Cool Shelter

Nov. 1, 2005
Cool metal roofs can be a cost-effective and energy-conscious alternative for school buildings.

With energy costs continuing to climb and budgets getting squeezed from all sides, administrators at school districts, colleges and universities are looking for every avenue that will save money.

At the same time, education institutions are encouraged to promote sustainable communities. Cool metal roofing can save schools money and promote sustainable design at the same time.

Cool metal roofing keeps the sun's heat from collecting on a roof. This lowers the cooling costs for a building, and these savings enable education facilities to reap the benefits for years to come.

The science of it all

The more that heat builds up on a roof, the greater the heat infiltration into the building. Cool roofing keeps the building itself cooler. This results in less demand for air conditioning systems and lowers electricity bills. A cool roof results from two major roof characteristics: the thermal emissivity of the roof and its solar reflectivity.

Emissivity is the ability of a material to emit heat, via infrared radiation, to the surrounding atmosphere. It is measured in values between 0 and 1: the higher the value, the higher the emissivity. Roofs with higher emissivity values stay cooler because the heat is released from the roof to the surroundings. Cool painted metal roofs have a high emissivity — about 0.85.

The more critical factor affecting the roof temperature is reflectivity, which is the measure of solar reflectance of a surface. Values also range from 0 to 1, with higher values indicating higher reflectivity. A typical asphalt shingle roof has a reflectivity value between 0.05 and 0.12. Cool non-white metal roofs have values ranging from 0.25 to as high as 0.40. The solar reflectance for white painted metal is about 0.65.

Certification requirements for reflectivity vary by state and organization. The EPA's Energy Star Program certifies high sloped roofs with a reflectivity of 0.25 or higher. This program does not have an emittance requirement.

Cool metal roofs contain a special grade of polyvinylidene fluoride (PVDF) resin typically referred to as Kynar, its original trade name. Kynar is related to Teflon and has become a catchall phrase for the latest versions of PVDF resins, Kynar 500 and Hylar 5000. These coatings are referenced most often in relation to cool roofs because their finishes are durable and soil-resistant, and the reflectance remains constant over the life of the roof.

Combining the PVDF resin with cool infrared reflective pigments improves the product's thermal performance and takes 80 to 85 percent of the heat out of the coating before it has an opportunity to penetrate into the building. This leaves only a small amount in the metal to attack the building, which affects the efficiency of the thermal envelope.

Cool infrared reflective pigmented coatings contain complex inorganic color pigments (CICPs). These are resistant to heat and aging, and are one of the main reasons that cool roofs reflect more solar energy. Ceramic pigments are added to the resin to make the colors. The sun does not oxidize ceramic, so the color stays true well beyond 35 years.

Previously, white was the only cool roof color as it was the only naturally reflective coating that could stand up over time. Researchers developed CICPs to allow dark roofs to reflect as if they were white through the use of pigments. These colored coatings meet Energy Star requirements, with a reflectivity value of at least 0.25. Since the development of the new CICPs, the ability of manufacturers to offer cool roofs in various colors has become widespread.

Built to last

Typical cool metal roof materials have warranties that last as long as 35 years. Most cool metal roofs are expected to last from 35 to 50 years. Research conducted by Oak Ridge National Laboratories showed that pre-painted metal roofing retained 95 percent of its initial solar reflectance over a three-year period. (The three-year studies are required by the Energy Star program and the Cool Roof Rating Council. This timeframe assumes that a roof will experience most of its degradation within the first three years after construction.)

In addition, fencepost exposure studies from South Florida showed that PVDF-based paint metals maintain their resistance to soiling for at least 30 years. A cool metal roof will cost more than a conventional roof, but the long-term gains can outweigh the upfront costs.

Additional benefits

In addition to lower energy costs, metal roofs have other benefits including the roofs' recycled content and their ability to be used on most buildings.

Many local and state governments encourage — and in some cases require — a certain amount of recycled content and energy efficiency in new public building construction. Cool metal roofs can help to meet those requirements. As noted, these roofs also meet the EPA's Energy Star Program guidelines. Most metal roofs have at least 25 percent recycled content.

If a roof needs to be replaced, the metal is 100 percent recyclable. This helps reduce waste and could even bring in a return on the sale of the scrap.

Metal roofs can be built onto many existing structures — even flat-top roofs. With some retrofitting, a lightweight structural system can be constructed on top of an existing roof (to add slope), and the metal roof would be placed on that.

Praeger is the assistant general manager of the Metal Building Manufacturers Association, Cleveland, and chairman of the Cool Metal Roofing Coalition.

A cool roofing analysis

In 2002-2003, the Paulding County (Ga.) school district built two new elementary schools: Bessie L. Bagget Elementary and Lillian C. Poole Elementary. The schools were nearly identical: same size (90,000 square feet), same HVAC units, and the same orientation to the sun. Both have metal building systems for the gym and cafeteria. The remainder of each school uses conventional structural steel with metal purlins and a metal roof.

It was proposed that the schools be used to study the value of cool roof technology. The goal was to see if one roof was truly more energy efficient than another. Poole Elementary was constructed with a metal roof that contained a Kynar (PVDF) coating that used an infrared reflective pigment (reflectivity of 0.29). Baggett Elementary was built with a metal roof that had a non-cool Kynar (PVDF) coating (reflectivity of 0.12). The roofs are both the same dark-green color. Thermostats for both buildings are controlled at the district office.

The two buildings opened in November 2003. In the first year these schools were open, the building with the cool metal roof, Poole Elementary, saved $8,054 more in energy costs than Baggett Elementary. Because the painted metal finish is warranted for 35 years, the savings for this building could be as high as $282,000, not including the escalating cost of energy.

Solar Reflectance Baggett SR (12%) Cost Poole SR (30%) Cost Total/Annual Savings Annual Electric $67,251 $58,447 $8,804 Annual Gas $16,837 $17,587 -$750 Annual Expense $84,088 $76,034 $8,054 35-year Expense $2,943,080 $2,661,190 $281,890

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