Cool Metal Roofs - Frequently Asked Questions
What is a cool roof and how does it work?
Why is COLOURCOIL Kynar 500 resin type paint system the best for a cool metal roof coating?
What should I ask for when selecting a cool metal roof?
Are there cost savings and financial benefits of using a cool COLOURCOIL Kynar 500 based pre-painted metal roof?
Cool roofing is defined differently by unique performance criteria established in different codes, standards, and incentive programs. In general, a cool roof is one that does not absorb solar energy which allows the surface temperature to remain lower.
During the daylight hours, a roof is constantly subjected to solar energy striking its surface. The term “Solar Reflectance” is a measure of the amount of that solar energy that is immediately reflected from the surface. It is not reflected heat, but reflected electromagnetic energy from the sun. Solar reflectance is reported as a decimal (0 – 1.00) or as a percentage (0-100%). The solar energy that is not reflected away from the surface is absorbed into the outer surface of the roof product and is converted into heat. The heat can be removed by convection as air flows over the surface, or by conduction through the roof material into the sheathing below. The energy that is left can also be re-emitted to the night sky in the form of infrared energy. That re-emitted energy is referred to as thermal emittance, which is also expressed as a decimal (0 – 1.00) or as a percentage (0-100%).
COLOURCOIL Kynar 500 based prepainted steel (i.e. colour coated steel) products for metal roofing are formulated with special inorganic cool pigments which boost the solar reflectance by reflecting more infrared solar energy. These pigments do not affect the visible color, so that darker colors can now perform like lighter colors in terms of their solar reflectance values.
A cool metal roof with high solar reflectance and high thermal emittance would have a lower surface temperature as compared to a roof with low reflectance and low emittance. In the case of a cool metal roof, a lower surface temperature translates into less heat gain into the attic space or living space below the roof. The result is a cooler building and lower cooling/heating energy usage.
The proven durability of full strength 70% Kynar 500 resin type paint systems allows for stable color and film integrity over decades of exposure. This durability is also seen in the retained amount of initial solar reflectance. Thus, paint systems based on the Kynar 500 resin have a surface that does not retain dirt, remains cool, and retains its original color better than any other resin type.
In the design and performance of a building, the owner wants to know that the initial energy cost savings projections are maintained over the life of the roof. Most other roof types have been documented to lose some percentage of their initial solar reflectance due to dirt pick up and overall degradation from the weather. In those cases, the aged performance of these cool roofs is far from what the building experiences in the early years. The strong performance of Kynar 500 based paint system which is maintained over time is especially apparent in the retention of the initial solar reflectance of cool Kynar 500 based prepainted paint systems. In research conducted by Oak Ridge National Laboratory on behalf of the Cool Metal Roofing Coalition Kynar 500 based prepainted metal test panels exposed in South Florida were found to retain 95% of their original solar reflectance over more than 30 years. Typically, dirt pick-up and retention is responsible for many roof products’ solar reflectance dropping over time. This is also true for some conventional resins used for prepainted metal roofing. However, Kynar 500 based prepainted metal roofing readily sheds dirt and does not promote the formation of algae or other microbial growth on the surface which could lower the solar reflectance.
No other prepainted metal roofing product has the high levels of gloss retention, color and chalk performance. The aesthetic appeal of using a Kynar 500 based prepainted cool metal roof is second to none when a building owner is considering what type of cool metal roof to specify.
ARKEMA’s research indicates that there are many reasons why Kynar 500 based paint systems are the product of choice for cool roofing applications. The most weatherable coatings for metal roofing are based on polyvinylidene difluoride resin technology. These resins have superior resistance to photo-chemical degradation. They maintain their gloss and color better than any other resin technologies used in today’s exterior paint systems used in the metal roofing industry. Actual exposure of panels in South Florida’s harsh climate after nearly 40 years substantiates the outstanding performance of the Kynar 500 resin.
