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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. In order to maintain consistency with the other requirements of the draft standards, we used the 90.2 Envelope Sub-committee DOE-2 prototype building and operating schedules which were supplied to us. The parametric simulations were performed for the following scenarios and combinations thereof: 3 heating systems, 4 duct and duct insulation configurations, 5 levels of ceiling insulation, 4 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 give equivalent reductions in roof insulation levels corresponding to increased roof reflectivity. The equivalence is designed such that the net energy use (cooling plus heating) of the building stays constant when compared with energy use of a dark-colored roof. Results indicate that in hot climates, increasing the roof reflectivity from 20% to 60% is worth over half of the roof insulation.

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Updates on Revision to ASHRAE Standard 90.2:

Including Roof Reﬂectivity for Residential Buildings

H. Akbari, Lawrence Berkeley National Laboratory

S. Konopacki, Lawrence Berkeley National Laboratory

D. Parker, Florida Solar Energy Center

ABSTRACT

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. In

order to maintain consistency with the other requirements of the draft standards, we used

the 90.2 Envelope Sub-committee DOE-2 prototype building and operating schedules

which were supplied to us. The parametric simulations were performed for the following

scenarios and combinations thereof: 3 heating systems, 4 duct and duct insulation

conﬁgurations, 5 levels of ceiling insulation, 4 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 give

equivalent reductions in roof insulation levels corresponding to increased roof

reﬂectivity. The equivalence is designed such that the net energy use (cooling plus heat-

ing) of the building stays constant when compared with energy use of a dark-colored

roof. Results indicate that in hot climates, increasing the roof reﬂectivity from 20% to

60% is worth over half of the roof insulation.

Introduction

Most commercial and residential buildings have dark roofs. Dark roofs are heated

by the summer sun and this raises the summertime cooling demand. For highly absorp-

tive (low-solar reﬂectance) roofs, the difference between the surface and ambient air tem-

peratures may be as high as 50° C (90° F), while for less absorptive (high-solar

reﬂectance) roofs, such as white paint, the difference is only about 10° C (18° F). For this

reason, "cool" roofs (which absorb little "insolation") are effective in reducing cooling

energy use. Numerous experiments in several residential and small commercial build-

ings in California and Florida show that painting roofs white reduces air-conditioning

energy use (compressor and condenser unit) between 10 and 50% (ranging from $10 to

$100 per year per 100m2 ), depending on the amount of thermal resistance of insulation

under the roof (Akbari et al. 1997, Parker et al. 1998). The savings, of course, are strong

functions of the thermal integrity of a building and climate conditions.

The American Society of Heating, Refrigeration, and Air-conditioning Engineers

(ASHRAE) develops voluntary standards to improve energy efﬁciency in buildings. In

many applications, the voluntary ASHRAE standards are modiﬁed by states, federal, and

other governmental organizations and used as codes and standards. Two such standards

address energy efﬁciency in new buildings: ASHRAE Standard 90.1 (Standards for

Buildings Except Low-Rise Residential Buildings) and ASHRAE Standard 90.2

(Energy-Efﬁcient Design of New Low-Rise Residential Buildings). In 1998, Standard

90.1 adopted modiﬁcation to the existing standards (Akbari et al. 1998, ASHRAE 1999).

Prior to adoption of the standards for inclusion of reﬂective roofs, ASHRAE sponsored a

symposium to discuss present results from ﬁeld application and modeling (ASHRAE

1998). The Envelope Subcommittee of ASHRAE Standard 90.2 also recognized the

importance of roof reﬂectivity in residential buildings in reducing the net energy con-

sumption of a given building, and it organized a task group to develop a proposal to

modify the existing standards. In order to be consistent with other sections of the pro-

posed standards, the task group planned a detailed simulation approach to study the

impact of reﬂective roofs on heating and cooling energy use of of several prototypical

buildings over a wide range of climates. This paper summarizes the results of the simu-

lation effort in support of ASHRAE SSPC 90.2 for inclusion of reﬂective roofs in the

proposed standard.

Methodology

Reﬂective roofs reduce the ﬂow of heat into the building by reﬂecting most of the

incident solar radiation during hot summer days. Having a well-insulated roof will also

reduce the heat gains during the day. During those hours of the day when the ambient

temperature is lower than the inside temperature, having high insulation in the roof

would block the path of heat ﬂow out of the building. During the winter when the days

are short and cloudy and the sun angle is low, a reﬂective roof may add a heating penalty.

Therefore, we analyzed the impact of the roof reﬂectance in terms of a trade-off with roof

insulation. On that basis, the Envelope Subcommittee directed us to perform comprehen-

sive simulations to analyze cooling energy savings and heating energy penalties of

several prototypical buildings over a wide spectrum of climatic conditions. The DOE-

2.1E building energy simulation program was selected as the tool to perform this

analysis.

We used a residential building prototype that ASHRAE has used extensively in

support of developing criteria for Standard 90.2. The details of the prototypical building

are summarized in Table 1 . The building was simulated with electric cooling, electric

heat pump, electric resistance heating, and gas heating systems.

Our simulations included prototypes with and without attics. These building were

simulated for a variety of roof insulation and roof reﬂectances. The roof insulations

included ceiling insulation levels: R-1, R-11, R-19, R-30, R-49. Parametric for roof

reﬂectivity included reﬂectance of 0.10, 0.25, 0.50, and 0.75. In addition we modeled

distribution system conﬁgurations with ducts in the attics with three levels of duct insula-

tion (R-2, R-4, and R-6) and ducts in the conditioned space. For the prototypical build-

ings with an attic, a fractional leakage area of 1:300 for the attic was assumed.

The simulations were performed for a wide range of climatic conditions from very

hot to very cold. A total of 36 climates were considered for these simulations; weather

data for ﬁve of these locations were not available. Also, for the Los Angeles area, simu-

lations were performed for both LAX, and Long Beach. Hence, in total, the simulations

were performed for 32 climates. These climate conditions are shown in Table 2.

