Reflectivity, or albedo, is the percentage of the sun’s energy that is reflected by a surface. Another important measurement of a roof membrane’s performance is emittance. Emittance deals with how effectively a surface releases heat; it is the percentage of absorbed energy that a material can radiate away.
Most authorities have concentrated on reflectivity as the prime measurement of energy performance of cool roofing. However, with even the most reflective materials some energy is absorbed, and if that absorbed energy is not released efficiently it can cause a roof to heat up.
There is another measurement, called the solar reflectance index (SRI), that is beginning to get some attention. SRI combines reflectivity and emittance to measure a roofs overall ability to reject solar heat. The calculation of this index is defined by ASTM E 1980-01 and is based on some rather complicated math that includes values for solar absorptance, solar flux, thermal emissivity, the Stefan Boltzmann constant, and various other coefficients. Standard black (reflectivity 5%, emittance 90%) has an index of 0, and standard white (reflectivity 80%, emittance 90%) has an index of 100. Very hot materials can actually have negative values and very cool materials can have values greater than 100.
When all is said and done, a specific value can be calculated for any roofing product. Materials with the highest SRIs are the coolest choices for roofing.
Here is a sampling of products measured by Lawrence Berkeley Labs:
|Duro-Last Cool Zone
|Atlanta Metal Products, Kynar Snow White
|White Granular Surface Bitumen
|Trocal Roofing Systems, White
|Light Gravel on BUR
Noted roofing authority, Richard L. Fricklas, discusses PVC roofing in Buildings Magazine’s December 2009 Newsletter.
For the last year, attention seems to be more on cool roofing, LEED, and vegetated roofs rather than what the roofing system is made of or what it can do. Maybe because current roofing systems are all well established, so they’re no longer newsworthy? Several claims are being made as to which manufacturer has lowest carbon footprint and which products are truly recyclable.
To read the full article click on the link below.
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Numerous terms and ideas are used to describe products, processes and techniques that are said to be sustainable or provide high performance. Terms such as “green,” “environmentally-friendly,” “recyclable” and “long life-cycle” attempt to define the concept of sustainability. But what really is sustainability? How do you determine whether a product is truly sustainable or not?
Along with the growth of green design programs such as LEED® and Green Globes, there have been efforts by state and local governments to add sustainability guidelines to building codes. The federal government has indicated it plans to add green design guidelines to its building requirements as well.
Guidelines for many performance criteria are established based on standards. There are ANSI standards for PVC sheet roofing, for measuring emittance and for calculating a solar reflectance index. There are LEED standards that attempt to set the bar for high performance building design and construction. There are standards for wind loads and for material strength and thickness. Sustainability standards already exist for a few building products, but not for single-ply roofing. Without specific, complete standards for single-ply roofing, PVC-based products could become part of a broad category that would not accurately or effectively present the complete green picture for vinyl roofing.
The Vinyl Roofing Division of the Chemical Fabrics and Film Association has undertaken an ambitious effort to develop and obtain approval for an ASTM standard for Sustainable Thermoplastic and Thermoset Single-Ply Membrane Roofing. The process is anticipated to take another 6 – 12 months, but in the end the standard will provide solid guidelines as to what constitutes a sustainable single-ply roofing system. Stay tuned for updates as the project progresses.
The term “green roof” has become narrowly defined in recent years to refer to “vegetative roof.” But “green roof” can also mean “sustainable roof” – one that provides long-term environmental benefits that building owners want roofing systems to deliver for their high-performance facilities: high reflectivity; recyclability; able to accommodate photovoltaic systems; able to help facilities obtain LEED credits; etc. This brief video discusses these benefits and more. For additional information about green roofing, visit www.whiteequalsgreen.com.
