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What’s So Cool About A Cool Metal Roof?

If you haven’t heard of cool metal roofs, keep reading. This article is certainly not an exhaustive description of the topic, but it will give you a general idea of what a cool metal roof is and how it may affect you. A builder’s familiarity with cool roofs likely depends on the area of the country in which he works. If you live in a large metropolitan area, you have probably already run into cool roof requirements through the permitting process. In Texas, the cities of Dallas, Houston and Austin already require cool metal roofs in their area of control. If you work primarily outside a metropolitan area, you may have heard the term “cool metal roof” but you are not completely sure what it is.

A brief note about the limitations of this article: A cool roof is not limited to any one specific construction material, but for purposes of this article I am addressing metal panels specifically and metal building systems in general. It is also important to realize that a building located in Minot, North Dakota and one located in Matamoros, Texas will not have the same roof design requirements. The first is primarily designed to keep warm air in and cold air out while the other is vice versa. This article is solely addressing the later case with roof systems in the southern United States. Finally, this article is addressing commercial construction only. Residential construction has a world of energy requirements.

A cool roof is simply a roof that reflects and emits the sun’s heat back to the sky instead of transferring it to the interior of the building envelope. The “coolness” of the roof is measured by two properties, solar reflectance and thermal emittance (image below is provided courtesy of the Cool Roof Rating Council).

 

There are two different measures you need to be familiar with when referring to cool roofs:

  • Solar Reflectivity (SR)
    The SR value is reported as a number in a range from 0 to 1.0. A value of 0 indicates that the material absorbs 100% of the solar energy that strikes it. Conversely, a value of 1.0 indicates a material reflects 100% of the solar energy striking it. The higher the number the better the cool roof rating. The SR value is used by Energy Star. Both new and used metal panels are tested to obtain their SR value. To be Energy Star rated, new roof panels must have an SR value of 0.25 or greater for steep slope roofs (greater than 2:12) and 0.65 or higher for low slope roofs (2:12 and less). For more information about SR ratings go to www.energystar.gov.
  • Solar Reflectance Index (SRI)
    The SRI is used to determine compliance with Leadership in Energy and Environmental Design (LEED®) requirements. To meet LEED requirements, a roofing panel must have an SRI of 29 or higher for steep slope roofs (greater than 2:12) and a value of 78 for low slope roofs (2:12 and less). Similar to the SR value, the higher the SRI value the better. For more information about SRI go to www.isgbc.org.

Permitting authorities and architects / engineers will use either SR or SRI. It is basically a personal preference for them. On LEED projects, the SRI value will be used exclusively. Metal panel manufacturers typically provide both an SR and SRI value for panels so you have the information you need whichever value is being used.

Cool metal roofs are a small component of the overall green building and sustainable construction movement. The ultimate purpose of cool metal roofs is to reduce the amount of energy consumption in the United States. According to the MBMA, Energy Design Guide for Metal Building Systems, “The entire building inventory in the United States is responsible for 39% of the nation’s total energy consumption, and two-thirds of the electricity (Alliance to Save Energy, 2005). In contrast, the transportation sector consumes 33% of the energy, and the industrial sector consumes 28%. Commercial buildings use 17% of the total energy consumed.” The idea is that by reducing the amount of heat entering a building, electricity demand to cool the building is reduced. Less electricity demand translates into less demand for natural gas, coal or etc. to produce electricity.

So far we have addressed what a cool metal roof is. The next important question is, “How are cool metal roofs going to affect me?” If your job involves obtaining building permits and/or purchasing metal building systems, you will definitely be affected sooner or later. There are a couple of avenues where you may run into a cool metal roof requirement soon.

