Cool Roof Q Amp A Lawrence Berkeley National Laboratory-Books Download

Cool roof Q amp A Lawrence Berkeley National Laboratory

2020 | 8 views | 14 Pages | 272.31 KB

Lawrence Berkeley National Laboratory ... the surface of a standard gray roof that reflects only about 20% of incident ... Cool roof Q+A 011.doc 29 July 2009 2, ,



Cool roof Q A 011 doc 29 July 2009
A A roof with a clean smooth bright white surface can reflect about 85 of incident sunlight R
0 85 and emit thermal radiation with 90 efficiency E 0 90 This surface has an SRI of
107 and will be only 9 F 5 K warmer than the outside air on a typical summer afternoon For
comparison the surface of a standard gray roof that reflects only about 20 of incident sunlight
R 0 20 E 0 90 has an SRI of just 19 and a surface temperature elevation surface
temperature outside air temperature T of 69 F 38 K
A roof with a clean smooth cool color surface such as a cool red tile can reflect about 35 of
incident sunlight R 0 35 and emit thermal radiation with 90 efficiency E 0 90 This
surface has an SRI of 38 and a T of 56 F 31 K The cool red tile is much warmer than the
bright white roof but still cooler than a standard red tile R 0 10 E 0 90 SRI 6 T 78 F
A roof with a clean bare zincalume steel surface can reflect about 75 of incident sunlight R
0 75 but emits thermal radiation with only 5 efficiency E 0 05 This surface has an SRI of
68 and a T of 36 F 20 K The surface of the bare zincalume steel is warmer than that of the
bright white roof SRI 107 but still cooler than that of the cool red tile SRI 38 and the
standard red tile SRI 6
Figure 1 shows the initial solar reflectance solar reflectance index and surface temperature
elevation surface temperature minus outside air temperature on a summer afternoon of cool
products for low sloped roofs pitch 2 12 such as single ply membranes field applied
coatings and modified bitumen Figure 2 does the same for cool materials for steep sloped roofs
pitch 2 12 including white and cool color tile shingle and metal products
Q What is a cool color
A About half of all sunlight arrives in the invisible near infrared spectrum Standard light
colored surfaces strongly reflect both visible and near infrared sunlight while standard dark
colored surfaces reflect modestly in both spectra Special dark and medium colored surfaces that
strongly reflect near infrared sunlight are called cool colors The solar reflectance of a cool
dark color can exceed that of a standard dark color by about 0 4
Q Do cool roofs stay cool
A Over time the deposition of airborne particles the growth of microorganisms and or the
oxidation of bare metal can reduce the solar reflectance of a cool roof These events can also
increase the thermal emittance of certain materials especially those with bare metal surfaces
Eventually an equilibrium is reached between processes that soil the surface such as particle
deposition and processes that clean the surface such as wind and rain Since the solar
reflectance and thermal emittance of a outdoor surface typically reach steady values within one
to three years the aged solar reflectance and thermal emittance of a roofing product are
measured after three years of natural exposure
Figure 3 shows the aged solar reflectance solar reflectance index and surface temperature
elevation surface temperature minus outside air temperature on a summer afternoon of cool
products for low sloped roofs
Cool roof Q A 011 doc 29 July 2009
Three years of exposure reduces the solar reflectance of an initially bright white low sloped roof
to 0 65 from 0 85 and leaves the thermal emittance unchanged at 0 90 This lowers its SRI to 79
