[BLDG-SIM] "Cool Roofing"

Jeff Haberl jhaberl at esl.tamu.edu
Tue Oct 8 08:13:18 PDT 2002

Here's my $0.05 worth.
First, FSEC, LBNL and others have done lots and lots of work on this...and
the "results" are very dependent on what is going on 
below the roof. For example, if there are ducts, what model is used, etc.
Information about what they have
done can be found in papers by Parker, and Fairey, etc.
Second, to be correct, one needs a simulation model that has radiation and
natural convection calculated 
correctly in the attic -- usually outside the domain of DOE-2, or powerDOE,
for example see the work by 
Mario Medina et al. at the University of Kansas.
Third, if one insists on using DOE-2 we have found that it makes a big
difference when one turns on the
CUSTOM-WEIGHTING-FACTORS by setting the floor weight equal to zero.  This,
of course needs
lots of other things, like a different floor model, and real materials,
etc., not for the faint-of-heart.
Finally, I have my doubts about DOE-2's radiative coupling to the sky, its
"rain" simulation, and its "dewpoint" assumptions,
based on some simulations that we have made where we compared the data from
measured attic temperatures from a Habitat House that
showed the simulation losing more heat than the real attic did --
interesting stuff that will be presented in a paper someday, report
available from the ESL soon.
Here's an annotated bibliography that we recently prepared for the
California Energy Commission on the topic.  
Akbari, H. (1998). Cool Roofs Save Energy. ASHRAE Transactions, Vol.104, Pt.
1B, pp. 783-788. <?xml:namespace prefix = o ns =
"urn:schemas-microsoft-com:office:office" />


This paper discusses about field data documenting the effect of white roofs
that has been found to reduce the air conditioning load in individual
buildings in California and Florida by between 10% and 50%, depending on the
thickness of insulation beneath the roof. In addition 'cool' roofs can limit
or reverse the urban heat island effect and can reduce low-level ozone
concentrations. The paper also presents simulated savings for several U.S.
metropolitan areas and briefly discusses policy and implementation issues
such as ratings and ASHRAE standards.


Akbari, H., Gartland, L. M., & Konopacki, S. J. (1998). Measured Energy
Savings of Light-colored Roofs: Results from Three California Demonstration
Sites. ACEEE 1998 Summer Study on Energy Efficiency in Buildings: Efficiency
& Sustainability, Vol. 3, pp. 3.1-3.12. 


This study demonstrated the impact of roof albedo in reducing cooling energy
use in three commercial buildings in California. Increasing the roof
reflectance from about 20% to 60% dropped the roof temperature on hot summer
afternoon by about 45°F. Savings are a function of both climate and the
amount of roof insulation. The cooling energy savings for reflective roofs
are highest in hot climates. A reflective roof may also lead to higher
heating energy use. Reflective coatings are also used in commercial building
to protect the roofing membrane. Reflectivity of coatings changes with
weathering and aging which have an effect on building cooling-energy


Akbari, H, & Konopacki, S. J. (1998). The Impact of Reflectivity and
Emissivity of Roofs on Building Cooling and Heating Energy Use. Proceedings
of the ASHRAE/DOE Conference on Thermal Performance of the Exterior
Envelopes of Buildings VII, Clearwater Beach, FL., pp. 29-39. 


This paper summarizes the result of computer simulations and analyses the
impact of roof albedo and emissivity on heating and cooling energy use. The
simulations are performed for eleven representative climates throughout the
country. Several residential and commercial prototypical buildings are
considered for these simulations. In hot climates, changing the roof
emissivity from 0.9 (emissivity of most nonmetallic surfaces) to 0.25
(emissivity of fresh and shiny metallic surfaces) can result in a net 10%
increase in annual utility bills. In colder climates, the heating energy
savings approximately cancel out the cooling energy penalties from
decreasing the roof emissivity. In very cold climates with no summertime
cooling, the heating energy savings resulting from decreasing the roof
emissivity can be up to 3%.


Akbari, H., Konopacki, S. J., Eley, C. N., Wilcox, B. A., Van Geem, M. G. &
Parker, D. S. (1998). Calculations for Reflective Roofs in Support of
Standard 90.1. ASHRAE Transactions, Vol. 104, Pt. 1B, pp. 976-987. 

