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Cool Roofs Cool Cities Cool Planet. Ronnen Levinson, Ph.D . Acting Leader, Heat Island Group Environmental Energy Technologies Division Lawrence Berkeley National Lab; tel. 510-486-7494;

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cool roofs cool cities cool planet

Cool Roofs Cool Cities Cool Planet

Ronnen Levinson, Ph.D.

Acting Leader, Heat Island Group

Environmental Energy Technologies DivisionLawrence Berkeley National Lab; tel. 510-486-7494;

Presented to the China NDRC Delegation to Berkeley Lab22 April 2010

bird s eye view of urban land use
Bird’s eye view of urban land use
  • The surface of Sacramento, CAis about
    • 20% roofs
    • 30% vegetation
    • 40% pavement

~ 1 km2

what makes a surface cool
What makes a surface cool?

net emittedthermalradiationE σ (T4 - Tsky4)




opaque surface at temperature T


  • High solar reflectance (Rsol) lowers solar heat gain (0.3 - 2.5 µm)
  • High thermal emittance (E) enhances thermal radiative cooling (4 - 80 µm)

high solar reflectance + high thermal emittance = low surface temperature

sunlight more than meets the eye
Sunlight — more than meets the eye


(clear sky)air mass 1

global horizontal

Solar reflectance Rsol = 6.6%  ultraviolet reflectance Ruv+ 44.7%  visible reflectance Rvis + 48.7%  near-infrared reflectance Rnir

white cool color warm color
White, cool color, warm color

white roof

cool red roof

gray roof

types of cool roofs
Types of cool roofs



flat, white

pitched, cool & colored

pitched, white

cool colored roofing
Cool colored roofing

cool concrete tile R ≥0.40


standard concrete tile

(same color)







solar reflectance gain =

cool clay tile

R ≥0.40

CourtesyMCA Clay Tile

cool metal

R ≥0.30

CourtesyBASF IndustrialCoatings

cool fiberglass asphalt shingleR ≥0.25

CourtesyElk Corporation

prototype shingles
Prototype shingles

Solar reflectance >= 0.25

(for conventional shingles,

SR ~ 0.05 – 0.25)

prototype concrete tiles
Prototype concrete tiles

Solar reflectance >= 0.40

(for conventional concrete tiles, SR ~ 0.1 – 0.4)

  • Plants cool air by evaporating water
    • sensible heat → latent heat
    • most effective in arid climates
  • Plant matter remarkably solar reflective
    • R ≈ 0.3 for leaves
    • R ≈ 0.4 – 0.5 for wood
    • reflectance results from cellular structure
  • Trees can shade buildings
  • Green roofs
    • high thermal mass
    • moderate solar reflectance
    • evaporative cooling
    • rainwater control
    • extensive (shallow soil) or intensive (deep soil)
cool pavement technology cement concrete
Cool pavement technology: cement concrete
  • Study by Portland Cement Association shows that cement concretes have solar reflectances of 0.30 – 0.65
  • LEED compliant (SRI ≥ 29)

Solar reflectances of 45 concrete mixes

cool pavement technology asphalt concrete
Cool pavement technology: asphalt concrete (?)
  • Cool asphalt concrete still in its infancy
  • We seek to identify cool solutions for resurfacing asphalt concrete pavement
potential benefits of white roofs on commercial buildings
Potential benefits of white roofson commercial buildings

DOE-2.1E building energy simulations

Two roof types

aged gray roof(solar reflectance=0.20)

aged white roof(solar reflectance=0.55)

Four building prototypes

new office, old office

new retail, old retail

235 U.S. cities

Local energy prices

Local emission factors

Local building stock

Local population density

Results: local, state, national

cooling energy saving

heating energy penalty

energy cost saving

reductions in emissionof CO2, NOx, SO2, Hg

annual energy cost saving m 2
Annual energy cost saving ($/m2)

R-19 roof

R-13 wallsEER10 A/C

annual co 2 emission reduction kg m 2
Annual CO2 emission reduction (kg/m2)

R-19 roof

R-13 wallsEER10 A/C

(from energyconservation)

potential white roof benefits to u s
Potential white-roof benefits to U.S.

Retrofitting 80% of U.S. air-conditioned commercial buildings (2.1B m2) would annually save


6.2 Mt CO2 (=1.2M cars)

9.9 kt NOx (=0.6M cars)

26 kt SO2

126 kg Hg

through energy conservation

Product lifetime energy savings has present value of $11B

New York Times, 30 July 2009

global cooling
Global cooling
  • “Global cooling” offers further CO2 reductions
    • negative radiative forcing: high Rsol lowers T, reducing both convection and thermal radiation into the atmosphere
    • 80% of reflected sunlight escapes into space
    • 100 m2 (1000 ft2) of white roof retrofit offsets 10 t of CO2 emission (once)
    • retrofitting 80% of U.S. commercial buildings yields one-time offset of 200 Mt CO2 (= 4M cars x 10 years)
on the web
On the web

Cool Colors Project

Heat Island Group

Cool Communities Project

Roof Savings Calculator

Cool Roof Rating Council

Cool California

EPA Heat Islands

Energy Star Cool Roofs

Thank You!