from 107 and increases its T to 28 F 16 K from 9 F 5 K Exposure also reduces the solar
reflectance of a bare zincalume steel roof to 0 50 from 0 75 and increases its thermal emittance
to 0 20 from 0 05 This lowers its SRI to 26 from 68 and increases its T to 65 F 36 K from
36 F 20 K Exposure has no net effect on a standard gray roof which retains R 0 20 E
0 90 SRI 19 and T 69 F 38 K
Because they are newer to market these is less information currently available about the effects
of exposure on the solar reflectances and thermal emittances of cool materials for steep sloped
roofs such as white and cool color tile shingle and metal products However preliminary data
indicate that the radiative properties of cool color products for steep sloped roofs are minimally
changed by exposure
Q Are green roofs cool What about thermally massive super insulated and or ventilated
A There are a variety of roofing assembly technologies that can reduce the flow of heat into a
building including but not limited to solar reflective thermally emissive surfaces vegetative
cover green roofing thermally massive construction super insulation and ventilation We
reserve the term cool roof to refer to one that stays cool in the sun by virtue of high solar
reflectance and preferably high thermal emittance
Q When should I install a cool roof
A We recommend selecting a cool product for new construction or when an old roof is
scheduled to be retrofitted It is rarely economical to replace a mechanically sound roof just to
increase its solar reflectance
Q Can I paint my roof white or a cool color
There are specialized white elastomeric coatings available for low sloped products and cool
color polymer coatings available for tiles that can be sprayed on existing roofs
Q What does a cool roof cost
A The price of a cool roof is typically comparable to that of a standard hot roof of the same
type because coloration is responsible for a very small portion of the total installed cost of any
roofing product
Q What are the benefits of a cool roof
A Selecting a cool roof instead of a standard hot roof directly benefits the occupants of the
building and indirectly benefits the entire community Installing a solar reflective roof can also
immediately slow global warming through a process called global cooling
DIRECT BENEFITS
Substituting a cool material for a standard hot material lowers the daytime surface of
temperature of the roof which in turn reduces the flow of heat into the occupied space This
Cool roof Q A 011 doc 29 July 2009
conserves energy in an air conditioned building and makes an unconditioned building more
comfortable
Reducing the need for cooling doesn t just save money it also reduces the emission of
pollutants from electric power plants including carbon dioxide CO2 nitrogen oxides NOx
sulfur dioxide SO2 and mercury Hg
Reducing the building peak cooling load with a cool roof can allow the installation of a smaller
less expensive air conditioner This is referred to as a cooling equipment saving Smaller air
conditioners are also typically less expensive to run because air conditioners are more efficient
near full load than at part load
Choosing a cool roof instead of a standard roof can slightly increase the need for heating energy
in winter However winter penalties are often much smaller than summer savings even in cold
climates because the northern mainland U S latitude 40 N receives about 3 to 5 times as
much daily sunlight in summer as in winter
The tables at the end of this document show in each California climate zone the present value of
