of UV radiation was well
documented even in 1994. Studies conducted by our
researchers and reported at the Third Symposium
on Roofing Research and Standards, sponsored by
ASTM, defined the fundamental durability-enhancing
mechanism of these coatings when applied over
conventional roofing substrates.4
The costs and benefits of elastomeric roof coatings
continue to be topics of study. While the energy
savings associated with these coatings may be
ancillary rather than primary, the coatings do help
extend roof life and greatly reduce life cycle costs of
the roofing composite. Today, there is a heightened
focus on “sustainable architecture,” which describes
buildings that can be easily maintained without
removing and replacing major components, such
as the roof. This approach is certainly more cost
effective for building owners and reduces the
amount of material going into landfills. A “cool
roof” clearly can help meet the goals of both “cool
buildings” and “sustainable architecture.”
Monthly Power Consumption of RH-ERC Building
Date
Control
Power
RH-ERC
Power
Power
Savings
BTU/sq.ft.
Savings
Cost
Saved ($)
Percent
Saved (%)
9-85
479.35
395.11
84.24
1,223
6.91
17.57
10-85
311.04
256.95
54.09
786
4.44
17.39
11-85
134.28
112.88
21.40
311
1.75
15.94
12-85
784.65
760.43
24.22
352
1.99
3.09
1-86
1,006.60
971.98
34.62
503
2.84
3.44
2-86
725.37
705.95
19.42
282
1.59
2.68
3-86
349.64
329.77
19.87
289
1.63
5.68
4-86
91.64
91.64
0.00
0
0.00
0.00
5-86
234.20
202.61
31.59
459
2.59
13.49
6-86
634.70
529.30
105.40
1,531
8.64
16.61
7-86
810.07
676.19
133.88
1,944
10.98
16.53
8-86
681.70
414.66
267.04
3,878
21.90
39.17
6,243.24
5,447.47
795.77
11,557
$65.25
Conclusion
Acrylic reflective roof coatings have proven beneficial in
increasing the albedo of buildings. Their cost effectiveness is
dependent on building design parameters and location, and the
ability of the coating to remain reflective. The coating’s ability to
function successfully is a composite of its initial reflectivity, dirt
pickup resistance, mildew resistance and its ability to maintain its
functional properties after years of in-field service.
TECHNICAL PAPER: FIELD STUDY
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Appendix
Accelerated Dirt Pickup Resistance Test
(Iron Oxide Method)
1 Clean the metal panels with MEK or acetone.
2 Roof coatings are cast at 40 wet mils (one coat) on
9" x 3" aluminum panels.
3 Coated panels are dried at ambient conditions three
days prior to testing.
Bibliography
Building for the Future, Boutwell, C.J., et al., 1987
1
”Technical Issues for the Development of Cool Building Materials,”
Cool Building Materials Workshop, Gaithersburg, MD, 1994, Berdahl, P.
2
Implementation of Solar-Reflective Surfaces: Materials and Utility
Program, Bretz, S., et al., 1992
3
“The Effects of Acrylic Maintenance Coatings on Reducing Weathering
Deterioration of Asphaltic Roofing Materials,” Roofing Research and
Standards Development: 3rd Volume, 1994, Antrum, R., et al.
4
4 Test panels are placed outside in the sunlight for
three days.
5 Allow test panels to equilibrate two hours in the CTR
(Constant Temperature Room) or (75°F/50% RH).
6 Iron oxide slurry* is brushed on one half of each panel,
then dried one hour at room temperature.
7
Panels are washed under running lukewarm tap water.
Use moderate pressure with a cheesecloth pad to
wipe off all excess iron oxide. Use a fresh cheesecloth
pad for each sample.
8 Allow panels to air dry two hours, then measure %
reflectance on tested and nontested portions using
Gardner Colorgard II Reflectometer** at 45° angle.
9 Report % reflectance retained with 45° head:
dirty side reading x 100% = % reflectance retained
clean side reading
*Brown Iron Oxide (Mapico 422, from Columbian Chemical Co.) slurry is
56% iron oxide mechanically mixed until smooth in DI water.
**Colorgard II Reflectometer
Gardner Neotec Division
Pacific Scientific
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