  • Municipal Permitting Authority
    As mentioned earlier, several cities in Texas already require cool roofs. I know for certain that Dallas, Austin and Houston do, and I am sure other cities in Texas as well as other southern states have similar requirements. The local permitting authority is the entity that will ultimately enforce the energy code in the United States. Of course the permitting authority’s enforcement power is the withholding of the certificate of occupancy.
  • There is a slow but steady trend for municipalities that do not currently require cool metal roofs to begin doing so. The reason for this is that the International Building Code includes a chapter on energy efficiency. In the IBC 2012 edition, Chapter 13 “Energy Efficiency” is the shortest chapter in the entire code book. The criteria simply states that, “Buildings shall be designed and constructed in accordance with the International Energy Conservation Code.” As of the 2009 edition of the IECC, a cool roof has not been a requirement of the code. However, the 2012 version of the IECC is rumored to have mandated a cool roof in certain areas of the country (I haven’t seen a copy of the IECC 2012 yet to be able to confirm that fact).
  • LEED Project
    Another area where a builder will definitely run into a cool roof requirement is when supplying a metal building system for a LEED project. Cool metal roofing can qualify for 1 point within ‘LEED-NC 2009, Sustainable Sites Credit 7.2, Heat Island Effect: Roofing’ by using roofing materials that have a Solar Reflectance Index (SRI) minimum of 78 for low sloped roofs (≤ 2:12) or a minimum of 29 for steep sloped roofs (> 2:12).
  • Project Specification
    Even if you are not building inside city limits and the project is not seeking LEED certification, a builder may still encounter a cool metal roof requirement in the Architects / Engineers specifications. Cool metal roof requirements are appearing more commonly each year in specifications as architects update their standard specification manual.

And finally, here a few facts and recommendations for you to keep in mind when you have your first job with a cool metal roof.

  • Cost to Comply
    You will be pleasantly surprised to know, there is not a high cost to comply with cool metal roof requirements. However, your color selection may be severely limited. For example purposes, I will use MBCI’s Commercial / Industrial color chart below.
  • Let’s assume you are building a low sloped metal building system (2:12 roof slope or less) in the city of Anywhere City, USA, and the city requires an SRI rating of 78 for low sloped metal roofs. That means you need a panel with an SRI value of 78 or higher.
  • Now refer to the MBCI color chart above. Below each color name is listed an SR and SRI value (you may have to zoom in to see the numbers). We are interested in the SRI value for this example. Notice that there is only one color option in the Signature® 200 line of colors that meets the city of Anywhere City, USA SRI requirement – Solar White with an SRI value of 91.
  • The cost for Solar White is the same as all the other colors in Signature® 200 line of colors (Polar White may cost slightly less than the others). Therefore, if you were already planning to purchase a painted roof panel for your building in Anywhere City, USA the cool metal roof requirement does not impose an additional cost on you. However, your color options are severely limited. It’s Henry Ford’s Model T color option, you can have any color you want – as long as its Solar White. (It is important to note that an unpainted Galvalume panel would not meet the SRI rating in this example either.)
  • Consider Roof Slopes Greater than 2:12
    Refer again to the example building in Anywhere City, USA. Suppose you have informed your customer that a Solar White finish panel will meet the permitting requirements for a cool metal roof, but your customer tells you he doesn’t want a white finish roof panel. Is there another option? Yes there is. By simply changing the roof slope from a low slope roof (2:12 or less) to a high slope roof (greater than 2:12), you lower the required SRI value to 29. Now instead of one color meeting the SRI requirement all fourteen of the Signature® 200 colors meet the requirement. As cool roof requirements become more common place, expect the new standard roof slope increase to something above 2:12 in an effort to provide customers more color options.
  • Communicate with the Permitting Authority
    There are several caveats regarding cool metal roofs and permitting authorities. First, determine whether your permitting authority has a cool roof requirement. The best time to determine this is before you purchase your roof system. You definitely want to know before the roof panels are manufactured. The absolute worst time to find out that your permitting authority requires a cool roof is after you install the roof system. The only fix for the non-complying roof panels is to remove them and replace them with a color that meets the cool roof requirements.
  • Second, determine your permitting authority’s specific requirements regarding a cool roof. For instance, do they require an SR or SRI value? It is extremely important to determine the specific value required for low slope and high slope roofs. Do not assume that since you have successfully met one city’s cool roof requirements all other cities will have the same requirements. They may not. Each municipality is free to set their own specific requirements.
  • Communicate with Your Metal Building Manufacturer
    Finally, communicate with your metal building system manufacturer regarding the cool roof requirement. Verify that the supplier has panel finishes that will meet the permitting authority requirements. It is also a good idea to determine how many color options the supplier has that will meet the requirements.

Cool roofs are here to stay. In the near future, the color pigment technology will probably improve to the point that all colors meet cool roof requirements. Until then, it pays to pay close attention to specifications and check with permitting authorities before you build.

Posted in: Roofing

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