lifetime energy cost savings the sum of the cooling equipment savings and the energy cost
savings and annual emission reductions for installation of a cool low sloped roof on a
nonresidential office building and also for the installation of cool color shingle tile and metal
steep sloped roofs on homes with and without attic radiant barriers
Finally lowering the peak daytime temperature of soft roofing products such as single ply
membranes and asphalt shingles may help them last longer by reducing the stress resulting from
thermal expansion during the day and thermal contraction at night
INDIRECT BENEFITS
The citywide installation of cool roofs can lower the average surface temperature which in turn
cools the outside air Cool roofs thereby help mitigate the daytime urban heat island by making
cities cooler in summer This makes the city more habitable and saves energy by decreasing the
need for air conditioning in buildings For example a program to install cool roofs cool
pavements and trees over about 30 of the surface of the Los Angeles basin has been predicted
to lower the outside air temperature by about 5 F 3 K Additional annual building energy
savings expected from the cooler outside air are estimated to be about half those resulting from
the cool roof itself
Cooler outside air improves air quality by slowing the temperature dependent formation of
smog Decreasing the outside air temperature in the Los Angeles basin by 5 F 3 K is predicted
to reduce smog ozone by about 10 worth about 300M yr in avoided emissions of smog
precursors e g NOx
Cool roofs decrease summer afternoon peak demand for electricity reducing the strain on the
electrical grid and thereby lessening the likelihood of brownouts and blackouts
GLOBAL COOLING
Cool roof Q A 011 doc 29 July 2009
Replacing a hot roof with a cool roof immediately reduces the flow of thermal radiation into the
troposphere negative radiative forcing Substituting 1000 ft2 100 m2 of cool white roofing
for standard gray roofing provides a one time not annual offset of about 10 metric tons of CO2
worth 250 at a price of 25 t CO2 Similarly substituting 1000 ft2 100 m2 of cool color
roofing for standard color roofing provides a one time offset of about 5 metric tons of CO2
worth 125 While the roof must be kept cool to retain this benefit the offsets are based on aged
rather than initial values of solar reflectance
Low sloped roofing materials initial properties
white field applied coating 10
100 white single ply membrane SRI reference white
white modified bitumen
Surface temperature elevation F
aluminum modified bitumen
Solar reflectance index SRI
70 bare zincalume steel
aluminum field applied coating
20 standard gray roof 70
0 SRI reference black
0 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 1 0
Solar reflectance
Figure 1 Initial solar reflectance solar reflectance index SRI and surface temperature elevation surface
temperature outside air temperature of products for low sloped roofs
Cool roof Q A 011 doc 29 July 2009
Steep sloped roofing materials initial properties
100 SRI reference white
Surface temperature elevation F
white metal
Solar reflectance index SRI
80 white tile
white shingle
40 cool color tile
cool color metal
cool color shingle
standard color tile or shingle
standard color metal 80
0 SRI reference black
0 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 1 0
Solar reflectance
Figure 2 Initial solar reflectance solar reflectance index SRI and surface temperature elevation surface
temperature outside air temperature of products for steep sloped roofs
Cool roof Q A 011 doc 29 July 2009