Significant Savings Drive Demand for Cool Roofing
Cool Roofing Options and Choosing the Best Cool Roofing System
There are two primary types of cool roofing products on the market today: (1) reflective paints and coatings; and (2) single-ply roofing systems. Paints and coatings based on either acrylic or elastomeric chemistry can be an effective short-term solution for reducing energy costs, but most facility owners looking for long-term, low-maintenance solutions opt for a complete single-ply roofing system.
Many roofing products are on the market, which can make choosing the right one a challenge. Fortunately, several objective tools are available to help with the process. Choosing a system from the approved list of products in the EPA’s ENERGY STAR® Roof Products Program or from the Cool Roof Rating Council (CRRC) ratings chart is a good way to narrow down the selection process. Products on the ENERGY STAR list undergo rigorous testing before approval to ensure that they meet the established standard for reflectance.
In addition, the ENERGY STAR Roof Products Program has developed an energy savings calculator that projects the potential savings from installing a cool roof compared with alternative “non-cool” systems.
Here is a short list of important factors to consider when selecting a cool roofing system:
- Reflectance/emittance performance, both initial and after three years.
- Long-term track record of durability and performance.
- A good warranty backed by a solid, well-established manufacturer.
- Climate and weather extremes in a given location.
- Maintenance requirements and ease of repair.
In our next installment we will discuss: Single-Ply Cool Roofing Systems
How Cool Roofing Works
The trend may be new, but mankind has understood for centuries that white or light-colored surfaces are cooler than dark surfaces. Those stunning, ancient, all-white Mediterranean and Middle Eastern cities are not only aesthetically pleasing, they are also surprisingly cool and comfortable even on the hottest days of summer. Economic and environmental pressures have inspired a renewed interest in the heat-reflective properties of white surfaces, and recent research into the dynamics of urban heat islands, or UHIs – the phenomenon where even small cities are typically three to ten degrees warmer than nearby suburbs and countrysides.
The UHI chain of cause and effect is clear: As temperatures increase, more electric power is needed for air conditioning and more fossil fuel is consumed, which leads to higher levels of air pollution. The probability of smog rises five percent for each one-half degree increase in ambient temperature above 70°F.
Meanwhile, Oak Ridge National Laboratory (ORNL) has conducted several related studies to evaluate various materials for solar reflectance and emittance.
- Reflectance, also known as albedo or reflectivity, is the percentage of solar energy reflected by a surface. The higher the percentage of reflectance, the more heat energy will be reflected from the surface.
- Emittance, or emissivity, is the percentage of heat energy a material can absorb and then shed in the form of infrared radiation. Materials with low emittance tend to heat up more easily because they collect and trap heat. It is interesting that while many black materials have very low reflectance, they can exhibit very high emittance.
Although there is no industry-wide definition of a cool roof per se, the Environmental Protection Agency’s (EPA) ENTERGY STAR® Roof Products Program has established a minimum standard for products to qualify. The EPA standard requires that low-slope roof products have an initial reflectance of at least 65 percent, and a reflectance of at least 50 percent after three years of weathering. The ENGERGY STAR Program also requires products to carry warranties similar to, or better than, those offered by the same manufacturer for similar non-reflective roof products. ENERGY STAR ratings can be found on their web site, www.energystar.gov.
In our next installment we will discuss: Cool Roofing Options and Choosing the Best Cool Roofing System.
With the continuing volatility of oil and gas prices, two things have become increasingly important to the owners and managers of buildings of all shapes, sizes and locations: energy management and cool roofing. And yet, the two are seldom discussed as related issues. If you ask a building owner or manager about their energy management strategies, chances are they’ll mention a variety of “high-tech” solutions for improving building automation, systems interoperability, and the energy efficiency of their lighting, office equipment, security systems, and the biggest electricity consumer of all – air conditioning. The U.S. Environmental Protection Agency (EPA) estimates that $40 billion is spent annually to air condition commercial buildings, which is one-sixth of all electricity consumed in the United States.