The locations of the distribution ducts have a signiﬁcant impact on the energy per-

formance of cooling systems. Leaky ducts in attics with a low level of duct insulation

can signiﬁcantly reduce the efﬁciency of duct systems. Jump and Modera (1994) have

measured the duct efﬁciency and reported a reduced efﬁciency of as much as 50% in

some residences. The higher the temperature of the attic, the higher the inefﬁciencies of

the duct systems. Parker et al. (1998) have developed a model to account for the impact

of attic temperature on the performance of the cooling systems. In our simulations, we

augmented DOE-2 with the algorithm developed by Parker et al.

Upon completion of simulated heating and cooling energy use, we regressed the

results into quadratic functions of roof absorptance (1 - reﬂectance), α , and u-value, U, of

the roof system. The equation used is:

Ei =C0 +C1 U +C2U2 +C3U α(1)

Where, Ei is either annual electricity use in kWh, annual gas energy use in therms, or net

energy use in $. To obtain the net energy-use cost, we used the 1998 national average of

$0.0826/kWh and $0.691/therm for the price of electricity and gas, respectively (EIA

1998). This linear correlation proved to be adequate for our analysis; the 95%

conﬁdence accuracy is 2%, the 98% accuracy is 3%.

We used these correlations to estimate the equivalency of the u-values and roof

absorptance. That is: given the energy use of a building with a dark roof (high absorp-

tance = α 1 ) and an overall u-value of U1, what will be the new overall u-value (U2 ) if the

roof had a higher reﬂectivity (α 2 <α 1 ), such that the annual energy use remains the

same? Applying this equivalency condition, the level of roof insulation requirements in

most hot cities could be reduced by a factor of 2.

To optimize the energy use of the building, Akbari et al. (1998) recommended

using a square root correlation of

(U1

U2 ) Recom = ( U 1

U2 ) 1/2

Equivalent (2)

where, Recom is the recommended value and Equivalent is the equivalency of the roof

absorptance and u-value obtained from the correlations.

Results

Table 3 shows the simulated annual energy expenditure for three climate regions:

Phoenix (hot and dry), Sacramento (moderate and dry), and Madison (cold). The results

are shown for heat pump, electric resistance, and gas heating systems. In Phoenix, for gas

heating systems, savings in the range of 6-17% (for various level of roof insulation) are

estimated by increasing the roof reﬂectance from 0.10 to 0.50, In Sacramento, the sav-

ings are in the range of 4-11%. Savings for Madison are nil. Since the price of gas per

unit of delivered energy is smaller than that of electricity, the savings are smaller for

electric heat pump and resistance heating.

The impact of roof reﬂectivity on the required level of roof insulation is shown in

Table 4. In hot climates, a signiﬁcant amount of roof insulation can be saved by increas-

ing the roof reﬂectivity. For example in Phoenix, a roof system with a reﬂectivity of

10% and ceiling insulation of R-30 has an equivalent annual energy performance of a

roof system with a reﬂectivity of 50% and ceiling insulation of R-14; over 50% savings

in required R-value of the insulation. Lower levels of insulation savings are observed in

moderate climates such as Sacramento. In cold climates, the saving in roof insulation is

obviously nil.

We performed a detailed sensitivity analysis in looking at the impact of variation

in duct R-value and attic leakage area fractions on the overall U2/U1. In general, in most

cases the impact was smaller than 10%. Hence, for the reminder of the analysis for the

prototypes with an attic, we assumed a duct insulation of R-4 and a leakage area fraction

of 1:300.

The results of the analysis for all climate regions are shown in Table 5 ,inan

ascending ratio of heating-degree-days (base 65F) over cooling-degree-days (base 65).

Our recommended U2/U1 values are signiﬁcantly lower than those obtained from the

correlations. Finally, we grouped the results into bins of similar modiﬁcation based on

the heating-degree-days.

ASHRAE Proposal

Based on this analysis, the ASHRAE Envelope Subcommittee of Standard 90.2

voted unanimously to adopt the following proposal as a modiﬁcation to SSP 90.2.

"Section 5.3.1.1: Single-Family Buildings (Ceiling with attics)

Exception to 5.3.1.1: For roofs where the exterior surface has either: a) a minimum total

solar reﬂectance of 0.65 when tested in accordance with ASTM E903 or E1918, and has a

minimum thermal emittance of 0.75 when tested in accordance with ASTM E408 or C1371;

or b) has a minimum solar reﬂectance index (SRI) of 75 calculated in accordance with

ASTM E1980 for medium wind-speed conditions, the U-value of the proposed ceiling shall

be permitted to be adjusted using Equation 5-3.1 for demonstrating compliance:

Uceiling_adj = Uceiling_proposed X Multiplier (5-3.1)

Where:

Uceiling_adj = the adjusted ceiling U-value for use in demonstrating compliance

Uceiling_proposed = the U-value of the proposed ceiling, as designed

Multiplier = the ceiling U-value multiplier from Table 5.3.1.

Section 5.3.1.2: Single-Family Buildings (Ceilings without attics)

Exception to 5.3.1.2: For roofs where the exterior surface has either: c) a minimum total

solar reﬂectance of 0.65 when tested in accordance with ASTM E903 or E1918, and has a

minimum thermal emittance of 0.75 when tested in accordance with ASTM E408 or C1371;

or d) has a minimum solar reﬂectance index (SRI) of 75 calculated in accordance with

ASTM E1980 for medium wind-speed conditions, the U-value of the proposed ceiling shall

be permitted to be adjusted using Equation 5-3.1 for demonstrating compliance:

Uceiling_adj = Uceiling_proposed X Multiplier (5-3.1)

Where:

Uceiling_adj = the adjusted ceiling U-value for use in demonstrating compliance

Uceiling_proposed = the U-value of the proposed ceiling, as designed

Multiplier = the ceiling U-value multiplier from Table 5.3.1.