Low sloped roofing materials aged properties
100 SRI reference white
Surface temperature elevation F
white single ply membrane
Solar reflectance index SRI
80 white field applied coating
60 aluminum modified bitumen
50 aluminum field applied coating
bare zincalume steel
20 standard gray roof 70
0 SRI reference black
0 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 1 0
Solar reflectance
Figure 3 Aged solar reflectance solar reflectance index SRI and surface temperature elevation surface
temperature outside air temperature of products for low sloped roofs after three years of natural exposure
Cool roof Q A 011 doc 29 July 2009
Nonresidential office building
based on LBNL s 2005 Title 24 CASE study
Roofing product built up roof
R in T24 simulation 0 35
R for CARB table 0 35
result scaling factor 1 00
15 yr NPV of of TDV NOx SO2
TDV energy energy cooling energy heating energy CO2 emission emission emission Hg emission
savings savings savings penalty reduction reduction reduction reduction
climate zone 1000ft2 1000ft2 kWh 1000ft2 yr therm 1000ft2 yr kg 1000ft2 yr g 1000ft2 yr g 1000ft2 yr g 1000ft2 yr
1 109 176 115 8 2 5 29 20
2 442 542 295 6 87 76 74 228
3 294 370 184 5 48 42 46 128
4 380 470 246 4 76 66 61 198
5 300 383 193 5 52 46 48 139
6 596 707 388 4 133 115 97 341
7 489 614 313 3 111 96 78 283
8 647 772 413 4 145 125 103 371
9 618 719 402 5 137 118 100 351
10 521 610 340 4 117 101 85 299
11 411 486 268 5 81 71 67 212
12 438 533 286 5 86 75 71 226
13 547 643 351 5 113 98 88 293
14 536 641 352 5 116 100 88 298
15 583 665 380 2 143 122 95 361
16 309 399 233 11 37 35 58 112
Cool roof Q A 011 doc 29 July 2009
Residential building standard insulation no radiant barrier
based on LBNL s 2008 Title 24 CASE study
Roofing product shingle
R in T24 simulation 0 15
R for CARB table 0 15
result scaling factor 1 00
30 yr NPV 30 yr NPV
of TDV of TDV NOx SO2
energy energy cooling energy heating energy CO2 emission emission emission Hg emission
savings savings savings penalty reduction reduction reduction reduction
climate zone 1000ft2 1000ft2 kWh 1000ft2 yr therm 1000ft2 yr kg 1000ft2 yr g 1000ft2 yr g 1000ft2 yr g 1000ft2 yr
1 97 80 4 4 5 22 17 1 48
2 179 206 98 6 2 6 8 24 27
3 5 20 18 2 7 7 5 4 13
4 58 86 45 3 9 3 1 11 1
5 12 34 23 2 7 5 3 6 8
6 40 68 22 1 4 1 2 5 6
7 80 111 39 2 2 4 4 10 14
8 247 284 91 2 5 23 21 23 63
9 295 324 103 2 1 30 26 26 79
10 490 529 169 3 2 51 44 42 133
11 469 512 159 2 6 50 43 40 129
12 316 344 116 3 1 30 27 29 81
13 587 632 196 2 8 63 55 49 164
14 452 490 171 4 9 42 38 43 115
15 761 790 234 0 9 89 76 59 224
16 209 259 92 4 1 15 14 23 45
Cool roof Q A 011 doc 29 July 2009
Roofing product tile
R in T24 simulation 0 30
R for CARB table 0 25
result scaling factor 0 83
30 yr NPV 30 yr NPV
of TDV of TDV NOx SO2
energy energy cooling energy heating energy CO2 emission emission emission Hg emission
savings savings savings penalty reduction reduction reduction reduction
climate zone 1000ft2 1000ft2 kWh 1000ft2 yr therm 1000ft2 yr kg 1000ft2 yr g 1000ft2 yr g 1000ft2 yr g 1000ft2 yr
1 156 133 3 6 9 35 28 1 76
2 189 229 129 10 2 2 2 32 13
3 17 17 24 4 1 12 9 6 22
4 46 88 56 5 9 9 5 14 11
5 10 18 28 4 3 12 8 7 21
6 39 76 28 2 3 1 0 7 2
7 95 129 51 3 2 4 4 13 15
8 300 351 117 3 8 27 24 29 74
9 374 415 136 3 3 37 32 34 98
10 633 690 226 5 2 63 56 56 167
11 626 681 215 3 8 65 57 54 171
12 414 453 157 4 7 38 34 39 104
13 797 863 269 4 3 84 74 67 220
14 588 645 232 7 8 51 46 58 142
15 1029 1068 319 1 6 119 102 80 301
16 274 348 126 6 3 17 17 31 55
Cool roof Q A 011 doc 29 July 2009
Roofing product metal
R in T24 simulation 0 30
