solar reflectance index Archives

Urban Heat Island, Part 2: How Cool Metal Roofs Benefit Building Owners

Posted on August 22, 2016 by Candy McNamee

In our prior blog post, Urban Heat Islands, Part 1: How Cool Metal Roofs Benefit the Community, we identified the existence of urban heat islands and their contribution to higher air temperatures that are found in urban areas compared to surrounding locations. We also identified a high Solar Reflectance Index (SRI), on a scale of 0-100, as the means to specify materials that can help reduce urban heat islands and benefit entire communities. In this post, let’s focus on the specific benefits to the building owner when cool metal roofs are used.

The Boundy Residence features a cool metal roof

Energy Savings for Cool Metal Roofs

In many commercial and industrial buildings, energy use is one of the largest ongoing operating expenses, meaning that building owners and operators are usually quite interested in lowering or controlling that expense. Cool metal roofs with a high SRI rating can help with that quest. For instance, since air conditioning is commonly a larger cost that heating for many such buildings, it is a natural place to target. Lowering the temperatures at the roof means there is less heat surrounding the building, reducing air conditioning load and directly impacting energy costs.

Comfort in Outdoor Areas

Some building types, such as restaurants, retail, and entertainment facilities, rely on outdoor seating or gathering areas to support their business. If urban heat islands make these spaces uncomfortable to spend time in, the business usually suffers too. Providing these buildings with high-SRI metal roofing can improve the situation.

Long-Term Durability

Building materials can degrade prematurely if they routinely exposed to high heat. The heat can cause them to dry out, become brittle, or simply decompose faster than expected. Using high-SRI roofing is not only good for the longevity of the roofing, it can be good for the durability of the materials directly under the roof as well. Roof sheathing and other substrate materials directly in contact with the roofing receive the same intense solar radiation that the roofing surface does.

Attic spaces below the roofing plane also receive the heat, making attic temperatures in excess of 130 degrees common, causing degradation of materials in those spaces, including mechanical and electrical equipment. That could mean more expansion and contraction of connections and joints or it could mean that air conditioning duct work is being heated, contrary to the efficient operation of the system. In any of these cases, a cool metal roof will help alleviate the negative impacts of solar heat and allow materials to achieve full life expectancy.

Supports LEED Certification

In the Sustainable Sites category of the LEED rating system, Heat Island Reduction can be selected as a credit to receive either one or two points toward certification. This credit relies on both roof and non-roof strategies and looks for calculations of solar reflectance (SR) and demonstrated Solar Reflectance Index (SRI) levels on specified products.

Favorable Payback

All of these benefits above can translate to financial benefits to the building owner or operator. Any cost premium incurred for selecting a high-SRI cool metal roof can likely be realized very quickly in energy cost savings, increased business, or maintenance and durability savings. In addition, the benefits of human comfort and achievement of LEED or other sustainability goals can be realized for the life of the building.

Urban Heat Islands, Part 1: How Cool Metal Roofs Benefit the Community

Posted on August 4, 2016 by Candy McNamee

Summer in the city usually means it’s hot – hotter than surrounding areas. Those who have investigated this phenomenon have identified the presence of “urban heat islands” – places that heat up disproportionately to those nearby.

One reason for this is the predominance of dark asphalt pavement and dark-colored roofing. The significance is that dark surfaces are known to absorb sunlight and re-radiate it back as heat. That’s how thermal solar panels work, but it is also dramatically apparent when walking across a black asphalt parking lot in the summer sun. The heat is coming not only from the sun above, but from the pavement below.

If nearby buildings have dark-colored roofs, the same is happening there. Studies have shown that this re-radiated heat can build up in urban areas and raise the surrounding air temperature by up to 5 degrees Fahrenheit on average. So while it might be a tolerable 85 degrees and pleasant a few miles away, the urban core could be sweltering in a self-induced 90 degrees – even higher on those dark roofs and parking lots.

Measuring Solar Heat

How do we know what materials help or hinder these urban heat islands? First, all materials will absorb and reflect varying amounts of solar radiation based primarily on the color and reflectance of a material. The way to measure that variation is based on ASTM test standards E903 and C1549. These tests are used to determine the solar reflectance (SR) of materials, which is expressed as the fraction of solar energy that is reflected on a scale of 0 to 1. Black paint, for example, has an SR of 0 and bright white titanium paint has an SR of 1 (highest reflectance).

