Keeping It Cool

40 June 2010 www.professionalroofing.net

After 18 years of exposure to extreme desert heat,

the roof system on the Nationwide® Scottsdale

Insurance Co. headquarters building in Scotts-

dale, Ariz., began to deteriorate. The maintenance staff

had been chasing leaks for nearly five years when Nation-

wide Scottsdale Insurance decided to hire Alan Stevens

Associates Inc., Cedar Rapids, Iowa, to solve the prob-

lem and replace the roof system.

The building’s original roof system was a ballasted

EPDM membrane over 4 inches of polyisocyanurate

insulation on a concrete roof deck blanketed with a

2-inch-thick layer of river rock. The roof system was

becoming a liability with regard to structural integrity

(from water damage), maintenance costs and energy

consumption.

The new roof system was installed by Starkweather

Roofing Inc., Phoenix, in December 2008 and consists

of 3 inches of polyisocyanurate board insulation set in

roof adhesive over the concrete roof deck and a self-

adhering base sheet and self-adhering white laminated

polymer-modified bitumen roof membrane over the

top. The membrane’s surface is a reflective white film

laminate with a solar reflectance index (SRI) value of

84.

In July 2009, the Arizona Cool Roof Council asked

Starkweather Roofing to conduct a case study to deter-

mine whether white roofs truly are cooler and more

energy-efficient than nonwhite roofs. The Nationwide

Scottsdale Insurance building’s roof was an ideal choice

for the in-depth study.

A cool roof case study investigates white roofs’ energy ef�ciency

by Chris Walker

Financial Services building in the samecorporate plaza (which still has its originalballasted EPDM roof system in place, in-cluding the same amount of insulationthat was on the Nationwide Scottsdale In-surance building’s original roof system). Obtaining rooftop and interior temper-

ature data from two different buildingswas beneficial because the results would be based on identical weather conditionsinstead of data from one year to the next

on the same building where many vari-ables could affect the results. The twobuildings are almost identical in heightand construction.The temperature sensors on the roof sur-

faces were secured under the new materialsused in each of the current roof systems toprevent direct sunlight from affecting thereadings and provide an accurate surfacetemperature for each building. The tem-perature sensors placed in the stairwellswere secured 3 feet from the roof hatchesand directly to the underside of the concreteroof decks to measure the roof systems’ en-ergy efficiencies; in other words, they meas-ured how much heat was being transferredthrough the concrete deck into the build-ings’ interiors.The sensors measured the roof surface

(the top of the membrane) and stairwelltemperatures for both buildings once perhour, 24 hours per day from Aug. 6, 2009,to Sept. 4, 2009. Official temperature datafor Scottsdale during the same time periodand at the same time intervals also were obtained. During the data collection period, Scottsdale primarily had hot, sunny days—with a few monsoon stormssprinkled in—allowing for excellent testconditions.The study’s conditions were near-

perfect for various reasons. First, we hadvirtually identical buildings that are sideby side. Second, the Wells Fargo FinancialServices building had an old roof systemand the Nationwide Scottsdale Insurancebuilding had a new roof system—wecould do a before-and-after analysis at thesame time and under the same conditions.Finally, concrete decks are not typical inArizona (plywood decks are more com-mon), so if the data proved any signifi-cant increase in energy efficiency in abuilding of this construction (with a con-crete deck), we would know even betterresults could be gained in a building thathas a plywood deck because heat transfersthrough plywood at a faster, more intenserate than it does through concrete.


Energy efficiency

On the morning of Aug. 6, 2009, ShermRobison, lead estimator for StarkweatherRoofing, and I installed HOBO® tempera-ture data loggers from Onset ComputerCorp., Bourne, Mass., directly onto theroof surfaces and in the low-traffic, non-air-conditioned and nonvented stairwellsof the Nationwide Scottsdale Insurancebuilding and the neighboring Wells Fargo

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The Nationwide Scottsdale Insurance building before (top) and after (bottom) Starkweather Roofinginstalled the cool roof system

Analysis and conclusions

The first thing that caught our attentionwhen observing the study’s results was thesignificant reduction of internal thermalvariation, which is caused directly by thecool roof system’s consistent temperaturerange. In Figure 1, notice that between 10

p.m. and 6 a.m., the buildings have simi-lar internal temperatures but the WellsFargo Financial Services building always is hotter than the Nationwide ScottsdaleInsurance building during the day. At 8a.m., the Wells Fargo Financial Servicesbuilding begins to heat up dramatically

while the Nationwide Scottsdale Insurancebuilding remains virtually unchanged. Fig-ure 2 shows between 5 p.m. and 8 p.m.,the Wells Fargo Financial Services build-ing cools down faster than it heated upand the Nationwide Scottsdale Insurancebuilding’s internal temperature again re-mains constant.Figure 1 also demonstrates how internal

temperatures change as a result of outsidetemperatures. Notice how the NationwideScottsdale Insurance building’s tempera-ture holds steady all day while the WellsFargo Financial Services building’s tem-perature fluctuates rapidly. There is an

average range of only 2 degrees in internaltemperature from the hottest to the coolestparts of the day inside the NationwideScottsdale Insurance building. Conversely,the Wells Fargo Financial Services buildingtypically has an 8-degree range of internaltemperature during a 24-hour period. Forbuildings as large as these two, a nearly 8-degree range means a lot in terms of theadditional energy required to maintain aconsistent temperature for the buildings’tenants. During August 2008, the Nationwide

Scottsdale Insurance building consumed719,000 kilowatts (kW) of electricity.