R for CARB table 0 25
result scaling factor 0 83
30 yr NPV 30 yr NPV
of TDV of TDV NOx SO2
energy energy cooling energy heating energy CO2 emission emission emission Hg emission
savings savings savings penalty reduction reduction reduction reduction
climate zone 1000ft2 1000ft2 kWh 1000ft2 yr therm 1000ft2 yr kg 1000ft2 yr g 1000ft2 yr g 1000ft2 yr g 1000ft2 yr
1 166 135 7 8 39 30 1 83
2 343 391 177 11 15 17 44 58
3 3 40 32 5 12 8 8 21
4 120 171 84 7 2 1 21 7
5 26 67 41 5 8 5 10 12
6 78 130 41 3 3 4 10 13
7 154 212 73 4 9 9 18 30
8 458 524 167 4 44 39 42 119
9 545 598 188 4 56 49 47 147
10 892 963 304 5 94 82 76 244
11 863 943 291 5 92 80 73 239
12 586 636 213 5 57 50 53 152
13 1068 1148 354 5 116 100 88 298
14 828 894 308 8 78 70 77 212
15 1388 1441 427 2 161 138 107 408
16 383 470 166 7 28 26 41 83
Cool roof Q A 011 doc 29 July 2009
Residential building standard insulation radiant barrier
based on LBNL s 2008 Title 24 CASE study
Roofing product shingle
R in T24 simulation 0 15
R for CARB table 0 15
result scaling factor 1 00
30 yr NPV 30 yr NPV
of TDV of TDV NOx SO2
energy energy cooling energy heating energy CO2 emission emission emission Hg emission
savings savings savings penalty reduction reduction reduction reduction
climate zone 1000ft2 1000ft2 kWh 1000ft2 yr therm 1000ft2 yr kg 1000ft2 yr g 1000ft2 yr g 1000ft2 yr g 1000ft2 yr
1 70 59 1 3 1 16 12 0 34
2 102 122 56 3 5 4 4 14 16
3 6 11 12 2 0 6 4 3 11
4 27 49 27 2 7 3 2 7 4
5 2 17 15 2 0 5 3 4 8
6 23 42 14 1 1 0 0 4 2
7 44 61 24 1 5 2 2 6 7
8 142 167 54 1 7 13 11 14 35
9 185 207 66 1 4 19 17 16 50
10 292 321 101 1 9 30 26 25 79
11 303 328 104 1 9 32 28 26 83
12 204 224 76 2 1 19 17 19 52
13 357 385 120 1 9 38 33 30 98
14 279 304 106 3 1 26 23 26 70
15 502 524 154 0 6 58 50 39 147
16 113 144 55 2 9 7 7 14 22
Cool roof Q A 011 doc 29 July 2009
Roofing product tile
R in T24 simulation 0 30
R for CARB table 0 25
result scaling factor 0 83
30 yr NPV 30 yr NPV
of TDV of TDV NOx SO2
energy energy cooling energy heating energy CO2 emission emission emission Hg emission
savings savings savings penalty reduction reduction reduction reduction
climate zone 1000ft2 1000ft2 kWh 1000ft2 yr therm 1000ft2 yr kg 1000ft2 yr g 1000ft2 yr g 1000ft2 yr g 1000ft2 yr
1 109 93 2 4 8 25 19 0 53
2 136 167 80 5 4 3 5 20 18
3 15 10 17 3 0 9 7 4 17
4 33 68 39 4 1 6 4 10 7
5 11 10 19 3 1 9 6 5 16
6 21 48 18 1 7 2 1 4 1
7 56 79 34 2 4 1 2 8 6
8 190 228 75 2 6 16 15 19 46
9 255 288 92 2 2 25 22 23 67
10 413 455 145 3 0 42 37 36 110
11 437 471 151 2 8 45 39 38 118
12 288 318 110 3 3 27 24 27 72
13 526 570 179 2 9 56 49 45 145
14 401 438 154 4 8 36 32 38 99
15 729 761 226 1 0 85 73 56 214
16 155 204 79 4 5 8 8 20 27
Cool roof Q A 011 doc 29 July 2009
Roofing product metal
R in T24 simulation 0 30
R for CARB table 0 25
result scaling factor 0 83
30 yr NPV 30 yr NPV
of TDV of TDV NOx SO2
energy energy cooling energy heating energy CO2 emission emission emission Hg emission
savings savings savings penalty reduction reduction reduction reduction
climate zone 1000ft2 1000ft2 kWh 1000ft2 yr therm 1000ft2 yr kg 1000ft2 yr g 1000ft2 yr g 1000ft2 yr g 1000ft2 yr
1 122 103 3 6 28 22 1 60
2 187 221 99 6 8 9 25 30
3 8 22 22 3 9 7 5 17
4 52 91 49 5 5 3 12 4
5 5 33 26 3 8 5 6 13
6 41 76 26 2 0 1 6 4
7 78 109 42 3 3 3 10 12
8 256 301 97 3 24 21 24 65
9 333 373 117 2 34 30 29 89
10 519 568 179 3 54 47 45 142
11 543 588 186 3 57 50 46 149
12 363 400 135 4 34 30 34 92
13 632 683 213 3 67 59 53 175
14 497 539 187 5 46 41 47 125
15 894 933 275 1 104 89 69 263
16 199 254 97 5 11 11 24 37


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