Photo courtesy of Duro-Last

Selecting a Reflective Roof: Important Considerations

Although a suitable reflective roof may be available for almost any commercial roofing need, a number of additional considerations should be examined before selecting the best reflective roofing system for any particular application.

Reflective Roofing Standards

As a starting point, the roofing option selected should meet or exceed the best current codes and standards for reflective roofing in order to maximize long-term benefit and performance. For roofing products to be considered reflective by today’s standards, the product must be tested to achieve a minimum Solar Reflectance Index (SRI) rating.

This index combines two measures critical to evaluating the total reflective potential of a roofing product. The first measure is Solar Reflectance (SR), which quantifies the amount of solar energy that is directly reflected by the material. The second measure is Thermal Emittance (TE), which quantifies the amount of solar energy that is indirectly released to the atmosphere or adjacent spaces after the material is heated by the sun. Looking back at Figure 2, the solid arrows represent the SR portion of solar energy that is directly reflected, while the wavy arrows represent the TE portion of energy that is indirectly emitted after the material has been heated.

In order to evaluate the total amount of energy removed by a reflective roof, it is necessary to combine the SR and TE of the material into a single SRI index. Almost all national energy codes and standards now use SRI as the reference standard for roof reflectance, and many of these codes and standards have established minimum SRI levels, both initial and after aging for 3 years. Table B provides a brief summary of both initial and aged minimum SRI values for three of the most-recognized U.S. building codes and standards—the International Energy Conservation Code, the ASHRAE 90.1 building energy standard, and the State of California Title 24 energy standard.

The building owner and designer should use the SRI range in this table as a guideline for product selection. To help verify that a product selected meets or exceeds the relevant SRI value, most roof system manufacturers identify the SRI of their products on published product information sheets and other technical literature. In addition, a free online listing of the tested and certified SRI of hundreds of roofing products is maintained by the Cool Roof Rating Council, which also maintains the building code-recognized standard for testing and reporting SRI. Additional information about the Cool Roof Rating Council is included in the “For More Information” section of this article.

Reflective Roofs and Aging

As shown in Table B, reflectivity values for roofing products are measured both initially and after an aging period. This is because the reflectivity of a roof surface tends to degrade due to aging of pigments and discoloration caused by surface accumulation of dirt, airborne pollutants, and biological growth. Although some roofing researchers originally suspected that the decrease in reflectivity over time might be significant, field studies conducted on a wide variety of reflective roofing types suggest that roof reflectivity tends to stabilize over time and probably never falls much below an SRI of 50 under typical field conditions.¹⁴

Because most reflective roofing systems maintain a relatively high level of reflectivity over time, it is likely that any attempts to wash or clean the surface of the roof will not offer any significant economic benefit. In fact, many roofing professionals do not favor the cleaning of roofing surfaces because the cleaning process may accelerate the overall aging of the system and reduce service life.¹⁴

Finally, it is important in any discussion of roof aging to draw a distinction between installing a high-performance commercial roofing system and merely painting an existing roof. In general, most commercial-grade roofing systems are available with in-service performance warranties that may extend for up to 20 years or more. In the case of paint or a commodity roof coating obtained from a local building supply, however, the available warranty may be non-existent or less than a few years. Although warranty terms and conditions may vary widely from product-to-product, it is important that the building owner or designer carefully verify the type and extent of warranty coverage that is available for the roofing system or product selected.

Comparing Reflective Roofing Options

Because reflective roofs are available in all major technologies, the selection of the best reflective roof for any application may be made based on economics and performance. Perhaps the best approach is to combine economics and performance by looking at the long-term life cycle cost of the roofing system. Such an approach may be especially useful if there are periodic maintenance requirements for one type of reflective roofing system as compared to another. As an example, if a reflective roof coating system requires recoating every so many years, it may have a higher cost than a reflective single-ply system that requires minimal maintenance over its service life.

Reflective Roofs and Roof Insulation

After finalizing a decision to install a new reflective roofing system, it is important not to neglect the underlying thermal insulation. In terms of return on investment, the best time to increase insulation levels is at the time a new roof is installed, especially when reroofing over an existing roof. As a general guide for roof insulation levels, the same major codes and energy standards mentioned previously may serve as the best references. In these codes, the thermal performance of roof insulation is measured by R-value; and in general, all of these codes and standards recommend R-value levels from between R-20 to R-35 for typical commercial roofs with insulation above the roof deck. Table C provides a summary by climate zone of minimum above-deck R-value levels in the most recent editions of the International Energy Conservation Code and the ASHRAE 90.1 building energy standard. In addition to designing for at least the minimum amount of roof insulation required by code, it is also important to install the insulation in two layers with staggered joints, in order to limit air infiltration and thermal loss.

Reflective Roofs and Adjacent Surfaces

Some building researchers have reported incidents when the heat and light reflected from a reflective roof may impinge on nearby surfaces such as walls and windows. In some cases, the increased heat on adjacent surfaces appeared to accelerate the aging of the surface; in other cases the reflected light caused an observation of glare through nearby windows.¹⁵ Although the reports to date appear to be limited in scope and anecdotal, it may be prudent for the building or roofing designers to consider the potential for heat and light reflection on adjacent structures during the design stage of any project involving a reflective roof.

Reflective Roofs and Adjacent Surfaces

Some building researchers also have reported incidents where moisture may have condensed and accumulated within reflective roofing systems, mostly in very cold climates. Similar to reports of the effects of reflective roofs on adjacent surfaces, many of the reports of roof condensation are anecdotal and appear to frequently be associated with other roof design or quality issues.¹⁵ For additional information, a recent paper reviewing the issue of roof condensation in reflective roofs is included in the “For More Information” section of this article.

Photo courtesy of Duro-Last