ProfessionalRoofing June 2010 43

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Avg Outside Temp 88.28 86.78 85.45 84.61 82.61 81.65 81.03 81.73 84.85 88.77 91.71 94.15 96.66 98.67 100.44 101.46 102.21 102.17 101.19 99.55 96.13 93.98 91.99 90.04

Avg PolyKool Stairwell Temp 84.24 84.11 84.01 83.93 83.79 83.72 83.67 83.68 84.01 84.15 84.28 84.50 84.69 84.92 84.98 85.06 85.13 85.61 85.47 85.40 85.09 84.69 84.44 84.33

Avg EPDM Stairwell Temp 85.81 85.60 85.29 84.97 84.94 84.86 85.22 85.60 86.61 88.14 89.66 90.51 91.60 92.23 92.78 92.94 92.96 92.88 92.13 89.97 87.44 86.92 86.44 86.05

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Figure 1: Average internal stairwell temperatures versus average external temperatures

Internal temperatures versus external temperatures

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Avg PK SW Temp Rate of Change (0.12) (0.10) (0.08) (0.14) (0.07) (0.05) 0.02 0.33 0.14 0.13 0.22 0.18 0.23 0.06 0.08 0.07 0.48 (0.14) (0.07) (0.32) (0.40) (0.25) (0.11)

Avg EPDM SW Temp Rate of Change (0.21) (0.31) (0.32) (0.03) (0.08) 0.36 0.39 1.00 1.53 1.51 0.86 1.09 0.62 0.55 0.16 0.02 (0.08) (0.75) (2.16) (2.53) (0.52) (0.48) (0.38)

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Figure 2: Average rates of change of internal stairwell temperature

Comparison of internal temperature change

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cooler now that you can place your hand on it midday without discomfort.Sunglasses now are mandatory for allworkers because the reflectivity is like sunshine on snow.“Our utility bill has shown a consistent

monthly drop in kilowatt-hour (kWh) usesince the roofing project’s completion,” he continues. “A conservative estimate ofpower reduction is around 50,000 kWhper month during midsummer. The build-ing’s air-conditioning demand has droppedtremendously. The cooling tower load hasdecreased so much I rarely have all fourtowers online. With the old roof sys-tem, we used all four towers from June to October to satisfy the building demand.”So the answer to the Arizona Cool Roof

Council’s question is clear: By obtainingrooftop and temperature data from anEPDM roof system and a self-adheringwhite laminated polymer-modified bitu-men roof membrane and comparing theresults, we now know without a doubtthat white roof systems truly are coolerand more energy-efficient than nonwhiteroof systems.

Chris Walker is a senior technologies projectmanager contracted by American Express,New York. He is a former employee of Starkweather Roofing Inc., Phoenix.

According to Chris Peterson, lead facilitiestechnician for the Nationwide ScottsdaleInsurance building, all other items remainedvirtually unchanged when compared with2008, and the self-adhering white laminatedpolymer-modified bitumen roof membraneinstallation was the only substantial differ-ence in 2009. During August 2009, thekilowatt consumption was 663,000 kW—a nearly 8 percent decrease (or $5,450saved) in electricity required to cool the building, deeming it more energy-efficient.Again, this is on a building with a con-

crete roof deck. Also, note that the coolroof cap sheet has an SRI value of 84. Ifthe building had a plywood roof deck andif the materials’ SRI value used was higher(100 is typical of most high-quality roofcoatings), the energy savings would beeven greater. Starkweather Roofing hasbegun performing similar analyses usingbuildings with plywood roof decks andhigher SRI value roof coatings to com-pare the results.On average, the Nationwide Scottsdale

Insurance roof system is 12 degrees coolerthan the Wells Fargo Financial Services roofsystem—but that includes overnight whenthere is no solar gain from direct sunlight.Therefore, the study results include temper-ature readings from during the night when

sunlight isn’t increasing the roof ’s surfacetemperature.Figure 3 shows that during the study,

the largest difference in roof surface temperature occurred at noon when theaverage outside temperature was 97 F. TheWells Fargo Financial Services roof systemon average was 156 F at that time, but theNationwide Scottsdale Insurance roof sys-tem was only 129 F. That’s a difference of27 degrees. Between 9 a.m. and 5 p.m.(maximum sunlight hours), there is an av-erage 22-degree difference in roof surfacetemperature, which is quite significant re-garding the long-term survival of rooftopequipment (most rooftop equipment isn’tconstructed or warranted to last as long in such extreme temperatures), as well as the health of the building’s facilitiesworkers who have to perform work on the roof.

Benefits

Peterson says the new roof system has givenNationwide Scottsdale Insurance, as wellas the building’s maintenance staff, peaceof mind.“The cool roof does not leak and is easy

to clean and keep free of debris,” Petersonsays. “We used to try to clean up the oldroof, but the rock ballast made this tasknext to impossible. The surface is so much


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Avg Outside Temp 88.28 86.78 85.45 84.61 82.61 81.65 81.03 81.73 84.85 88.77 91.71 94.15 96.66 98.67 100.44 101.46 102.21 102.17 101.19 99.55 96.13 93.98 91.99 90.04

Avg PolyKool Roof Temp 66.46 65.44 64.36 62.76 61.69 60.45 59.80 65.65 82.32 99.77 113.09 122.21 128.59 130.59 129.15 124.86 117.84 108.06 97.16 84.57 74.47 70.35 68.27 66.92

Avg EPDM Roof Temp 70.86 69.45 68.15 66.66 65.37 64.14 63.21 66.99 89.05 114.86 135.36 148.47 156.28 156.91 152.75 145.61 135.06 122.80 110.14 97.22 86.22 79.42 75.50 72.66

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Figure 3: Average roof surface temperatures versus average external temperatures

Roof surface temperatures versus external temperatures

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Keeping It Cool

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