Section 5.5.1.1: Multi-Family Buildings (Ceilings with attics)

Exception to 5.5.1.1: For roofs where the exterior surface has either: e) a minimum total

solar reﬂectance of 0.65 when tested in accordance with ASTM E903 or E1918, and has a

minimum thermal emittance of 0.75 when tested in accordance with ASTM E408 or C1371;

or f) has a minimum solar reﬂectance index (SRI) of 75 calculated in accordance with

ASTM E1980 for medium wind-speed conditions, the U-value of the proposed ceiling shall

be permitted to be adjusted using Equation 5-3.1 for demonstrating compliance:

Uceiling_adj = Uceiling_proposed X Multiplier (5-3.1)

Where:

Uceiling_adj = the adjusted ceiling U-value for use in demonstrating compliance

Uceiling_proposed = the U-value of the proposed ceiling, as designed

Multiplier = the ceiling U-value multiplier from Table 5.3.1.

Section 5.5.1.2: Multi-Family Buildings (Ceilings without attics)

Exception to 5.5.1.2: For roofs where the exterior surface has either: g) a minimum total

solar reﬂectance of 0.65 when tested in accordance with ASTM E903 or E1918, and has a

minimum thermal emittance of 0.75 when tested in accordance with ASTM E408 or C1371;

or h) has a minimum solar reﬂectance index (SRI) of 75 calculated in accordance with

ASTM E1980 for medium wind-speed conditions, the U-value of the proposed ceiling shall

be permitted to be adjusted using Equation 5-3.1 for demonstrating compliance:

Uceiling_adj = Uceiling_proposed X Multiplier (5-3.1)

Where:

Uceiling_adj = the adjusted ceiling U-value for use in demonstrating compliance

Uceiling_proposed = the U-value of the proposed ceiling, as designed

Multiplier = the ceiling U-value multiplier from Table 5.3.1.

Table 5.3.1. Ceiling U-value Multiplier

HDD 65 (HDD18) Ceilings with Attics Ceilings without Attics

0-360 (0-200) 1.50 1.30

361-900 (201-500) 1.30 1.30

901-1800 (501-1000) 1.20 1.30

1801-2700 (1001-1500) 1.15 1.30

2701-3600 (1501-2000) 1.10 1.20

> 3600 (> 2000) 1.00 1.00

Section 8.8.3.1: Exterior Absorptivity

Since the colors are subject to change over the life of the building, the exterior absorptivity

of all walls and roofs shall be 0.5 regardless of color, and the exterior absorptivity of roofs

shall be 0.2 regardless of color. If unconditioned spaces so as garages are not modeled,

walls between them and conditioned space shall be treated as exterior walls with an absorp-

tivity of zero.

Note: For low absorptivity roofs, the reference house may employ Exceptions 5.3.1.1 or

5.3.1.2 or 5.5.1.1 or 5.5.1.2."

Conclusion

In this study, we have documented the result of a building energy simulation

analysis to account for energy-saving beneﬁts of reﬂective roofs in residential buildings.

DOE-2 was used to calculate the annual electricity and fuel use for a prototypical single-

family one-story house in 32 climate regions. Parametric simulations were performed for

the following scenarios and combinations thereof: 3 heating systems, 4 duct and duct

insulation conﬁgurations, 5 levels of ceiling insulation, 4 levels of roof reﬂectivity, and 4

levels of attic air change rate.

The results are condensed into climate-dependent adjustment factors that give

equivalent reductions in roof insulation levels corresponding to increased roof

reﬂectivity. The equivalence is designed such that the net energy use (cooling plus heat-

ing) of the building remains constant when compared with energy use of a dark-colored

roof. Results indicate that in hot climates, by increasing the roof reﬂectivity from 20% to

60%, one can reduce the roof insulation by half and still have the same net annual energy

use.

Acknowledgement

This work was supported by the U.S. Environmental Protection Agency (EPA)

under IAG No. DW89938442-01-2 and by the Assistant Secretary for Energy Efﬁciency

and Renewable Energy, Building Technologies, of the U.S. Department of Energy (DOE)

under contract No. DE-AC03-76SF00098.

References

ASHRAE. 1999. "ASHRAE/IESNA Standard 90.1-1999: Energy Standard for Buildings

Except Low-Rise Residential Buildings," Page 20, American Society of Heating,

Refrigerating and Air Conditioning Engineers 1791 Tullie Circle, NE, Atlanta, Geo.

30329.

ASHRAE, 1998. "ASHRAE Technical Bulletin, Energy Savings of Reﬂective Roofs,"

American Society of Heating Refrigerating and Air-Conditioning Engineers,

Atlanta, Geo., Volume 14, Number 2, January.

Akbari, H., S. Konopacki, D. Parker, B. Wilcox, C. Eley, and M. Van Geem. "Calcula-

tions in Support of SSP90.1 for Reﬂective Roofs," ASHRAE Transactions, , 104(1),

pp. 984-995, January 1998.

Akbari, H., S. Bretz, H. Taha, D. Kurn, and J. Hanford. 1997. "Peak Power and Cooling

Energy Savings of High-albedo Roofs," Energy and Buildings , Vol. 25, No. 2, pp.

117-126.

EIA 1998. Energy Information Administration (EIA). 1998. http://www.eia.doe.gov.

Washington, DC.

Jump, D. and M. Modera. 1994. "Energy Impacts of Attic Duct Retroﬁts in Sacramento

Houses," Lawrence Berkeley National Laboratory Report LBNL-35375, Berkeley,

CA.

Parker, D., J. Huang, S. Konopacki, L. Gartland, J. Sherwin and L. Gu. 1998. "Measured

and Simulated Performance of Reﬂective Rooﬁng Systems in Residential Build-

ings". ASHRAE Transactions 104 (1):963-975.

Table 1. Prototypical construction, equipment, and operation characteristics for a single-

family one-story ranch house.