To achieve superior resistance to weathering, Kynar 500 based paint systems commonly use durable grades of inorganic pigments and tough acrylic resins. The structure of a Kynar 500 based paint system is thermoplastic in nature, thermodynamically stable and based mainly on the photochemically inert PVDF resin. This combination has been shown to retain the aesthetic and cool properties of Kynar 500 based paint systems for decades. This durability is in stark contrast to the common crosslinked binder chemistries used in polyester and silicone modified polyester paint systems often used in the metal construction industry.
|TSR Values of PVDF Coatings|
|22 Year Florida Panels|
Other research has found that 70% Kynar 500 based paint systems actually achieve higher solar reflectance when compared to identical colors of polyester and urethane resin based paint systems.
|TSR Values of Orange Paints|
|22 Year Florida Exposure|
|70% KYNAR 500 PVDF||0.463||0.439||0.426|
This significant advantage is in addition to the expected superior retention of gloss and fade, and the prevention of chalking of the paint film.
Hence, a cool Kynar 500 based prepainted metal roof product has the best protection against color fade, chalk and erosion of gloss AS WELL AS having the highest solar reflectance of a color formulated in any other paint resin type. There is no better choice than Kynar 500 resin when selecting a durable exterior paint system for cool metal roofing.
A supplier of cool Kynar 500 based prepainted metal roofing products should have either an ENERGY STAR Roof Products label or a product that is listed on the Cool Roof Rating Council directory. These programs certify that the measured cool properties of the product have been substantiated by approved ASTM testing methods. To maximize the energy efficiency, one should ask about the solar reflectance value, and again the higher the better. One should inquire what colors are available in the cool roof category, and which could meet the ENERGY STAR criteria. Depending on where the project is being built, one should ask what the state or local energy code requires for cool roofing, what utility incentives might be available, what state/federal tax credits might be available, and any other rebates, credits, or incentives for which one might be eligible. The warranty of the paint system should be investigated to be sure that a cool Kynar 500 based prepainted product is being offered. Terms for color fade, chalk and film integrity need to be confirmed.
Cool metal roofs can reduce heat transfer to the indoor environment, thereby lowering air conditioning costs. Studies monitoring buildings in California and Florida demonstrate that cool roofs save residents and building owners 20 to 70 percent in annual cooling energy use, and earn LEED or GBI credits. In some countries, cool metal roofs can qualify for tax incentives.
The cost savings can often be determined using the DOE/ORNL calculator for steep slope roofing energy savings. (www.ornl.gov). In comparison to a black roof material, this user friendly calculator allows one to compare cooling/heating energy cost savings in any region of the country, varying the level of insulation, the TSR (Total Solar Reflectance) and TE (Thermal Emittance) of the roof product, and the local energy costs. Using this tool, one can estimate a relative cost savings for the use of a Kynar 500 based pre-painted cool metal roof of various colors and radiative properties.
There are real-world case studies that also demonstrate cool metal roofing’s ability to save money on cooling and heating bills. In 2003, a Georgia school district constructed two identical 90,000 ft2 schools and maintained the thermostats at the district office. The construction of the schools was identical except for the type of pre-painted metal installed on the roofs. On one school, a Kynar 500 based pre-painted metal roof from Architectural Metal Systems was installed with an Evergreen color (solar reflectance of 0.12) and on the second school, a COOL Kynar 500 based pre-painted metal roof was installed with the same color but featuring special pigments in the paint system that increased solar reflectance to 0.29. (paint systems were provided by BASF). After the first year of operation, the school district showed a $8000 savings in cooling/heating utilities at the school with the cool pre-painted metal roof.
In addition to the direct savings in energy cost, the financial benefits for residential roofing include an increase in the appraised value of the home and a higher resale value. A Kynar 500 based pre-painted metal roof is the best investment for a homeowner to make with regard to metal roofing coating. The roof will not depreciate with time and will likely be the last roof installed on the home. This concept maybe referred to as Investment Grade Roofing. Some roof systems begin to depreciate and degrade as soon as they are installed. However, with Kynar 500 based pre-painted metal roofing, the proven durability of the coating allows the performance to remain consistent over the life of the roof. In reality, a Kynar 500 based pre-painted metal roof appreciates the value of the home rather than depreciates as soon as it is installed.
COLOURCOIL PVF2 Series, METALLIC PVF2 Series & DUOTONE Series pre-painted steel for the downstream manufacturing of cool metal roofs are in compliance with Energy Star requirements. The coatings are manufactured from full strength 70% Kynar 500 in combination with high quality reflective pigments.