Surface Color & Solar Gain: White Is The Greenest Color
The U.S. Environmental Protection Agency says that for low-rise commercial buildings, the heat gain through the roof is about 50 percent of the heat gain for the entire building. This heat gain can be reduced not only by using more insulation but also by reflecting the suns radiation. The American Society of Heating, Refrigeration, and Air-conditioning Engineers states that the heat gain through a white roof is half of that of a black roof, and the heat gain through a white wall is two-thirds that of a black wall. Thus, in hot cli- mates, the walls should have a light color and the roof should be white.
Solar reflectivity, which is also known by the term albedo, is a number that indicates how much of the solar radiation is reflected from a surface. An albedo of 0 indicates that no sunlight is reflected or that all sunlight is absorbed, while an albedo of 1 indicates that all sunlight is reflected. The color white has the highest solar reflectivity with an albedo of about 0.9 if it is fresh, clean, and glossy. Clearly then, the most sustainable buildings would have white roofs and appropriately utilize the colors with an awareness of these aspects.
White is the Greenest Color
When it comes to solar-responsive design, choosing white can be the easiest and biggest bang for your buck. Selecting white design elements can save a lot of energy, resulting in saving money and being more efficient and comfortable.
U.S. Secretary of Energy Steven Chu
Do White Roofs Really Save Energy
There are mixed opinions on whether painting your roof white will save energy and money. White roofs reflect heat and cool homes in the summer but in the winter they drive up heating bills, which is not helpful for homes in northern climates. Thus, solar panels that create energy are the preferable option. There is accumulating worldwide evidence in support of white roofs. Keith Oleson of the National Center for Atmospheric Research in Boulder claims that if every roof in large cities around the world was painted white, thereby raising their reflectivity, it would limit the urban heat island effect by a third. Thus, white roofs have more of an advantage in warmer climates and should factor into your decision.
Do White Roofs Save Money
26 Jul 2021, 11:40am
What difference does the colour of your roof make? According to several recent studies a real difference both to your pocket book and the environment.
A report by Lawrence Berkeley National Laboratory in the U.S. finds that opting for a white roof can save you cooling costs in summer and reduce your propertys greenhouse gas emissions .
White roofs win based on the purely economic factors we included, and black roofs should be phased out, said study co-author Dr Arthur Rosenfeld, a Berkeley Lab distinguished scientist emeritus and former commissioner of the California Energy Commission.
In this U.S. study, roofs painted white were found to reflect up to 3 times more sunlight than a black-roofed building, minimising the build-up of heat in roof cavities. The result: white roofs returned a 50-year net saving of US$25 a square metre.
White roofs also outperformed so-called green roofs, which incorporate turf or other vegetation planted topside to reduce GHGs and manage storm water. Green roofs were US$96 a square metre more expensive to install and this premium outweighed the advantage of their longer working life, which is double that of standard rooftops.
The results of the U.S. study are similar to a 2010 South Australian government discussion paper which also found white roofs reduced cooling costs and GHGs.
A white roof can save you cooling costs in summer and reduce greenhouse gas emissions
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Black Or White: Roofing Color As An Energy Strategy
Reflective roofing has gained tremendous market share in recent years as energy costs and environmental concerns have increased.
Black roofing membranes have been the status quo for more than a century. And while they usually do a great job protecting the building beneath, they can also contribute to higher energy costs and the urban heat island effect. The solution may be a simple black-and-white decision.
How it Works
White roofs work because of the physics of sunlight. Dark roofs absorb and hold more than 80% of solar energy, while white ones can reflect 75% of it away. That makes a white-roofed building cooler and cheaper to air-condition.
Its an idea, writes David Fahrenthold, that depending on your perspective, is either fairly new, or as old as Mediterranean villages, desert robes and Colonel Sanderss summer suit. Not only do white roofs reflect more light, but they lose the energy they do absorb more quicklya principle known as thermal emittance.
These before-and-after photographs of a BigLots box store shows the striking difference a reflective roof can make. The white spray-on coating produced a seamless rubber-like surface that decreased roof temperatures by 100 degrees and will last for many years.
On a hot day, a standard black roof can be 100 degrees warmer than the ambient air temperature, while a cool roof might be only 20 degrees warmer. That translates to a cooler building and less air conditioning.
Benefits
Cost
Regulations and Standards
Low And Steep Sloped Roofs
Metal roofs are available with natural metallic finishes, oven-baked paint finishes, or granular coated surfaces.
How they can be made cool: Unpainted metals are typically good solar reflectors but poor thermal emitters, so they rarely satisfy low slope cool roof requirements. Painting a metal roof can increase its solar reflectance and thermal emittance, allowing it to achieve cool roof status. Alternatively, you can apply cool reflective coatings.
