hite roofs are certainly cool, butwhat about aged and dirtyroofs?

There is ample evidence thatsolar-reflective roofs can help

keep a building cool, particularly in hot climateswith a high level of direct sunlight. And while anumber of technologies are available that can deliv-er a reflective roof surface, such as single-ply roofmembranes, reflective aluminum-pigmented coat-ings, and even darker-colored coatings employinginfrared-reflective pigmented coatings, the basicwhite reflective roof still reigns supreme as the dom-inant cool-roof technology.

But what about the issue of roof aging, and itseffect on roof reflectivity? The factors affecting agingperformance and the measurement and certificationrequirements for cool roof coatings has been thesubject of previous JAC articles, including thisPassing the Test column (JAC, January, 2006, It’s aMatter of Degrees). The aging factor is taken into account in theU.S. Department of Energy’s Energy Star program, which requiresan initial solar reflectivity of at least 65%, with a 50% reflectivitylevel required after three years of outdoor weathering. The inde-pendent Cool Roof Rating Council (CRRC) has more stringent spec-ifications, and stipulates three-year outdoor weathering in the threedifferent climates of south Florida, Arizona, and northern Ohio aspart of its rating system.

Recent revisions to the California Energy Commission’s Title 24Part 6 cool-roof provisions for residential and non-residential low-slope roofs also include aged and initial reflectivity requirements.These requirements, set forth in the California Building Standards

Code, are included as part of the overall buildingenergy-efficiency standards set by the EnergyCommission. The new provisions also extend thestandards to some steep-slope roofs.

Title 24, as well as CRRC certification, are basedon the CRRC-1 Program Manual requirements. Oneimportant aspect for coatings, however, is that thesubstrate can alter a coating’s performance. Forexample, reflectivity would be compromised if acoating delaminated from the roof substrate. Apotential discrepancy within the CRRC-1 protocol isthat initial reflectance measurements are made oncoatings applied to standard aluminum test panels,but weather-aged performance was to be measuredas applied to various actual roof substrates. Thisdetail was initially overlooked until about two yearsinto the initial three-year weathering requirement forcertification; the exposures on standard aluminumpanels were to be accepted.

Clearly, however, data are needed to answer thequestion of how important this matter of substrate type is, in terms ofaged roof-surface reflectivity. Currently, several interested parties areworking to address this need for data. The Reflective Roof CoatingsInstitute (RRCI), whose members consist primarily of white roof-coat-ings manufacturers, has launched a three-year minimum weatheringstudy to determine how coating chemistry, film thickness, geograph-ic location, and substrate type affect performance. Commercial for-mulations of acrylic, silicone, polyurethane, and acrylic/fluoropoly-mer coatings are being exposure tested in three separate film thick-nesses. These coatings are applied to various common roof sub-strates, including PVC and EPDM sheet, alumimum and cold rolledsteel, spray polyurethane foam (SPF), and several asphalt types.

W

J o u r n a l o f A r c h i t e c t u r a l C o a t i n g s / J u n e / J u l y 2 0 0 8 6 3

Passing the testAllen Zielnik

Atlas MaterialTesting

Technology LLC

Effect ofweathering cancut into energysavings, butresearch seeksto shed morelight on thesubject

D u r a b i l i t y

Pausing for reflection on aging of the cool roof

64 J o u r n a l o f A r c h i t e c t u r a l C o a t i n g s / J u n e / J u l y 2 0 0 8

Samples are being exposed in three loca-tions—Florida, Arizona, and Minnesota—to determine changes in reflectivity andemittance.

The Roof Coatings ManufacturersAssociation (RCMA), which represents awider range of coatings types than theRRCI, has launched a similar but broaderstudy to determine the effect of substrate onaged coating performance. The question isnot academic; if the substrate proves to bea significant factor in rating coatings, thiswill necessitate much more compre-hensive (and expensive) testing thancurrently required. At this time, both ofthese studies are less than one yearalong in their respective programs,and data is not yet available.

Roof weatheringand reflectivity loss

Roof reflectivity also can be negativelyimpacted both by erosion of the coat-ing or membrane due to weathering,and by soiling due to dirt retention ormildew growth. A research study con-ducted by Oak Ridge National Laboratory(ORNL) for the Single Ply Roofing Industry(SPRI) trade group compared major com-mercial roof membranes for aging charac-teristics and energy performance. It wasdetermined that dirt and mildew weremajor issues for all membrane types, espe-cially after the first three to four years of ser-vice, but that periodic maintenance clean-ing usually restored reflectivity perfor-mance. Does this also hold true for roofcoatings, some of which are used to rehabmembrane roofs? Does it make economicsense to perform roof cleaning? And if itmakes sense, how should it be done, andhow often, without damaging the roof?

Typical initial solar-reflectance values foraluminized roof coatings are in the range of0.50 to 0.60 on a scale of 1.00.Reflectance of white reflective roof coatingsis typically higher, with most having valuesof 0.70 to 0.85, numbers that meet thestandards set by the EPA Energy Star pro-

gram and California Title 24. Researchersat the Lawrence Berkeley NationalLaboratory (U.S.) and the NationalResearch Council (Canada) concluded thatmembrane roofs, on average, lost about20% of their initial solar reflectance over thefirst one to three years, but that 70% to100% of that loss could be regainedthrough washing with water or detergents,and in some cases with an algaecide.

