Sustainable Roofing Strategies

Lowering Energy Costs, Advancing Environmental Goals
This course is no longer active
[ Page 4 of 6 ]  previous page Page 1 Page 2 Page 3 Page 4 Page 5 Page 6 next page
Advertorial course provided by: American Hydrotech, Inc., Cool Roof Rating Council, Evergreen Slate, NEOGARD®, division of Jones-Blair Company, and Petersen-Dean Roofing Systems
Barbara A. Nadel, FAIA

Radiative Properties 

Cool roofs are measured by solar reflectance and thermal emittance. Both properties are measured from 0 to 1, with higher values representing cooler roofs. As the sun's radiation hits the roof surface, some heat is absorbed by the roof and transferred to the building below. Solar reflectance is the fraction of solar energy that is reflected by the roof. Thermal emittance is the relative ability of the roof surface to radiate absorbed heat. When combined, these two measurements comprise the Solar Reflective Index (SRI).

Roof rating programs offer radiative performance data for roofing products. In the private sector, a credible and unbiased third-party rating program measures and reports the radiative properties of roof surfaces. Independent testing laboratories, not manufacturers, are generally best suited to measure relative property values. The two primary values measured are solar reflectance and thermal emittance values.

The U.S. Environmental Protection Agency's ENERGY STAR® program allows manufacturers to rate their products as long as they meet ENERGY STAR's minimum specifications. This government program accepts ratings from the manufacturer's own testing or from a third-party laboratory, uses only solar reflectance as a measure, and only includes products exceeding a minimum reflectance.

The difference in radiative properties of roofing materials can result in large differentials in surface temperatures. A dark colored roofing material may measure 60°F to 100°F above the ambient outdoor temperature, while a cool roof is generally 10°F above ambient temperature.

Energy savings depend on climate, roof performance, insulation, and other design elements. Savings generally average from 10 to 30 percent of the cooling energy needed, and total building electric bill reduction of three to ten percent. These ranges apply to low-rise buildings in regions with significant cooling loads.

FLUID-APPLIED COATINGS AND FOAM ROOF SYSTEMS

During the summer, flat black roofs can reach temperatures ranging from 150°F to 200°F. If dark roofs represent 20 percent of an urban area, ­shading becomes difficult. Typically, 80 percent of low slope roofs are black. White roofs can average 120°F, significantly cooler than black roofs. White surfaces can be applied on black roofs, but this often results in a short term benefit, where effectiveness diminishes quickly.

White roofs provide several advantages when combined with fluid-applied roof coating systems, including durability, sustainability, low-maintenance, cost efficiency, and reflectivity. Their superior resistance to impact, fire and mildew; flexibility, adhesion, and ease of application make this system an excellent protective elastomeric barrier for extending the life of most roofing substrates. (Elastomeric refers to an elastic substance resembling rubber.) Additionally, white roofs can reflect up to 85 percent of solar rays, reduce temperatures of interior spaces, and mitigate the urban heat island effect. They qualify as a cool construction material.

 

Mississippi Coast Coliseum & Convention Center, in Biloxi White roofs with seamless fluid-applied coating
The dome roof of the Mississippi Coast Coliseum & Convention Center, in Biloxi, Mississippi was severely damaged after Hurricane Katrina. It was repaired with a fluid-applied coating and foam roofing system.
Photo courtesy of BaySystems North America.
White roofs with seamless fluid-applied coating systems provide protection against leaks at roof penetrations and flashing transitions, as seen at the Las Vegas Marriott, Las Vegas, Nevada.
Photo courtesy of Jones-Blair Company

 

High-performance Coatings     
           
Roofing systems are available that can leave existing roofs in place, and fix problems after they have been identified, potentially doubling or tripling the service life of existing roofs. This approach decreases energy consumption by 10 to 20 percent, and reduces roof waste that is transported to landfills.

Fluid-applied coating systems provide roof protection for low slope roof applications over various systems, including spray polyurethane foam, EPDM/single ply, modified bitumen, built-up roofs, metal, and concrete. They can increase and extend the life of existing roofs, and provide a cost-effective, sustainable alternative to traditional roofs that require expensive and disruptive tear-off and replacement.

Fluid-applied roof coating systems are appropriate for commercial, industrial and institutional uses. They don't have seams or fasteners, the two major sources of roofing failure. A base coat optimizes adhesion, bond strength, and fire retardant properties. A top coat provides high-reflective coating. Seams are often found at flashing transitions, and when the slope changes, causing leaks to occur. Fluid-applied roof coating performance properties include:

• Seamless, fully adhered:Provides a one-piece finished roof and protection from leaks.

• Light weight:Goes over existing roof without tear-off, reduces re-roofing costs and weather exposure.

• Elastomeric: Accommodates thermal expansion and contraction, no cracking or splitting.

• Light reflective:Keeps buildings cooler, reduces urban heat island effect, lowers energy costs.

• Insulation: Highest R-value, continuous insulated surface reduces energy consumption.

• Sustainable: Renews, not replaces, systems; reduced recycling costs, less landfill waste.

• Hail and wind up-lift resistant:Performs in adverse weather conditions, fewer repairs, reduced insurance claims.

• Environmentally friendly: Low odor, no hazardous waste disposal costs.

Foam Roofing Advantages

With the proper surface coating, polyurethane foam is a re-roofing option with several advantages:

• Thermal insulation:Foam roofs can provide energy cost savings of up to 33 percent.

• Light weight:A finished application weighs about 50 lbs. per 100 square feet, compared to as much as 2,000 lbs. for conventional roofing systems.

• Reduced thermal shock:The insulating properties of foam roofing systems reduce substrate cracking caused by thermal expansion and contraction. • Seamless: No laps or joints to expand, contract, crack or tear, and no seams to fail.

• No migration:Water will not migrate through the closed cell foam. Damage is quickly repaired. • Adaptable: For re-roofing or new construction, polyurethane foam is adaptable to most substrates.

• Fast and economical:No costly, inconvenient tear-off of existing roofing.

• Design flexibility:Polyurethane foam adapts to any slope and shape.

 

 

[ Page 4 of 6 ]  previous page Page 1 Page 2 Page 3 Page 4 Page 5 Page 6 next page
Originally published in GreenSource Magazine.
Originally published in July 2007

Notice

Academies