This paper summarizes the results of a simulation effort in support of
ASHRAE SSPC 90.1 for the inclusion of reflective roofs in the proposed
standard. Simulation results include the annual electricity and fuel use for
two building types, residential and non-residential. The 90.1 Envelope
Subcommittee DOE-2 prototype building and operating schedules were used. The
parametric simulations were performed for 19 climate bins, as defined in the
current 90.1 draft; a range of roof absorptivities from 0.25 to 0.95; and
three roof U-factors (corresponding to roof insulation of R3, R11, and R38).
The results are condensed into climate-dependent adjustment factors to
reduce roof insulation for buildings with reflective roofs such that the net
energy use of the building stays constant when compared with the energy use
of a dark-colored roof.


Akbari, H., Konopacki, S. J.,  & Parker, D. S. (2000). Updates on Revision
to ASHRAE  Standard 90.2: Including Roof Reflectivity for Residential
Buildings. ACEEE 2000 Summer Study on Energy Efficiency in Buildings:
Efficiency & Sustainability, Vol. 1, pp. 1.1-1.11.


This paper discusses the results of a simulation effort in support of ASHRAE
SSPC 90.2 for inclusion of reflective roofs in the proposed standard.
Simulation results include the annual electricity and fuel use for a
prototypical single-family one-story house. The 90.2 Envelope Subcommittee
DOE-2 prototype building and operating schedules were used. The parametric
simulations were performed for the following scenarios and combinations
thereof: 3 heating systems, 4 duct and duct insulation configurations, 5
levels of roof reflectivity, and 4 levels of attic air change rate. The
simulations were performed for 32 climate regions. The results are condensed
into climate-dependent adjustment factors that equivalent reductions in roof
insulation levels corresponding to increased roof reflectivity. Results
indicate that in hot climates, increasing the roof reflectivity from 20% to
60% is worth over half of the roof insulation. 


Akbari, H., Levinson, R. & Berdahl, P. (1996). ASTM Standards for Measuring
Solar Reflectance and Infrared Emittance of Construction Materials and
Comparing their Steady-State Surface Temperatures. ACEEE 1996 Summer Study
on Energy Efficiency in Buildings: Efficiency & Sustainability, Vol. 1, pp.


This paper describes the technical issues relating to development of two
American Society for Testing & Materials (ASTM) standards, E 903 – Test
Method for Solar Absorptance, Reflectance, and Transmittance of Materials
Using Integrating Spheres, and E 408 – Test Methods for total Normal
Emittance of Surface Using Inspection-Meter Techniques. The study addresses
the measurement of the solar reflectance of the horizontal surfaces in the
field and translating the results into a comparative index. SRI is an
excellent predictor of relative surface temperature for materials with high
infrared emittance and is generally insensitive to variations in convection
coefficients, ambient temperature, and sky temperature.


Akbari,H., Taha, H., & Sailor, D. (1992). Measured Savings in Air
Conditioning from Shade Trees and White Surfaces. Proceeding of the ACEEE
1992 Summer Study on Energy Efficiency in Buildings: Efficiency &
Sustainability, Vol. 9, pp. 9.1-9.10.

This study discusses the measured saving in air-conditioning electricity use
which resulted from painting roofs white and planting shade trees for six
houses and a school bungalow in Sacramento, CA. Preliminary data indicate
the painting roof white of one of the houses eliminated air-conditioning
energy use about 12 kWh/day and 2.3 kW in peak power. Painting the roof and
one wall of a school bungalow white reduced its air-conditioning energy use
by over 50%. Shading the west windows, south windows, and air-conditioning
condenser units of two houses with trees appear to have lowered cooling
electricity use by 10 to 40%.


Akridge, J. M. (1998). High-albedo Roof Coating - Impact on Energy
Consumption. ASHRAE Transactions, Vol. 104, Pt. 1B, pp. 957-962. 