Construction

zones interior: conditioned

attic: unconditioned and naturally ventilated

ﬂoor area 1540ft2

perimeter 166ft

aspect ratio 1:1

wall height 8ft

roof 1/4" asphalt shingle over 3/4" plywood decking (4/12 slope)

solar absorptance: 0.90, 0.75, 0.50, or 0.25

infrared emittance: 0.9

overhang 2ft around entire perimeter

ceiling frame (15%) and R-1, 11, 19, 30, or 49 ﬁberglass insulation (85%)

over 1/2" drywall

exterior wall stucco over frame (15%) and R-11 ﬁberglass insulation over 1/2" drywall

windows 185ft2 (14% of exterior wall area) double clear with operable shades, U-

IP 0.57, and shading coefﬁcient 0.88

foundation slab-on-grade with carpet and pad

Equipment

sizing based on peak cooling and heating loads

sizing ratio 1.25

cooling direct expansion: SEER 10

heating (1) gas furnace: AFUE 78%

(2) electric heat pump: HSPF 6.8

(3) electric resistance

distribution constant-volume forced air system

10% duct leakage

duct insulation R-value: 2, 4, 6 (attic), or 0 (interior)

supply duct area 370ft2

return duct area 69ft2

Operation

cooling thermostat 78°F

heating thermostat 68°F

natural ventilation enthalphic controlled window operation: 68° F min and 5 ACH max

inﬁltration Sherman-Grimsrud: ﬂa 1:2000 (interior) and ﬂa 1:300 (attic)

peak internal heat gain 0.68 W/ft2

Table 2. Selected locations, TMY2 weather ﬁle availability and degree-days.

id location tmy2 weather ﬁle cdd 65 hdd 65

1 Adak, AK not available

2 Albuquerque, NM Albuquerque, NM 1211 4361

3 Brownsville, TX Brownsville, TX 3563 659

4 Bangor, ME not available

5 Bismarck, ND Bismarck, ND 408 8666

6 Bryce, UT not available

7 Charleston, SC Charleston, SC 2010 2209

8 Denver, CO Boulder, CO 623 6007

9 Dodge, KS Dodge City, KS 1371 5353

10 El Paso, TX El Paso, TX 2046 2597

11 Fort Worth, TX Fort Worth, TX 2415 2304

12 Fairbanks, AK Fairbanks, AK 29 14095

13 Fresno, CA Fresno, CA 1884 2602

14 Fort Smith, AR Fort Smith, AR 1895 3351

15 Honolulu, HI Honolulu, HI 4329 0

16 Jacksonville, FL Jacksonville, FL 2657 1437

17 Kwajalein, PI St.Paul Island, AK 0 11126

18 Lake Charles, LA Lake Charles, LA 2624 1683

19 Laredo, TX not available

20 Las Vegas, NV Las Vegas, NV 3067 2293

21 Los Angeles, CA LAX 470 1291

21 Los Angeles, CA Long Beach 943 1309

22 Miami, FL Miami, FL 4127 141

23 Madison, WI Madison, WI 521 7495

24 North Omaha, NE Omaha, NE 1051 6047

25 New York, NY New York, NY 1002 5090

26 Phoenix, AZ Phoenix, AZ 3815 1154

27 Redmond, OR Redmond, OR 194 6732

28 Roswell, NM not available

29 Tucson, AZ Tucson, AZ 2763 1554

30 Sacramento, CA Sacramento, CA 1144 2794

31 San Diego, CA San Diego, CA 766 1076

32 Seattle, WA Seattle, WA 127 4867

33 San Francisco, CA San Francisco, CA 69 3239

34 St. Louis, MO St. Louis, MO 1437 5021

35 Washington, DC Sterling, VA 1044 5233

36 Winnemucca, NV Winnemucca, NV 604 6444

Table 3.Simulated annual cooling and heating total energy base use [$/1000ft2 ] and the direct savings [%] from the use of high-

albedo roofs for a typical single-family one-story ranch house with gas heat, R-4 attic ducts and 1:300 attic fractional leakage area.

Phoenix Sacramento Madison

Ceiling insulation→ R-1 R-11 R-19 R-30 R-49 R-1 R-11 R-19 R-30 R-49 R-1 R-11 R-19 R-30 R-49

Gas furnace

base use,

=0.90 639 411 379 359 345 422 255 229 214 202 779 572 532 508 491

savings,

=0.75 6 4 3 3 242 2 2 1 00000

savings,

=0.50 17 10 8 7 6 11 6554 10000

savings,

=0.25 28 17 14 12 10 17 10877 1-1 -1 -10

Electric heat pump

base use,

=0.90 687 444 409 388 372 525 346 316 298 283 1166 997 953 925 904

savings,

=0.75 6 3 3 3 231 1 1 1 00000

savings,

=0.50 16 9 8 7 684 3 3 2 1-1-1-1-1

savings,

=0.25 26 15 13 11 10 12 6554 1-1 -1 -1 -1

Electric resistance

base use,

=0.90 804 513 468 442 423 856 557 504 473 449 1947 1522 1425 1367 1324

savings,

=0.75 5 3 2 2 211 0 0 0 00000

savings,

=0.50 13 8 6 5 532 1 1 1-1 -1 -1 -1 -1

savings,

=0.25 21 13 10 9 852 2 2 2-2 -2 -2 -2 -1

Table 4. Estimated roof-composite U_factor (U2/U1) and equivalent cool-roof ceiling insulation R-value (shown in parentheses) as

a function of roof solar absorptance (

) and ceiling insulation R-value. Dark roof absorptance is 0.90. All results for R-4 attic ducts

and 1:300 attic fractional leakage area.