Important as these high-tech solutions are, the enormous energy savings potential from smart “low-tech” roofing decisions are typically regarded as a traditional “building envelope” issue. But smart roofing decisions can reduce annual air conditioning consumption by 10 to 40 percent, depending on location, building design, climate, and other factors. This not only reduces air conditioning loads and utility bills, but can also allow facilities to downsize their air conditioning equipment considerably.
The Cool Roofing Trend
Roofing can contribute to energy efficiency in two ways – proper insulation, and reflective surfaces. Thermal roofing insulation became a major consideration during the oil embargoes of the 1970s. Proper insulation helps keep warm air in during the winter and out during the summer. Insulation needs vary from climate to climate, and most local building codes today mandate minimum roofing R-values – a material’s ability to resist heat flow.
A more recent trend has been the phenomenal growth of “cool roofing” – the use of white or light-colored roof surfaces that reflect solar energy and keep building relatively cooler in summer months. Although the overall market for commercial low-sloped (flat, or nearly flat) roofing has been stagnant in recent years, demand for cool roofing systems has continued to grow strongly. More than just a sensible, long-term, “green” building design solution, cool roofing is considered by many scientists, industry experts, and government officials to be an effective means of addressing critical national energy efficiency and environmental challenges.
In our next installment we will discuss: How Cool Roofing Works.
There are many misconceptions when it comes to the perceived benefits of black roofs compared with white roofs.
Those living in a northern climate may think that summertime energy savings realized with a white roof will be offset by the elimination of a beneficial heating gain that might occur with a black roof in the winter; that the heating benefit of a black roof is greater than the cooling benefit of a white roof. However, several factors make any potential wintertime heat gain from a black roof relatively immaterial:
- The laws of physics dictate that hot air will always rise. Thus, any heat that is transferred to the interior of a building structure from the outside will remain at the top of the structure, providing minimal heating benefits to occupants below.
- In all parts of North America in the winter, there are fewer hours of sunlight available to contribute to warm a building. In fact, in some areas, there is more than a six-hour difference between peak-summer and peak-winter sunlight. Plus, the angle of the sun is less direct, which also helps to minimize potential warming.
- In many areas, roofs are covered with snow for much of the winter, turning them “white,” and eliminating any potential black roof heat gain.
- The energy required to air condition a building in the summer is usually considerably greater than the energy to heat it in the winter, making the potential for summer energy cost savings much greater with a highly reflective white roof than winter savings with a heat absorbing black roof.
More commercial developers and owners of commercial buildings are installing “cool” roofs to increase the life of the roof and potentially lower operating costs.
Conventional flat roofs are black, and black materials absorb more heat, leading to warmer building. A cool roof is lighter in color and made of materials that tend to reflect light and heat. The result is a cooler building, which cuts air-conditioning costs in the summer. In fact, on a hot day, a black roof can be 70 degrees hotter than the air temperature, compared with only 20 degrees hotter for a cool roof.
In the past, some in northern climes have resisted cool roofs because they were thought to increase heating costs in the winter. Now a recent study of cool and conventional roofs by the Lawrence Berkeley National Laboratory has found that in virtually the entire country, the summer air-conditioning savings of cool roofs are greater than the increased heating cost in the winter.
There are other benefits of cool roofs that make them attractive, particularly for commercial structures:
- Cool roofs tend to last longer because they suffer less thermal stress than conventional roofs do.
- Cool roofs reduce the urban heat island, because the building structure is cooler. The heat island effect in one of the causes of the formation of smog and greenhouse gases.
- A cool roof helps a building qualify as environmentally friendly by the U.S. Green Building Council, which is of interest to a growing number of tenants.
Cool roofs typically cost the same or only slightly more than conventional roofs to install.
Gray, Thomas O. AIA, DRS Architects. “Cool Roofs are Hot.” Post-Gazette Now
3 Sept. 2008. Business. 3 Sept. 2008 <http://www.post-gazette.com/pg/