Keeping roofs cool: in part two of Cool Roofing, George R Pilcher, examines why aesthetics are...

In general, roofing and roofing materials are characterised as being 'low-slope' or 'steep-slope.' Low-slope is generally considered in the industry as a roof with a 'rise-over-run' of two feet in 12 ('2:12') or less. (Flat roofs are de facto considered to be in the 'low-slope' category). Steep-slope would be any roof with a rise-over-run relationship of greater than 2:12 (>10[degrees]). In general, steep-slope roofs are far more common in residential applications, while low-slope roofs are more common in commercial applications, although this is certainly not a hard-and-fast rule.

Another significant difference between low-slope and steep-slope roofing has to do with aesthetic considerations. Generally, low-slope roofing cannot be seen from ground level, so the appearance of such roofing is not a major factor. Steep-slope roofing, however, is easily visible and 'looks count; As a result, manufacturers of steep-slope roofing products invest considerable resources addressing aesthetic issues.

Asphalt shingles are now commonly supplied with dimensional characteristics and variegated colour schemes. Painted metal roofing comes in thousands of different colours, shapes, sizes, and textures. Concrete tile, clay tile, and cedar shake roofing are all designed with performance and aesthetics in mind. Because of the different requirements for steep-slope vs low-slope roofing i.e., steep-slope roofs are typically designed as water-shedding systems, and low-slope roofs are designed to form a seal against water penetration--different materials are typically used in their composition and construction.

National initiatives

The environmental community, and those concerned with 'sustainability,' recognise the significant benefits of cool roofing. In the U.S. Green Building Council's Leadership in Energy and Environmental Design Rating System (LEED), (5) cool roofing comes into play in two different ways:

* Qualified cool roofing materials receive a direct credit under the 'Sustainable Sites' category, in recognition of the benefits of cool roofing in mitigating heat island effects.

* Cool roofing can help buildings to attain energy efficiency levels needed to obtain points in the 'Energy and Atmosphere' section. This credit recognises the environmental benefits of reduced electric power consumption.

LEED represents an incentive-based initiative that promotes energy-efficient design in all aspects of building construction. Roofing is just one of the components of a building that can be used to maximise the effective use of energy.

The U.S. EPA's Energy Star program was begun in 1992 to provide guidance to consumers on a wide array of products. A key goal of the Energy Star Program is to assist consumers in identifying specific products, within any general class of products, which are more energy-efficient than the others. For any given product, Energy Star chooses an appropriate rating system (with substantial input from its manufacturing and other partners) and the level of performance Energy Star performer. So consumers know that products bearing the Energy Star label, whether computers, kitchen appliances, air conditioning units, or roofing materials, are efficient performers.

For roofing materials, the Energy Star program (6) has been available since early 1999. Energy Star focuses exclusively on the solar reflectance of materials, including both initial values and values obtained following three years of weathering. Manufacturers self-report reflectance values, although they are required to use an appropriate ASTM test method for determining reflectivity, as previously described. Three-year aged values are determined by field measurement of actual roofs (chosen by the manufacturers) that have been in place for at least three years.

The numerical average of multiple samples from different regions of the country determines the final three-year value. Energy Star also allows the use of 'test farm' samples (coupons), provided that the samples are _24 sq. inches in area and exposed at a weathering facility which is in compliance ISO/IEC 17025:1999 General Requirements for the Competence of Testing and Calibration Laboratories. Energy Star permits washing of samples prior to testing. Again, these aged reflectance values are self-reported by the participants. For low-slope materials, Energy Star qualification requires an initial total solar reflectance of at least 0.65 (65%) and a three-year value of at least 0.50 (50%). Note: A solar reflectance value of 0.65 is basically a white material. For steep-slope materials, initial and 'aged' values of 0.25 and 0.15, respectively, qualify. The allowance of substantially lower TSR values for steep-slope roofing recognises the broad appeal of and desire for--coloured roofing (i.e., not a white material) in applications where the roof is visible to the casual observer. As of January 28, 2004, there were 625 products listed as Energy Star qualified roof products.