Phoenix Sacramento Madison

ceiling insulation→ R-11 R-19 R-30 R-49 R-11 R-19 R-30 R-49 R-11 R-19 R-30 R-49

gas furnace

= 0.75 1.14 1.16 1.17 1.18 1.07 1.08 1.08 1.08 1.00 1.00 1.00 1.00

(9) (15) (23) (37) (10) (17) (26) (42) (11) (19) (30) (49)

= 0.50 1.48 1.54 1.59 1.64 1.21 1.23 1.24 1.25 1.01 1.01 1.01 1.01

(5) (9) (14) (21) (8) (14) (21) (33) (11) (19) (30) (48)

= 0.25 1.97 2.16 2.33 2.50 1.39 1.42 1.45 1.47 1.01 1.01 1.01 1.01

(2) (5) (7) (10) (6) (11) (16) (25) (11) (19) (29) (48)

electric heat pump

= 0.75 1.13 1.14 1.15 1.16 1.05 1.06 1.06 1.06 1.00 1.00 1.00 1.00

(9) (15) (24) (38) (10) (17) (27) (44) (11) (19) (30) (49)

= 0.50 1.42 1.47 1.51 1.55 1.16 1.17 1.17 1.17 1.00 1.00 1.00 1.00

(6) (10) (15) (23) (8) (15) (23) (37) (11) (19) (30) (49)

= 0.25 1.85 2.00 2.12 2.25 1.29 1.30 1.31 1.31 1.00 1.00 1.00 1.00

(3) (6) (8) (12) (7) (12) (19) (30) (11) (19) (30) (49)

electric resistance

= 0.75 1.10 1.11 1.11 1.12 1.02 1.02 1.02 1.02 0.98 0.99 0.99 0.99

(9) (16) (25) (40) (11) (18) (29) (47) (11) (19) (31) (50)

= 0.50 1.31 1.34 1.36 1.38 1.05 1.06 1.06 1.06 0.96 0.96 0.96 0.96

(7) (12) (18) (28) (10) (17) (27) (44) (12) (20) (32) (53)

= 0.25 1.60 1.67 1.73 1.79 1.09 1.09 1.10 1.10 0.94 0.94 0.94 0.94

(4) (8) (12) (18) (9) (16) (26) (41) (12) (21) (33) (55)

Table 5. Estimated roof-composite U_factor (U2/U1) for a reﬂective roof with a solar absorptance of 0.45. Dark roof absorptance is

0.90. All results for R-4 attic ducts and 1:300 attic fractional leakage area.

Duct in Attics Ducts in Conditioned Space Roofs with no Attics

Location cdd_65 hdd_65 hdd/cdd U2/U1 (U2/U1)1/2 Recomm U2/U1 (U2/U1)1/2 Recomm U2/U1 (U2/U1)1/2 Recomm

