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 savings.
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. 1.1-1.9.
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 described.
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.
1626-1642.
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 reflectance.
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. 963-975.
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.
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 Panhandle.
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.
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Jeff S. Haberl, Ph.D.,
P.E.............................jhaberl@xxxxxxxxxxxx
Associate Professor....................................Office
Ph: 979-845-6507
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
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