Research by Oak Ridge NationalLaboratory showed that the economic ben-

efit of washing reflective roof membranesvaried, depending on the expense associat-ed with washing and the resulting reductionin energy costs. These benefits are oftenlocation-dependent, with climate playing amajor role. Reflective roof coatings, howev-er, are often used to extend the service lifeof traditional built-up roofs (BURs) andmembrane systems, so these positive eco-nomic and environmental impacts shouldalso be taken into account.

To wash or not to wash?So do roof coatings need washing, and isthere a worthwhile economic result? Theanswer to those questions may be suppliedin part by data generated from the RRCIand RCMA field-exposure trials. But at leastanecdotal field information from currentproduct installations appears to shed somelight on the subject. Jim Leonard of elas-tomeric roof-coatings manufacturerERSystems has compiled informal data on

61 roofs where the company’s white acrylicor acrylic-polyurethane coatings wereapplied. These were applied to a variety ofsubstrates, including thermoplastic, EPDM,and Hypalon membranes, granule surface-modified bitumen, smooth bitumen, smoothBUR, and SPF systems located in nine states,including Florida and Texas and locations inthe Midwest and the Pacific Northwest. Hisfield survey results for unwashed white roofcoatings suggest;• a loss of solar initial reflectivity of about 20-

25% during the first few months to oneyear, with little change beyond that;• a loss of solar reflectivity consistentwith other reported studies; and• generally little variation by sub-strate.

Leonard concludes that washing ofwhite-coated, reflective roof surfacesas a maintenance measure does notconstitute a cost-effective means ofretaining or restoring solar-reflectancevalues, especially considering that“most solar reflectance loss will reoccurin three to six months.” He also con-

cludes that energy calculations should bebased on at least one-year aged reflectancevalues, which are typically 75-80% of the ini-tial reflectance values.

A compilation of information from variousstudies suggests that for all reflective roofingtypes, about 95% of aging occurs during thefirst two years, and 98% occurs within thefirst three years of installation. Initial first-yearsolar reflectance losses of about 20-25% canbe expected. It is important to realize that itis not the percentage loss of solar reflectivitythat is significant, but rather the impact ofthat change on the energy efficiency of thebuilding. Solar-reflectance changes of plus orminus 2% have a negligible on energy effi-ciency. While washing can usually restoremost of the solar reflectivity, at least duringthe first several years of aging, the effects areshort lived (typically three to six months),yield limited economic payback, and havethe potential to damage roofs in the process.

What does it all mean? Primarily, it means

Photos courtesy of National Coatings Corp.

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that the actual economic benefit of washingwill vary by location. Energy-efficient, reflec-tive “cool roofs” are being used in all cli-mates, and their use is increasing, while tra-ditional, dark-colored, built-up roofs, thoughstill dominant, are declining. But the energysavings attributable to cool roofs will bemuch different in Phoenix than in Boston.

Reflective roofs generate the greatest ben-efit by reducing air-conditioning costs, whichare obviously higher in warmer climates withhigh levels of solar radiation. While reflectiveroofs suffer from a “heating penalty” in termsof energy-cost reductions in colder climates,this is often offset by high electricity costs dueto air conditioning. The following list of citiesshows EPA estimates of average annual eco-nomic savings realized by cool reflectiveroofs for every 1,000 square feet of air-con-ditional building space.

Philadelphia $4Chicago $9Dallas/Ft. Worth $11New York $17Houston $18Los Angeles $20Miami/Ft. Lauderdale $21Phoenix $34

Energy-savings economics are a combi-nation of local climate and the cost of elec-tricity. Therefore, the greater the energy sav-ings provided by a cool roof, the greater thepenalty paid due to reflectivity loss withaging. For example, ORNL calculated thatfor San Diego, the yearly energy coolingsavings per 1,000 square foot of a newreflective vs. a black roof was $72; a 20%reflectivity decrease due to roof agingreduced these savings to only $55, or a 30%decline in savings. Indianapolis, by contrast,provided a more modest initial cooling-ener-gy savings of $6 per 1,000 square feet, butbecause of the cold weather heating-penal-ty offset, a 20% reflectivity loss due to agingresulted in no net change to overall energysavings. So energy savings alone suggestedonly a modest benefit from installing areflective roof in Indianapolis, and cleaningit would provide no benefit at all. But the sit-

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uation in San Diego is quite different.Reflective roof coatings can be used to

rehabilitate traditional roofing such as BURs,as well as membrane systems. The econom-ic benefit can be determined by the overallbuilding energy savings, but these calcula-tions should be based on solar-reflectanceestimates after at least one year of aging.Reflective roof coatings can also prolong thelifetime of existing roof systems, since ther-mal degradation is a major cause of roofaging. It is possible that results from theongoing weathering field tests will provideadditional guidance in the coming years.

So, the answer to the question of whetherto clean or recoat cool roofs to maintain orrestore solar reflectivity can basically beanswered in two parts:• in hot, high solar-load climates where airconditioning cost dominates and heatingpenalty-offset cost is minimal, cleaning most

likely provides a benefit;• in colder climates where the cost of airconditioning is high enough to offset theheating-penalty effect, cleaning most likelyprovides a benefit; and• in cold climates where the annual energycost of air conditioning is relatively low andthe cost of heating is fairly high, cleaning orrecoating would not make economic sense interms of energy cost savings

Clearly the costs and frequency of clean-ing need to be carefully considered as well,as should the indirect benefit of prolongedroof life courtesy of the use of coatings, evenif the energy savings decline with age.

In any case, if cleaning is warranted, thecoating manufacturer should be consultedfor recommendations on methods and mate-rials to avoid damaging the coated roof sur-face.

JAC

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