Honolulu, HI 4329 0 0.00 2.62 1.62 1.5 2.62 1.62 1.5 1.69 1.30 1.3

Miami, FL 4127 141 0.03 2.19 1.48 1.5 2.19 1.48 1.5 1.67 1.29 1.3

Brownsville, TX 3563 659 0.18 1.6 1.26 1.25 1.6 1.26 1.25 1.61 1.27 1.3

Phoenix, AZ 3815 1154 0.30 1.53 1.24 1.2 1.53 1.24 1.2 2.39 1.55 1.3

Jacksonville, FL 2657 1437 0.54 1.52 1.23 1.2 1.52 1.23 1.2 1.59 1.26 1.3

Tucson, AZ 2763 1554 0.56 1.49 1.22 1.2 1.49 1.22 1.2 2.76 1.66 1.3

Lake Charles, LA 2624 1683 0.64 1.46 1.21 1.2 1.46 1.21 1.2 2.02 1.42 1.3

El Paso, TX 2046 2597 1.27 1.42 1.19 1.15 1.42 1.19 1.15 1.68 1.30 1.3

Los Angeles, CA 943 1309 1.39 1.38 1.17 1.2 1.38 1.17 1.2 1.64 1.28 1.3

San Diego, CA 766 1076 1.40 1.37 1.17 1.2 1.37 1.17 1.2 1.69 1.30 1.3

Las Veg as, NV 3067 2293 0.75 1.37 1.17 1.15 1.37 1.17 1.15 1.65 1.28 1.3

Fresno, CA 1884 2602 1.38 1.34 1.16 1.15 1.34 1.16 1.15 1.56 1.25 1.3

Charleston, SC 2010 2209 1.10 1.33 1.15 1.15 1.33 1.15 1.15 1.58 1.26 1.3

Fort Worth, TX 2415 2304 0.95 1.31 1.14 1.15 1.31 1.14 1.15 1.64 1.28 1.3

Fort Smith, AR 1895 3351 1.77 1.24 1.11 1.1 1.24 1.11 1.1 1.63 1.28 1.3

Sacramento, CA 1144 2794 2.44 1.22 1.10 1.1 1.22 1.10 1.1 1.61 1.27 1.2

Albuquerque, NM 1211 4361 3.60 1.19 1.09 1 1.19 1.09 1 1.4 1.18 1.2

Los_Angeles, CA 470 1291 2.75 1.16 1.08 1.2 1.16 1.08 1.2 1.55 1.24 1.2

St. Louis, MO 1437 5021 3.49 1.11 1.05 1 1.11 1.05 1 1.5 1.22 1.2

Washington, DC 1044 5233 5.01 1.09 1.04 1 1.09 1.04 1 1.23 1.11 1

Dodge, KS 1371 5353 3.90 1.09 1.04 1 1.09 1.04 1 1.34 1.16 1

North Omaha, NE 1051 6047 5.75 1.08 1.04 1 1.08 1.04 1 1.27 1.13 1

Denver, CO 623 6007 9.64 1.06 1.03 1 1.06 1.03 1 1.33 1.15 1

Winnemucca, NV 604 6444 10.67 1.06 1.03 1 1.06 1.03 1 1.28 1.13 1

New York, NY 1002 5090 5.08 1.05 1.02 1 1.05 1.02 1 1.28 1.13 1

Bismarck, ND 408 8666 21.24 1.02 1.01 1 1.02 1.01 1 1.25 1.12 1

Redmond, OR 194 6732 34.70 1.01 1.00 1 1.01 1.00 1 1.26 1.12 1

Madison, WI 521 7495 14.39 1.01 1.00 1 1.01 1.00 1 1.18 1.09 1

Seattle, WA 127 4867 38.32 0.97 0.98 1 0.97 0.98 1 1.12 1.06 1

Fairbanks, AK 29 14095 486.03 0.97 0.98 1 0.97 0.98 1 1.09 1.04 1

Kwajalein, PI 0 11126 ∞ 0.95 0.97 1 0.95 0.97 1 0.93 0.96 1

San Francisco, CA 69 3239 46.94 0.94 0.97 1 0.94 0.97 1 0.98 0.99 1

... Using reflective materials or coatings with high solar reflectivity in cool roofs can result in reducing the amount of insulation compared to black roofs. Akbari et al. (1999Akbari et al. ( , 2000 investigated the effect of rooftop solar reflectivity on the cooling and heating energy loads for residential and commercial buildings in the United States. Their results for onestory single family building showed that about half the amount of insulation can be saved as a result of increasing the rooftop solar reflectivity from 20 to 60% (Akbari et al., 2000). ...

... Akbari et al. (1999Akbari et al. ( , 2000 investigated the effect of rooftop solar reflectivity on the cooling and heating energy loads for residential and commercial buildings in the United States. Their results for onestory single family building showed that about half the amount of insulation can be saved as a result of increasing the rooftop solar reflectivity from 20 to 60% (Akbari et al., 2000). In another study by Ray and Glicksman (Ray and Glicksman, 2010) for onestory building having a modified-bitumen roof with insulation thermal resistance of 2.7 m 2 K/W (15.3 ft 2 F h/BTU) when this building was subjected to the weather conditions of Boston, 13% in energy use can be saved by doubling the amount of insulation, whereas 12% in the energy use can be saved by replacing the modified-bitumen roof by a green roof. ...

In regions with hot climatic conditions such as that in Saudi Arabia, a substantial share of energy is used for cooling the buildings. Many studies have shown that cool (white) roofs can help reduce the cooling energy load and thus the demand for energy over time. Also, cool roofs help reduce the urban heat island during the summer time. This research study focused on determining: (a) whether cool roofs lead to risk of condensation and mold growth in Saudi climates, (b) the amount of energy savings as result of using cool roofs instead of black roofs of same insulation amount, and (c) the reduction in the amount of insulation in cool roof having the same energy performance level as the black roof. As such, numerical simulations were conducted for a roofing system that is commonly used in low-rise buildings in Saudi Arabia in order to asses and compare the energy and hygrothermal performance of cool and black roofs. The roof was subjected to weather conditions of the Eastern Province of Saudi Arabia. The indoor conditions were taken based simple method of ASHRAE Standard 160. The results showed no moisture accumulation occurred from year-to-year after 6 years and 7 years for the black roof and cool roof, respectively, and the highest relative humidities in the black and cool roofs were well below 80% resulting in no risk of condensation and mold growth occurred in these roofs. The main outcome of this study has shown the capabilities of using reflective materials with different short-wave solar absorption coefficients for enhancing the energy performance of roofs and/or reducing the amount of insulation that resulted in same energy performance as black roofs. This study can be used in future for upgrading the Saudi Building Code so as to allow less roof insulation if cool roof is installed.

... Ref. [59] have shown in their study on a single-family one-storey building that by increasing the reflectivity of the roof from 20% to 60% is equivalent to a 50% reduction of the roof insulation thickness in hot climates. This comes to support the introduction of the reflective roof materials in the proposed ASHRAE SSPC 90.2. ...

Fadye Al Fayad
Wahid Maref
Mohamed M. Awad

This article performed a comprehensive review of the different state-of-the-art of roofing technologies and roofing materials and their impact on the urban heat island (UHI) and energy consumption of buildings. The building roofs are the main sources of undesirable heat for buildings, especially in warm climates. This paper discusses the use and application of white roofing material in emerging economies. The use of white roofing material is a suggestion because of its cooling, evaporative and efficiency characteristics compared to traditional black roofing materials. Many research studies have shown that the darker roofing surfaces that are prevalent in many urban areas actually can increase temperature by 1 to 3 degrees Celsius to the environment surrounding these urban areas. Additionally, improved temperature control and heat reflection also work to reduce the energy requirements for the interior spaces of the structures that have white roofing surfaces. The white or lighter colored roofs tend to reflect a part of the solar radiation that strikes the roof's surface. Consequently, one might believe that white roofing material would be commonplace and especially so within emerging economies. Yet, this is hardly the case at all. This paper examines the issue of white roofing materials in emerging economies from a dual perspective. The dual perspective includes the technical details of white roofing material and its impact on lowering the interior temperature of the affected structures, which consequently reduces hours of indoor thermal discomfort and use of air conditioners in indoor spaces. The other element in this study, however, involves the marketing aspect of white roofing material. This includes its adoption, acceptance and cost-benefit in emerging economies.

... When a building with modified-bitumen roof was subjected to the climates of Boston, Ray and Glicksman (2010) showed that it was possible to save 13% of energy by doubling the amount of insulation, however, saving 12% of energy was possible by replacing this roof by a green roof. For white roofing systems, Akbari et al. (1999Akbari et al. ( , 2000 investigated the effect of the short-wave solar absorption coefficient (α s ) of the rooftops on the heating and cooling energy loads of commercial and residential buildings in the US. Their results showed that about half the amount of the insulation can be saved by decreasing α s from 0.8 to 0.4 for a onestory single-family building. ...