The paper addresses the recent tests conducted on a 12,000ft² single-story
building used as an educational center identified high roof temperatures as
a significant problem. The galvanized roof frequently reached temperatures
above 180°F. Considerable heat energy reached the non-ventilated attic,
resulting in temperatures as high as 105degF during the peak of summer.
Although the HVAC units were equipped with insulated return ducts, these
temperatures increased energy conduction through the duct insulation and
through the ceiling insulation into the conditioned space. The roof was
coated on March 28 and 29, 1995, with a high-albedo acrylic coating
developed to control thermal gain and rust. Tests show installation of the
thermal-control roof coating reduced the peak roof temperature to 120°F and
significantly decreased the energy flow through the roof and ceilings. Tests
show that the high-reflectivity roof coating reduced HVAC energy consumption
in a range from 8.7% to 27.5%, depending on the solar radiation and the
ambient temperature.


Anderson, R. W. (1989). Radiation Control Coatings: An Underutilized Energy
Conservation Technology for Buildings. ASHRAE Transactions, Vol. 95, Pt. 2,
pp. 682-685.


The paper points out that the application of radiation control coatings
(RCCs) to exterior roof and wall surfaces can effectively block solar heat
gains and significantly reduce cooling energy consumption and the sizing of
cooling equipment in warm climates. The application of RCCs remains
underutilized and is not yet recognized in building energy codes. The paper
discusses the status of RCC technology and the benefits to be gained from
the use of RCCs, including estimated reductions in cooling requirements and
energy consumption and suggests projects to demonstrate the full potential
of RCCs technology for building applications.
Bretz, S. E., & Akbari, H. (1994). Durability of High-Albedo Roof Coatings.
ACEEE 1994 Summer Study on Energy Efficiency in Buildings: Efficiency &
Sustainability, Vol. 9, pp. 9.65-9.75.

The study addresses the aging characteristics of high-albedo roofs.
Twenty-six spot albedo measurements of roofs were made using a calibrated
pyranometer. The decrease in albedo depends on the coating itself, the
texture of the surface, the slope of the roof and the nearby sources of dirt
and debris. The largest decrease in albedo occurs in the first year, at a
reduction of about 20%. After the second year, the incremental decrease in
albedo can be small, lowering saving estimates by 10-20%. Washing the
high-albedo coatings returned the albedo to 90-100% of the estimated
original value but it is not cost-effective if only concerned with
cooling-energy saving. 


Carlson, J. D., Christian, J. E., & Smith, T. L. (1992). In Situ Thermal
Performance of APP-modified Bitumen Roof Membranes Coated with Reflective
Coatings. Proceedings of the ASHRAE/DOE Conference on Thermal Performance of
the Exterior Envelopes of Buildings V, Clearwater Beach, FL., pp. 420-428.


A multi-faceted field research program regarding seven atactic polypropylene
(APP) modified bitumen membrane roof systems and four reflective coatings
began in 1991. This long-term project is evaluating the performance of
various APP-modified bitumen membranes (both coated and uncoated), the
comparative performance of coating application soon after membrane
installation versus pre-weathering, coating performance, and aspects of
recoating. Reports progress on the in-situ thermal performance of the
various types of coated membranes compared to the thermal performance of the
exposed membranes. The thermal performance of an adjacent ballasted ethylene
propylene diene terpolymer (EPDM) roofing system is also described.


Fawcett, S. L., Shull, P.D., & Smith, D. (1992). Application and Experience
in the Use of Aluminium Chips as a Reflective Surface for Commercial
Roofing. Proceedings of the ASHRAE/DOE Conference on Thermal Performance of
the Exterior Envelopes of Buildings V, Clearwater Beach, FL., pp. 417-419.


The paper contains the information of using thin aluminium chips as a
reflective surface for commercial roofing. Chips can be field or factory
applied. Solar reflectivity measurements indicate that approximately 70% of
the full solar spectrum is reflected. A brief review of the early roofing
projects indicates that retained reflectivity and extended roof life are
being achieved. The paper describes characteristics of such roofing
surfaces, how the product is applied to roofs, and the experience to date in
their application and performance.


Fisette, P. (1996). “Roofing and Siding Rehabs Get an Energy Fix”, Home
Energy Journal, November-December, pp.25-31.


This paper explains the idea of using thin aluminum chips as a reflective
surface for commercial roofing. The paper states about reflectivity of the
chips (approximately 70%) that more than 96% is still retained over 10
years. Advantages of roof temperature reduction, roof life as well as
characteristics of roofing surface, how the product is applied to roofing,
and the experience to date in their application and performance are also


Gartland, L. M., Konopacki, S. J., & and Akbari, H. (1996). Modeling the
Effects of Reflective Roofing. ACEEE 1996 Summer Study on Energy Efficiency
in Buildings: Efficiency & Sustainability, Vol. 4, pp. 4.117-4.124.


