Roof Color and Reflectivity
Facts: Is White the Only
Option?
André Desjarlais
Oak Ridge National Laboratory
18 January 2008
Presentation Overview
‰ What is a Cool Roof?
‰ How to make low-slope roofs cool?
‰ What happens when color matters?
‰ Are there options to getting the energy
savings?
‰ What’s the future?
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
2
Why a Cool Roof?
‰ Reduces cooling loads
‰ Reduces peak energy demand
‰ Can reduce ambient temperature in an
urban setting (urban heat island)
• Improved air quality
• Further energy savings
‰ Cool roofs being included into energy
codes
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
3
What is a Cool Roof?
‰ Roof surface that has a high solar
reflectance and a high thermal emittance
‰ In CA, 70% SR and 75%TE
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
4
ρsolar and εIR are Both Important
Total Solar
Irradiation
It
ρsolar It
Reflected
Convection
hair(tair-ts)
Net Infrared
Radiation
εIR∆R
with ∆R=σ(Ts4-Tsky4 )
(αsolarIt Absorbed)
Net Heat Flux into Roof
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
5
Solar Energy Spectrum
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
6
When did this Hubbub Start?
Published in 1989
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
7
Membrane Temperature (°F)
Validating Benefit of Cool Roofs
180
Meas- Pre- Solar
IR
ure dict Refl. Emit.
R,% E,%
160
140
120
100
80
x
60
0 2 4 6 8 10 12 14 16 18 20 22 24
Hours into Day (8/17/99)
53
48
34
26
36
42
5
5
Tair
85
82
60
68
64
56
92
87
Effect of ±R, ±E:
±3°F at Peak
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
8
The Calculator
Dr. Tom Petrie
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
9
Roof Energy Savings
R70E90
Savings, $/ft² per year
$0.25
R65E10
R50E40
R-5
R-15
$0.20
$0.15
$0.10
$0.05
$0.00
Knoxville
Phoenix
Chicago
Knoxville
Phoenix
Chicago
City
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
10
Surfacing
Protects against UV,
Heat, and traffic
GRAVEL
OAK RIDGE N
ATIONAL LABORATORY
GRANULE
CAP
U. S. DEPARTMENT OF ENERGY
SHEET
REFLECTIVE ROOF
COATING
11
White single ply sheet
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
12
What About Snow?
Instant “Cool Roof”
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
13
Alternatives to a White Cool Roof?
‰ Cool colors
‰ More insulation
‰ Ballasted roofs
‰ Above sheathing ventilation
‰ Thermochromic surfaces
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
14
Camouflage Invisible to Night Vision
Conventional Film
Near Infrared Film
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
15
Solar Energy Spectrum
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
16
Conventional vs. Infrared Pigments
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
17
Higher Reflectance without
Sacrificing Color Choice
Regal White
Rawhide
Slate Blue
Standard SR .67
Standard SR .47
Standard SR .21
Cool SR .72
Cool SR .56
Cool SR .33
Slate
Bronze
Brick Red
Charcoal
Gray
Hartford
Green
Standard SR .25
Standard SR .14
Standard SR .11
Standard SR .08
Cool SR .28
Cool SR .26
Cool SR .30
Cool SR .28
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
18
The Proof: A Case Study
Baggett vs. Poole Elementary Schools
Paulding County Georgia
Baggett – Standard Roof
Evergreen 12% SR
90,000 S.F. footprint
Poole – Cool Roof
Evergreen 29% SR
90,000 S.F. footprint
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
19
Same Design……..Different Results!
Roof construction: R15 blanket insulation
over purlins
BESSIE L BAGGETT ELEMENTARY
DALLAS, GA
LILLIAN C POOLE ELEMENTARY
POWDER SPRINGS, GA
R-19 6” batt insulation
at ceiling level
Electric HVAC with
Gas-Fired Heating
Thermostats Controlled
at District Office
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
20
Energy Bills are Different!