Cool/white roofing systems use paints and membranes with high solar reflectivity to reflect a portion of the incident solar radiation resulting in lowering the temperature of the exterior surfaces in respect of the conventional/black roofing systems. This study focuses on the energy performance of roofing system that is being used in buildings of the Gulf Cooperation Council countries when this roof is exposed to a hot and humid climatic conditions in Saudi Arabia. The long-term moisture performance of the white and black roofing systems were investigated in a previous study in which the results showed that no risk of condensation and mold growth occurred in the roofs with different values of solar reflectivity of the rooftop and different initial construction moisture. With the same environmental conditions that were used in the previous study, the focus of this paper is on assessing the energy performance of white and black roofing systems for a wide range of: (a) thermal insulation thickness, and (b) solar reflectivity of the rooftop. Also, considerations are given in this study to develop a practical design tool that can easily be used by building engineers and architects for determining all pairs of the insulation thickness and the corresponding solar reflectivity of the reflective roofing materials/coatings that resulted in the same levels of the energy performance as those for the black roofing systems of thicker insulation thickness. The results of this study along with the developed practical design tool can be used in future to upgrade the Saudi building code to allow using less insulation in the roofs if white roofing systems are installed.

... Varios autores afirman, que el uso de la alta reflectividad tiene la misma influencia sobre la reducción de las temperaturas interiores, que el uso de materiales aislantes en este elemento [H. Akbari, S. Konopacki, 2000][ANSI/ . ...

Jefferson Torres-Quezada

The central scope of this thesis is the architectonic element THE ROOF within the context of regions with warm and humid climates near the equator. The constant population growth in the world has been reflected, in these regions, with a high horizontal expansion of the city, where the low height buildings are the most extended urban typology. This fact added to the high and constant solar radiation determines that the roof is the main source of heat gains inside the residential buildings. In the last decades, the Coast Region of Ecuador has experienced a significant change in the use of roofing materials, with a general tendency to increase heavy concrete roofs to the detriment of light metal roofs. There is a general belief that concrete roofs, given their material characteristics, offer better environmental conditions than metal roofs, and consequently a lower energy demand. Therefore, the increased use of heavy concrete roofs "seems to have" a thermal purpose. However, these covers have a higher investment cost for the user, and in addition, and very important, they have a higher embodied energy. Based on the above, the general objective of this research is to analyse and compare the thermal behaviour of these two covers, through measurements and simulations, in order to answer the question: WHAT ROOF TYPOLOGY OFFERS A LOWER OVERHEATING OF THE INTERIOR SPACE IN THE CLIMATE OF THIS REGION, THE LIGHT METAL ROOF OR THE HEAVY CONCRETE ROOF? After the measurements of the climatic conditions of this region made in situ, it has been determined that the CLOUDINESS plays a fundamental role in the amount and type of solar radiation received in the roofs, as well as in the cooling capacity of the sky. Consequently, the cloudiness is the climatic factor that has the greatest influence on the thermal performance of the roofs in this region. The consideration of this aspect in the energy analysis of the roofs has resulted in variables that in other climates have a high impact on the roof thermal behaviour, such as inclination, orientation or thermal resistance, can be cancelled in this climate. The interior surface temperature of the light metal roof drops more than that of the heavy one during the night period but, despite what is commonly believed, during the day the interior temperature of the metal roof maintains a thermal performance almost equal to the concrete roof, given the increase of reflectivity to the visible and in particular to the increase of thermal infrared emissivity contributed by the use of suitable paints. Therefore a light metal roof with values of thermal infrared emissivity and visible reflectivity above 0.70 has a better thermal performance than the heavy concrete roof with the same radiative properties and with a higher thermal resistance.

... Akhbari et al. [10] have studied the effect of roof reflectivity on different roof insulations for residential buildings. A parametric simulation for four different duct insulations, five different ceiling insulations, and four different levels of roof reflectivity over 32 different climatic regions was performed. ...

To make roofs energy efficient, typically two types of techniques are followed: Surface treatments (Cool Roofs, Radiant Barriers) and Thermal property modifications (Roof insulation). The interplay between these two techniques has been studied using energy simulations. A single storey, daytime operational, office building of 200 m2 area has been simulated for five climatic zones in India. A total of 88 different roof combinations have been studied for each climatic zone. An economic analysis using Internal Rate of Return has been performed to identify a suitable roof insulation thickness for a roof with high albedo, and radiant barrier combination. The incremental benefits in energy savings reduces by adding insulation after a limit. For a roof with Albedo of 0.6 and Radiant Barrier emittance of 0.2, the optimized roof R-value is 0.49 m2 K/W in Hot & Dry and Composite climates, 0.31 m2K/W in Warm & Humid and Temperate climates, and 1.02 m2K/W for cold climates.

... This study showed that the estimated annual electricity savings was about 10 TWh with a net savings of about $750 M in annual energy payments, and the peak electricity power reduction was about 7 GW. In support of inclusion of reflective roofs in the proposed ASHRAE standard SSPC 90.2, simulation results including the annual electricity and fuel use for a prototypical single-family onestorey house indicated that increasing the roof reflectivity from 20% to 60% is worth over half of the roof insulation in hot climates [12]. Recently, Levinson and Akbari [13] conducted energy simulations (no moisture transport was accounted for) to determine the annual heating and cooling energy uses of four commercial building prototypes in 236 US cities. ...

Andri Ottesen
Sumaya Banna

During 2012, the authors conducted a comprehensive survey that aimed to measure the market readiness for electric vehicles in Iceland as a predictive assessment for European and American markets that would be the most likely customer segment of electric vehicles for 2018. The earlier study made a prediction about a strong rise in the demand for electric vehicles until 2018. Hence, the predictions for 2018 are investigated and examined and followed up with in-depth qualitative interviews with several owners of electric vehicles and members of a think tank for application of new fuels in Iceland, and lastly with experts at the Ministry of Transportation. The main objective was to gather qualitative data about the implementation of electric vehicles in Iceland, its success and limitations, the outlook, conditions for success, discussion of future research, management implications, and how the case of Iceland is applicable as an early adoption model for countries within the region. Consistent with prior research, we found that financial, battery-charging infrastructure and battery-related concerns remain major obstacles to widespread electric vehicles market penetration in Iceland.