Reducing Heat Islands with Cool Metal Roofs

Taking things one step further, the Solar Reflectance Index (SRI) has been developed as a measure of the ability of a constructed surface, particularly roofs, to stay cool in the sun. It relies on both an initial SR value as well as a thermal emittance value being determined for a material or product. Using ASTM E1980 and values from the Cool Roof Rating Council Standard (CRRC-1), an SRI of between 0 (common black surface) and 100 (common white reflective surface) can be determined. The higher the SRI, the higher the amount of solar radiation that is reflected and thermal radiation minimized, thus creating a comparatively cool surface.

Metal roofing is particularly well suited to achieve high SRI values, minimize heat build-up, and reduce urban heat islands. Recognizing this, many manufacturers test metal roofing products and publish the SRI results, allowing professionals and consumers to make informed decisions. Of course, other roofing materials are tested for SRI values too, but few test as effectively and economically as metal roofing.

(For specific information about the radiative properties of MBCI’s colors, consult our listings in the respective databases on the CRRC and ENERGY STAR websites.)

Benefits to the Community

Specifying and building with high-SRI metal roofs has benefits beyond just the immediate building—reducing urban heat islands keeps excess heat from building up in the surrounding community too. Higher summer temperatures can be detrimental to plants, trees, and people who are outside in urban areas. By using cool metal roofs that reduce the surrounding air temperature, plants don’t lose water as quickly, people are more comfortable, and trees are less stressed. Cooler air temperatures around a building also means air conditioning does not need to work as hard or as often. That translates into less energy use and fewer greenhouse gas emissions from electricity to run the air conditioning—both of which could significantly contribute to cleaner air in the community.

Results

By recognizing the existence of urban heat islands and their impact on people and the environment, those of us in the design and construction field can choose to do something about them. By specifying and installing high-SRI cool metal roofs, the environment benefits, people benefit and our buildings benefit.

Learn more in our blog post, “Code Requirements for Cool Roofs with Climate Zone Specifics.”

Code Requirements for Cool Roofs with Climate Zone Specifics

Posted on June 8, 2015 by Brad Johnson

There is still a lot of discussion—some agreeable and some not so agreeable—about the necessary color of our rooftops. One side of the discussion revolves around keeping the surfaces of our built environment “cool,” so there’s a movement to make all rooftops “cool” by making them white, or at least light-colored. Those on the other side of the discussion claim that cool roofs are necessary to reduce a building’s energy use. Cool roofs can be a really good idea, but let’s not mix up the reasons why cool roofs matter—are we cooling the urban areas (that is, reducing urban heat islands), or are we saving energy costs for individual buildings?
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The average building height in the United States is less than two stories, but “white roofs” are mostly desired in dense, urban areas…and how many buildings here are less than two stories? Tall buildings are typically found in dense, urban areas, with shorter buildings dominating the fringe urban areas. In the suburbs and rural areas, one- and two-story buildings are more the norm. So we have a mix of building heights in the United States, but the conflict is that the “cool roof” focus is often where the tallest buildings exist.

And unfortunately, a cool roof on a 20-story building isn’t going to reduce its energy use, especially if the code-required amount of insulation exists on that roof. Rather, reducing energy use of a 20-story building hinges on the energy efficiency of the 20-story-tall walls—R-value of walls, percentage of windows, and solar blocking eaves, just to name a few items. Conversely, the energy efficiency of a one-story big-box store comes down to its roof. And for these buildings, roof color definitely can make a difference. However, our building codes don’t differentiate based on building proportions, but only on geographic location—and that’s problematic. But as designers, we can improve on the code requirements.

The 2015 International Energy Conservation Code provides specific information about cool roofs, which are required to be installed in Climate Zones 1, 2, and 3 on low-slope roofs (<2:12) directly above cooled conditioned spaces. There are two ways to prescriptively comply with this requirement: use roofs that have a 3-year-aged solar reflectance of 0.55 and a 3-year-aged emittance of 0.75. Notice that initial (i.e., new) reflectance and emittance are not specified; long-term values are more important. The second method to comply is to have a 3-year aged solar reflectance index (SRI) of 64. SRI is a calculated value based on reflectivity and emittance. It’s important to understand why a cool roof is desired and to make appropriate design decisions.

To locate metal roof products that meet the IECC requirements, go to http://coolroofs.org/products/results and use the search function to narrow your results or view our finishes’ SRI ratings on our Cool Metal Roofing page.