These different slope characteristics, combined with the wide range of materials used in roofing-from cedar shakes to asphalt shingles; from tile to painted metal shingles; from built-up roofs to various single-ply systems and many others--make 'one-size-fits all' measurement and rating systems tricky at a practical level, but, after years of effort, the extant methods seem to handle most situations quite well.

New Building Code Requirements for Cool Roofing in California

With an eye toward increasing the energy efficiency of buildings, state and municipal code bodies are increasingly requiring more effective roof radiative performance, and California has taken the lead. The California Energy Commission (7) (CEC) recently adopted the 2005 language for the state's building energy efficiency standard, Title 24. This update will have a very significant impact with regard to the use of cool roofing on low-slope, non-residential buildings. In essence, this change in the building code requires cool roofing for all significant roofing jobs for low-slope, non-residential buildings, including 'greenfield' new construction and re-roofing jobs. (It is estimated that there are three times as many re-roofing jobs as there are new installations.) In other words, in the State of California, any job involving roofing, and requiring a building permit, falls within the scope of this change. The final effective date for the 2005 update is not yet determined, but a date sometime in the latter half of 2005 is a reasonable estimate for now. This language can be found on the California Energy Commission's Title 24 website. (8)

Before discussing California any further, it is sensible to step back and ask if genuine benefit comes from these energy efficiency initiatives. The best quantitative answer to this question comes from a study done by the Natural Resources Defense Council (NRDC). In their work, Energy Efficiency Leadership in California: Preventing the Next Crisis, the graph shown as Figure 1 was developed to demonstrate the total effect of the overall efforts that have taken place in California, over the past few decades, to minimise energy consumption.

[FIGURE 1 OMITTED]

This figure demonstrates a number of interesting things. The first is that, in 1977, the per capita consumption of electricity in California was close to the per capita consumption of the other 49 states. The 1974 energy crisis was a wake-up call for California. The State launched a large number of initiatives to slow the growth in electrical power demand, and the graph clearly shows a lower slope for California after 1977 than for the rest of the U.S. Roofing, of course, is only part of the equation.

To achieve the results shown in Figure 2, greater utilisation of high-tech window technology, enhanced levels of insulation, development of energy-efficient appliances, and air conditioners, etc., was necessary. California has had much experience with the concept of a State Energy Code. Specifications for windows and air conditioning have been codified for some time. In the case of roofing materials, the CEC had no desire to create its own specified department to collect and disseminate radiative data relevant to materials proposed for use within the state to comply with the Energy Code. Instead, it provided the seed money to create the Cool Roof Rating Council (CRRC, see below), and has designated it as the sole supervisory entity for the purpose of maintaining a credible rating system for radiative properties of roofing materials.

[FIGURE 2 OMITTED]

Per Title 24, alternate code compliance methods are available for those situations where a cool roof is not feasible for some reason. Under one compliance method, it is possible to 'trade off' other (improved) building efficiency measures against roofing materials that fall short of the solar reflectance and thermal emittance requirements for cool roofing. For example, assuming that compliant roofing is not possible, more insulation or less window area may be accepted in exchange. There are other issues associated with Title 24 compliance, but which fall outside of the scope of this article. Readers interested in learning more are advised to visit the CEC website for details.

In future years, California is likely to consider cool roofing requirements for residential low-slope and steep-slope roofs as well as for nonresidential steep-slope roofs. As is the case with Energy Star, it is likely that the cool roofing definition in such cases will be different from the current nonresidential, low-slope requirements.

Cool Roof Rating Council (CRRC)

Formed in 1998, the CRRC9 was established with a commission to set up a fair, accurate, and credible rating system for the radiative properties of roofing materials. It requires that values be established for both solar reflectance (TSR) and thermal emittance (TE). As with Energy Star, the CRRC requires initial and three-year aged values.

Unlike Energy Star, the CRRC system does not define 'cool' in terms of any particular performance level; it simply establishes a system to collect credible radiative data (both initial and aged), and maintains a directory of rated products with their rated properties.

The CRRC rating program began on September 1, 2002; as of February 2004, the CRRC Directory of Rated Products has listed 143 products. In another significant departure from the Energy Star program, the CRRC protocols require the use of independent, third-party, CRRC-accredited testing laboratories to establish rated values, utilising appropriate ASTM test methods. Manufacturers cannot 'self-report' performance values. The CRRC system also requires that a portion of rated products be subjected to random testing each year, to assure system integrity.