Benefit
Penalty
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
21
Reflectivity/Insulation Study
‰ Model cooling and heating energy loads of
prescriptive or “cool” roofs
‰ Model cooling and heating energy loads of
“non-cool” roofs
‰ Adjust insulation levels of “non-cool” roofs
to match cooling energy requirements of
prescriptive roof
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
22
Compute “Non-Cool” Cooling
Loads – Zone 12 (Sacramento)
Load, Btu/ft^2
"Non-Cool Roof"
"Cool Roof"
5000
4000
3000
2000
1000
0
19 32 36 40 44 48 52
Insulation R-Value
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
23
Stone Ballast
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
24
Test Sections Configured for
the Ballast Tests
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
25
Ballast Study - Test Section Details
Test Section
Covering or
Loading
Thickness
Solar
Reflectance
Black Control
Bare EPDM
0.04 in.
0.06
White Control
Bare TPO
0.05 in.
0.78
10# Stone
10 lb/ft² on
EPDM
1.3 in.
0.22
17# Stone
16.8 lb/ft² on
EPDM
2.2 in.
0.22
24# Stone
23.5 lb/ft² on
EPDM
3.1 in.
0.22
Paver
23.5 lb/ft² on
EPDM
2.0 in.
0.41
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
26
Bare Black EPDM
Under 10 lb/ft² Stone
Under 24 lb/ft² Stone
Bare White TPO
Under 17 lb/ft² Stone
Under Uncoated Paver
150
16
130
12
110
8
90
4
70
0
50
-4
30
0
2
4
6 8 10 12 14 16 18 20 22 24 0
Hours into 10/4/2004
2
4
-8
6 8 10 12 14 16 18 20 22 24
Hours into 10/4/2004
Heat Flux through Insulation [Btu/(h·ft²)]
Membrane Temperature (°F)
Test Results –
Six Month Exposure Data
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
27
Integral Analyses
‰ Seek integral heating and cooling loads
for each roof system
• To study aging, perform analyses semiannually
• No load when Toutside is between 60 and
75ºF
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
28
Average Total Loads
Year 2
Year 3
10
9
8
ft^2
Load, Btu/h
Year 1
7
6
er
P
av
#
24
#
17
#
10
ck
la
B
W
hi
te
5
Roof System
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
29
Natural Convection Prevalent in
Counter-Batten Roof Systems
Parker, Sonne and Sherwin (ACEEE 2002)
Roof surface-to-deck ∆T’s ≈ 14°F (8°C)
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
30
Counter Batten Construction
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
31
Test Control is an Asphalt
Shingle (SR093E89)
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
32
Reflectance and Above-Sheathing
Ventilation (ASV) Effects
Light Gray Shake, (SR246E90)
Underside Unpainted
Batten & Counter batten
Dark Gray Shake, (SR08E90)
Underside Unpainted
Batten & Counter batten
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
33
ASV Reduced Peak Heat Flow by
30% of Asphalt Shingle
OAK RIDGE NATIONAL LABORATORY
August 2005
U. S. DEPARTMENT OF ENERGY
34
Above-Sheathing Ventilation
(ASV) Equal to 15 Points of SR
Attic Contains R-30 Insulation and AC Ducts with R-5.6 Insulation
Underside Emittance = 0.90
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
35
Next Step: “Smart” Reflective
Exterior Surface
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
36
“Smart” Reflective Exterior
Surface Test Panel
Switch at
84˚F
Switch at
65˚F
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
37
$0.40
$0.30
$0.20
Cool
$0.10
Heat
Total
$0.00
Knoxville
Minneapolis
it
ch
S
w
80
%
it
ch
S
w
80
%
S
w
it
ch
-$0.10
80
%
Savings ($/sq. ft. yr)
Potential Savings Using
“Smart” Technology
Phoenix
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
38
Summary
‰ Energy savings due to white cool roofs
are well documented
‰ Options are available that yield equal
savings and peak demand
• More insulation
• 17# ballast better than cool roof in
first year
• Above sheathing ventilation
‰ New technologies are under
development
OAK RIDGE NATIONAL LABORATORY
U. S. DEPARTMENT OF ENERGY
39
Comments and Questions?
Roof Color and
Reflectivity Facts: Is
White the Only Option?