This paper describes a function that was written to incorporate the attic
heat transfer processes into the DOE-2 building energy simulation. This
function adds radiative, convective and conductive equations to the energy
balance of the roof. Results of the enhanced DOE-2 model were compared to
measured data collected from a school bungalow in a Sacramento Municipal
Utility District monitoring project. The function improves the accuracy of
DOE-2 in modeling the effects of high albedo roofing but still over-predicts
the daily energy use of both high and low albedo roofs. The yearly energy
savings of a white roof may be as much as four times higher than is
currently predicted by DOE-2.


Griggs, E. I., & Shipp, P. H. (1988). The Impact of Surface Reflectance on
Roofs: An Experimental Study. ASHRAE Transactions, Vol. 94, Pt. 2, pp.


The paper is the study about the thermal effects of black versus white
membranes on an insulated low slope roof over an 18 month period. White or
black polyisobutylene (PIB) membrane was used. Seasonal distinctions in the
measured data between black and white membranes are reported. Included are
cumulative and instantaneous heat fluxes and hourly surface temperature
variations. Peak membrane temperatures were observed to differ by up to 50
ºF during the day. Nighttime differences in membrane surface temperatures
were negligible. Changes due to dirt accumulation and local environmental
factors were observed in surface reflectance values calculated from the
energy balance at the roof membrane and from reflectometer measurements.


Hildebrandt, E. W., Bos, & W., Moore, R. (1998). Assessing the Impact of
White Roofs on Building Energy Loads. ASHRAE Transactions, Vol.104, Pt. 1B,
pp. 810-818. 


The study states the impact of white roof coatings on energy loads in three
non-residential buildings in Sacramento. Hourly metered loads were designed
to isolate the effects of white roof coatings on building cooling loads from
changes in cooling loads due to variations in outdoor temperatures. Basic
multiple linear regression model used to weather normalize energy
consumption data was expanded to include hourly solar radiation or
insolation levels as explanatory variables, along with explanatory variables
representing outdoor temperatures. Results indicate that the effect of solar
insolation levels on cooling energy consumption was significantly decreased
after the application of white roofs in all three buildings. Savings
estimates based on this approach range from 17% to 39% of total cooling
loads, or .35kWh to .68kWh per square foot of treated roof area per year.


Kochhar, G. S., Osborne, R. W. A., & Lewis, E. R. (1992). Enhancement of
Thermal Performance of Domestic Roofing System for Tropical Climes.
Proceedings of the ASHRAE/DOE Conference on Thermal Performance of the
Exterior Envelopes of Buildings V, Clearwater Beach, FL., pp. 429-439. 


This paper studies a series of side-by-side tests using model roof
assemblies have been conducted to determine the potential of radiant
barriers for enhancing the thermal microclimate of local domestic,
single-storey low-cost housing, for tropical climates. A comparative system
was adopted, using two identical test models, one an unchanging reference
and the other a test unit for examining the behavior of various radiant
barrier and ceiling configurations. The paper presents the design details of
the outdoor testing system used and the results of comparative testing of
aluminium foil and aluminium paint with regard to their effectiveness as
radiant barriers. Experimental results showed that the low-cost aluminium
paint, although not as effective as aluminium foil, does have an enhancing
effect on the thermal performance of the roof assembly system. 


Konopacki, S., & Parker, D. (1998). “Saving Energy with Reflective Roofs”,
Home Energy Journal, November-December, pp. 9-10.


This paper contains information of five final case studies out of ten case
studies that the Florida Solar Energy Center conducted in Florida during
midsummer over a period of four years. The studies measured the effect of
increasing the roof surface solar reflectance on air conditioning energy
use. The studies did not recommend painting or coating a conventional
shingle roof white because it can lead to potential moisture damage. In all
locations, reflective roofs reduced space-cooling varying from 13% to 58%.
Heating consumption was increased only slightly, from 3% to 6%.    