The objective of this study is to evaluate the life cycle costs and market barriers associated with using reflective paving materials in streets and parking lots as a way to reduce the urban heat island effect. We calculated and compared the life cycle costs of conventional asphalt concrete (AC) pavements to those of other existing pavement technologies with higher reflectivity-portland cement concrete (PCC), porous pavements, resin pavements, AC pavements using light-colored chip seals, and AC pavements using light-colored asphalt emulsion additives. We found that for streets and parking lots, PCC can provide a cost-effective alternative to conventional AC when severely damaged pavements must be completely reconstructed. We also found that rehabilitating damaged AC streets and intersections with thin overlays of PCC (ultra-thin white topping) can often provide a cost-effective alternative to standard rehabilitation techniques using conventional AC. Chip sealing is a common maintenance treatment for low-volume streets which, when applied using light-colored chips, could provide a reflective pavement surface. If the incremental cost of using light-colored chips is low, this chip sealing method could also be cost-effective, but the incremental costs of light-colored chips are as of yet uncertain and expected to vary. Porous pavements were found to have higher life cycle costs than conventional AC in parking lots, but several cost-saving features of porous pavements fell outside the boundaries of this study. Resin pavements were found to be only slightly more expensive than conventional AC, but the uncertainties in the cost and performance data were large. The use of light-colored additives in asphalt emulsion seal coats for parking lot pavements was found to be significantly more expensive than conventional AC, reflecting its current niche market of decorative applications. We also proposed two additional approaches to increasing the reflectivity of conventional AC, which we call the chipping and aggregate methods, and calculated their potential life cycle costs. By analyzing the potential for increased pavement durability resulting from these conceptual approaches, we then estimated the incremental costs that would allow them to be cost-effective compared to conventional AC. For our example case of Los Angeles, we found that those allowable incremental costs range from less than dollar 1 to more than dollar 11 per square yard (dollar 1 to dollar 13 per square meter) depending on street type and the condition of the original pavement. Finally, we evaluated the main actors in the pavement market and the existing and potential market barriers associated with reflective pavements. Apart from situations where lifecycle costs are high compared to conventional AC, all reflective paving technologies face a cultural barrier based on the belief that black is better. For PCC, high first costs were found to be the most significant economic barrier, particularly where agencies are cons trained by first cost. Lack of developer standards was found to be a significant institutional barrier to PCC since developers are often not held accountable for the long-term maintenance of roads after initial construction, which creates a misplaced incentive to build low first-cost pavements. PCC also faces site-specific barriers such as poorly compacted base soils and proximity to areas of frequent utility cutting.

Danny Parker
Yu Joe Huang
Steven J. Konopacki
Lixing Gu

A series of experiments in Florida residences have measured the impact on space cooling of increasing roof solar reflectance. In tests on 11 homes with the roof color changed mid summer, the average cooling energy use was reduced by 19%. Measurements and infrared thermography showed that a significant part of the savings were 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 14 different climate locations around the United States.

Inefficiencies in air distribution systems have been identified as a major source of energy loss in US sunbelt homes. Research indicates that approximately 30--40% of the thermal energy delivered to the ducts passing through unconditioned spaces is lost through air leakage and conduction through the duct walls. Field experiments over the past several years have well documented the expected levels of air leakage and the extent to which that leakage can be reduced by retrofit. Energy savings have been documented to a more limited extent, based upon a few field studies and simulation model results. Simulations have also indicated energy loss through ducts during the off cycle caused by thermosiphon-induced flows, however this effect had not been confirmed experimentally. A field study has been initiated to separately measure the impacts of combined duct leak sealing and insulation retrofits, and to optimize a retrofit protocol for utility DSM programs. This paper describes preliminary results from 6 winter and 5 summer season houses. These retrofits cut overall duct leakage area approximately 64%, which translated to a reduction in envelope ELA of approximately 14%. Wrapping ducts and plenums with R-6 insulation translated to a reduction in average flow-weighted conduction losses of 33%. These experiments also confirmed the appropriateness of using duct ELA and operating pressures to estimate leakage flows for the population, but indicated significant variations between these estimates and measured flows on a house by house basis. In addition, these experiments provided a confirmation of the predicted thermosiphon flows, both under winter and summer conditions. Finally, average material costs were approximately 20% of the total retrofit costs, and estimates of labor required for retrofits based upon these experiments were: 0.04 person-hrs/cm{sup 2} of duct sealed and 0.21 person-hrs/m{sup 2} of duct insulated.

Hashem Akbari
Sarah Bretz
Dan M. Kurn
James Hanford

In the summers of 1991 and 1992, we monitored peak power and cooling energy savings from high-albedo coatings at one house and two school bungalows in Sacramento, California. We collected data on air-conditioning electricity use, indoor and outdoor temperatures and humidities, roof and ceiling surface temperatures, inside and outside wall temperatures, insolation, and wind speed and direction. Applying a high-albedo coating to one house resulted in seasonal savings of 2.2 kWh/d (80% of base case use), and peak demand reductions of 0.6 kW. In the school bungalows, cooling energy was reduced 3.1 kWh/d (35% of base case use), and peak demand by 0.6 kW. The buildings were modeled with the DOE-2.1E program. The simulation results underestimate the cooling energy savings and peak power reductions by as much as twofold.

Energy Savings of Reflective Roofs American Society of Heating Refrigerating and Air-Conditioning Engineers

ASHRAE, 1998. "ASHRAE Technical Bulletin, Energy Savings of Reflective Roofs," American Society of Heating Refrigerating and Air-Conditioning Engineers, Atlanta, Geo., Volume 14, Number 2, January.

American Society of Heating Refrigerating and Air-Conditioning Engineers

Ashrae

ASHRAE, 1998. "ASHRAE Technical Bulletin, Energy Savings of Reflective Roofs," American Society of Heating Refrigerating and Air-Conditioning Engineers, Atlanta, Geo., Volume 14, Number 2, January.

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Source: https://www.researchgate.net/publication/287915687_Updates_on_Revision_to_ASHRAE_Standard_902_Including_Roof_Reflectivity_for_Residential_Buildings

Iola Zappavigna

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