Under the CRRC requirements, 'aged' values are determined by examining exposure data generated at three designated 'test farms' representing hot/humid, hot/dry, and 'Midwestem' exposure conditions.

All rated products are exposed at all three test farm sites in exactly the same manner and then tested, after three years, using the same ASTM test methods that were used to establish the initial values. The CRRC protocol, unlike the Energy Star protocol, does not permit washing of samples prior to the aged test measurements. The CRRC aged testing program component was scheduled to begin in March 2004.

The Energy Star program for cool roofs is well designed to reinforce its primary mission of encouraging consumers to choose high-performance roofing products for their buildings, on a voluntary basis.

The CRRC system, on the other hand, is more focused on maintaining a technically rigorous rating system for use by various parties who may wish to use the CRRC system to assure conformance to performance levels that they choose.

The two organisations maintain a close and productive working relationship. U.S. EPA and DOE (sponsoring organisations of Energy Star) assisted in the establishment of the CRRC, and are well-represented on its Board of Directors.

ASHRAE and Other States

Currently, the American Society of Heating, Refrigeration, and Air-conditioning Engineers [(ASHRAE).sub.10] has established two energy efficiency standards 90.1 for commercial buildings and 90.2 for low rise buildings. The current ASHRAE standard requires a minimum total solar reflectance of 0.70 and a minimum thermal emittance of 0.75 when tested in accordance to ASHRAE's listed test methods. ASHRAE 90.1 and 90.2 are referenced by 37 states in their energy codes and are also included in the 2003 version of the International Energy Conservation Code (IECC). Other jurisdictions, notably the states of Arizona, Florida, Georgia, and the City of Chicago have either adopted or are considering adoption of cool roofing standards for inclusion in their building codes.

Excellent timing

The concept of cool roofing is based upon the sound premise that whenever energy can be saved, power generation will either be contained or decreased, and the environment will subsequently be improved. The roofing market is a $10+ billion market, and many opportunities exist for coatings to contribute to the success of this market, both aesthetically and with regard to energy conservation.

There are new challenges facing today's coatings chemists, but there are also new formulating options available to them as well, and responsible and productive use of all of the available tools at our disposal can have profound effects on the use of energy and the subsequent improvement of the environment. Can reflectance be increased on an entire customer's standard product line of a rainbow of colours? New pigment technology now exists that helps to answer this question. Can a thin-film coating take advantage of a metal substrate's inherently high solar reflectance, while at the same time maximising the thermal emittance, which is an area where bare metal suffers in this cool roof environment?

How about 'smart' coatings that change from dark, heat-absorbing colours when the air temperature is cold (i.e., during the winter months), to a white or light heat-reflecting colour during the hot summer weather? Such coatings would have heat-absorbing properties when heating is required, and heat-reflecting properties when cooling during the summer is required.

The thin film or coating on a roof--presumed by so many people to provide only aesthetic value controls all of the radiative properties of the entire roof. This is truly a new era for coatings technology, where a newly-recognised set of material properties solar reflectance and thermal emittance provides opportunities for all of us to improve the environment and minimise energy demands.

References

(1) http://www.sws.uiuc.edu/atmos /statecli/General/1995Chicago.htm.

(2) Klinenberg, Eric. 'Heat Wave: A Social Autopsy of Disaster in Chicago,' 2002.

(3) http://eetd.lbl.gov/Heatlsland/.

(4) http://www.ornl.gov/sci/ roofs+walls/facts/CoolCalcEnergy.htm

(5) http://www.usgbc.org/leed/ leed_main.asp.

6) http://www.energystar.gov/ index.cfm?c=roof_prods.pr_roof_ products.

(7) http://www.energy.ca.gov/.

(8) http://www.energy.ca.gov/ 2005_standards/rulemaking/documents/index.html.

(9) http://www.coolroofs.org/.

(10) http://www.ashrae.org/.

Author: George R Pilcher

Technical director, Coil & Extrusion

Coatings

Akzo Nobel Coatings Inc

Email: george.pilcher@cbs.

akzonobel.com

Tel: +1 614 294 3361

Fax: +1 614 421 4360