MacDonald, J. M., Courville, G. E., Griggs, E. I., & Sharp, T. R. (1989). A
Guide for Estimating Potential Energy Savings from Increased Solar
Reflectance of a Low-sloped Roof. Proceedings of the ASHRAE/DOE Conference
on Thermal Performance of the Exterior Envelopes of Buildings IV, Orlando,
FL., pp. 348-357. 


This paper describes the methodology and limitations of an easy-to-use guide
for calculating energy and cost saving resulting from a change in the solar
reflectance of a low-slope roof. The guide provides data and calculation
procedures for estimating energy and cost savings. In most instances, the
cooling cost savings associated with a change to a white roof surface (one
with higher solar reflectance) exceed the heating cost penalty. If the
difference between reduced cooling costs and increased heating costs is
significant, it can affect the choice of membrane for a new roof or a
re-roofed building. The guide helps the user estimate this energy cost
difference and also describes how various factors influence potential energy
savings and actual roof surface temperatures for different solar


Parker, D., & Barkaszi, S. (1994). “Saving Energy with Reflective Roof
Coatings”, Home Energy Journal, May-June, pp. 15-20. 


This paper contains information of six case studies out of ten case studies
that the Florida Solar Energy Center conducted in Florida during midsummer
over a period of four years. The studies measured the effect of increasing
the roof surface solar reflectance on air conditioning energy use and shows
that reflective roofs can reduce space-cooling energy consumption and demand
in Florida. The savings is about 10-40% that is around 440 to 1760 kWh per
year for household electricity use or an annual saving of $35-$140 at
current electricity rates (assuming 8 cents per kWh). The savings will vary
depending upon the severity of the cooling season and roof insulations. The
paper discusses the payback concern of reflective roofing which overall
application would cost about $1 per ft² or approximately $2,200 for a
typical home. With annual energy saving in Florida of $35-$140, the payback
times are long, usually, lasting longer than the roof itself.


Parker, D. S., Cummings, J. B., Sherwin, J. R., Stedman, T. C., & McIlvaine,
J. E. R. (1994). Measured Residential Cooling Energy Savings from Reflective
Roof Coatings in Florida. ASHRAE Transactions, Vol. 100, Pt. 2, pp. 36-49.


This study presents experiments about the impact of reflective coating on
air conditioning energy use that were applied to the roof of two residential
buildings in Cocoa Beach, Florida, in the summer of 1992. Site 1 with
approximately R-11 (RSI 1.9) ceiling insulation and Site 2 with a flat roof
with no insulation. Reflective coatings were applied to the roofs of both
residences in mid-summer. Analysis revealed substantial reductions in
space-cooling energy use in both homes. Air-conditioning energy use was
reduced by approximately 25% at Site 1. Utility coincident peak demand
between 5 and 6 p.m. was reduced by 28%. Cooling energy savings at the
uninsulated Site 2 home were approximately 43% and the coincident peak
reduction was 38%.


Parker, D. S., Huang, Y. J., Konopacki, S. J., Gartland, L. M., Sherwin, J.
R. & Gu, L. (1998). Measured and Simulated Performance of Reflective Roofing
Systems in Residential Buildings. ASHRAE Transactions, Vol.104, Pt. 1B, pp.


This paper contains information about a series of experiments in Florida
residences that have measured the impact of increasing roof solar
reflectance on space cooling. In tests on eleven homes with the roof colour
changed in mid-summer, the average cooling energy use was reduced by 19%.
Measurements and infrared thermography show that a significant part of the
savings is due to interactions when the duct system is located in the attic
space. An improved residential attic and duct simulation model, taking these
experimental results into account, has been implemented in the DOE-2.1E
building energy simulation program. The model was then used to estimate the
impact of reflective roofing in fourteen climatic locations around the
United States.


Parker, D. S., Sherwin, J. R., & Sonne, J. K. (1998). Measured Performance
of a Reflective Roofing System in a Florida Commercial Building. ASHRAE
Transactions, Vol.104, Pt. 1B, pp. 789-794.


The paper reports on the first results from tests on a reflective roofing
system on a commercial building in Florida. The building is an elementary
school with a sloped, modified bitumen roof. Air conditioning power was
measured in a base configuration prior to the roofing system being changed
to a white colour. Roof, decking, and plenum air temperatures were strongly
affected by the change to a white roof system. The school, which was
monitored for a full year in both the pre- and post-condition, saw the
measured annual chiller electric power reduced by 10%, or 13,000kWh/yr.
Cooling-load reductions during the utility summer peak were substantially
greater, more than 30% during the afternoon hours.

Petrie, T. W., Childs, P. W., &Christian, J. E. (1998). Radiation Control
Coatings Installed on Rough-surfaced Built-up Roofs: Initial Test Results.
ASHRAE Transactions, Vol. 95, Pt. 2, pp. 795-809.

This paper is a study on the solar reflectance and thermal performance of
small samples of various radiation control coatings on smooth surfaces that
have been tracked for several years on a roof test facility in East
Tennessee. The focus is on white coatings because of their potential to
weather, which causes the solar reflectance to decrease as the coatings age.
An extension of the study included more small samples on smooth surfaces and
entire rough-surfaced roofs at a federal facility in Florida. Two
rough-surfaced, moderately well insulated, low solar reflectance built-up
roofs (BURs) were spray-coated with a latex-based product including ceramic
beads. Only a small patch was left uncoated on each BUR to gather data
throughout the project on the performance with no coating for direct
comparison to data from instrumented coated areas. The average power demand
during occupied periods for the first month with the coating for the
building with the thermally massive roof deck was 13% less than during the
previous month without the coating. For the other building, with a
lightweight roof deck but high internal loads, there were no clear average
power savings due to the coating. 


Petrie, T. W., Childs, P. W., & Christian, J. E. (1998). Radiation Control
Coatings on Rough-surfaced Roofs at a Federal Facility: Two Summers of
Monitoring Plus Roof and Whole Building Modeling. Proceedings of the
ASHRAE/DOE Conference on Thermal Performance of the Exterior Envelopes of
Buildings VII, Clearwater Beach, FL., pp. 353-371. 


This paper updates and completes the presentation of data for this New
Technology Demonstration Programme (NTDP) project. The paper discusses the
effect of radiation control coatings on rough-surfaced, low-slope roofs at a
federal facility in the Panhandle of Florida. Two gravel-topped, moderately
well-insulated, low solar reflectance built-up roofs (BURs) were spray
coated with a white, latex-based product with ceramic beads. One roof was
significantly shaded and its building had high internal loads. The other had
a thermally massive deck but its building had little internal load.
Measurements show the history of coated and uncoated outside-surface
temperatures and solar reflectance of the roof surfaces from July 1996, when
the roofs were coated, through October 1997. Roof models based on
one-dimensional transient conduction through the roofs are used to compare
the heat fluxes through the roof deck for coated and uncoated roof surfaces.
DOE-2.1E whole building annual energy use predictions specific to the
buildings and their operating schedules show the effect of the coatings and
other building features for the climatic conditions of the Florida


Rosenfeld, A. H., Akbari, H., Taha, H., & Bretz, S. (1992). Implementation
of Light-Colored Surfaces: Profits for Utilities and Labels for Paints.
Proceeding of the ACEEE 1992 Summer Study on Energy Efficiency in Buildings:
Efficiency & Sustainability, Vol. 9, pp. 9.141-9.144. 


This study discusses about the problem of summer urban heat islands that
requires significant additional generating capacity and results in increased
pollutant emissions and higher energy bills. Urban areas can be lightened
through use of high-albedo materials for both building and urban surfaces.
The paper estimates that heat island reduction savings of $1 billion could
be realized through utility-sponsored demand-side management (DSM) programs
that promote the whitening and greening of cities. Assuming these utilities
are permitted to retain 10% of program savings, then they could earn about
$100 million/year.


8=!   8=)  :=)   8=)   ;=)   8=)   8=(   8=)   8=()   8=)   8=|   8=)   :=')
8=)   8=)   8=? 

Jeff S. Haberl, Ph.D., P.E.............................jhaberl at esl.tamu.edu 
Associate Professor....................................Office Ph:
Department of Architecture...........................Lab Ph: 979-845-6065 
Energy Systems Laboratory...........................FAX: 979-862-2457 
Texas A&M University..................................77843-3581 
College Station, Texas, USA...........................URL: www-esl.tamu.edu 

8=/   8=)  :=)   8=)  ;=)   8=)   8=()   8=)   :=)   8=)   8=!   8=)   8=?
8=)   8=)   8=0   


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