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The Heat Island Effect
Urban areas tend to be hot because weve replaced plants with asphalt. Not only that, but they are covered in other dark surfaces, like roofs, and a lot of people. All the dark surfaces absorb lots of heat and dont reflect enough of it.
The urban heat island effect, as its called, contributes to climate change by forcing people to use lots of energy to cool their homes. White roofs help solve this by absorbing significantly less heat, which lowers the overall temperatures in the area. Those cooler temperatures also lead to less smog!
What Advantages Do Cool Roofs Bring To Small Businesses
The primary advantage of a cool roofing system is that business owners can expect to use less energy to cool their small businesses. In hot climates, traditional roofs in the contiguous United States can reach temperatures of 150 to 185 degrees Fahrenheit. A cool reflective roof stays 50 to 60 degrees cooler when exposed to the same summer heat.
Generally speaking, the cost of installing cool roofing during construction is comparable to traditional roofing, as this energy-saving calculator demonstrates, so small business owners may save on energy costs in long-run by installing a cool roof.
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The Benefits And Controversy Of White Roofs
Much ado has been made in recent years about the benefits of installing white or cool roofs. The idea: Light-colored materials reflect heat, while dark roofs, such as those made of asphalt and tar, absorb heat, driving up air-conditioning consumption in order to offset it.
So, in other words, installing a roof that is white or reflective helps save energy and money.
Energy Secretary Steven Chu has pressed Americans to install cool roofs on their homes and business, even committing his own department to making all its new roofs white.
But is the hype about cool roofs legitimate? Should you consider one for your business?
First, the net energy savings generated by white roofs compared with dark roofs depends partly on where youre located. White roofs seem like no-brainers in warm climates like Phoenix, Las Vegas and Miami, where they can keep air-conditioning bills in check year-round. States like California even now mandate white roofs on commercial buildings.
Another question is the overall environmental benefit of reflective roofs and whether they actually do slow global warming. A Stanford study published last year found that the heat being reflected from white roofs may actually exacerbate climate change by contributing more heat to the atmosphere and absorbing more emissions. Earlier studies have suggested that whitewashing roofs can have a cooling effect.
What Does The Science Say
The technical name for a roof that reflects heat is that it has a high albedo.
High-albedo roofing has two scientifically proven benefits: it helps to mitigate the urban heat island effect and reduce cooling energy demand and costs. It can also increase thermal comfort in non-conditioned buildings.
Benefits are both direct and indirect.
Direct benefits to individual buildings occur by reducing absorbed shortwave radiation through the roof. Neighbourhood-scale indirect benefits result from reduced ambient air temperatures, particularly when high-albedo surfaces are deployed on a large scale.
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These conclusions come from several sources, including one, which comprised whole-building energy simulations of a set of archetypical single family residential buildings in three locations with distinct characteristics in the Los Angeles area . This location is about the same degrees latitude north of the equator as many Australian cities are south of it.
The simulations show that benefits from the indirect effect can be the same order of magnitude as the direct effects. The systematic replacement of dark surfaces with white could lower heat wave maximum temperatures by 2 degrees Celsius or more, according to other research from Yale University.
But these benefits depend on the climate and building characteristics. For heating-dominated climates, white roofs may not be appropriate.
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Disadvantages Of Cool Roofs
Many disadvantages of cool roofs can be offset by your choice of cool roofing system, or by adapting other aspects of your buildings heating/cooling system, but its worthwhile to be aware of the possible cons.
The disadvantages of cool roofs include potential:
- retrofitting costs, as converting an existing roof can become expensiveespecially when you include repair work, recoating and cleaning costs
- algae or mold growth, which can occur when a cool roof is installed in warm, humid locations
- condensation in hot, humid climates, which in turn can cause water damage
- glare, as the lighter colors associated with cool roofing materials and coatings can disturb occupants of taller neighboring buildings and
- increased winter heating, as the same reflective and thermal emittance features that make cool roofs so good at controlling temperatures in the summer can lead to increased heating needs in the winter .
What Roofs For Which Parts Of Australia
Research commissioned by the RTAA, performed by the University of Newcastle, found that light coloured tiles yielded energy savings between 25-36 per cent compared to dark coloured tiles.
Lower cooling energy demand was found even with insulation or sarking, although it is smaller.
The sustainability guide Your Home 5th edition states that light coloured roofs are estimated to reflect up to 70 per cent of summer heat gain around 50 per cent more than a dark roof. Dr Chris Reardon, principal author, said: In cooling-dominated climates, you need more than one layer of sarking two layers separated by 25 mm gap plus bulk insulation.
A dark roof could be beneficial in colder, heating-dominated climates like Canberra and possibly even Melbourne, but it shouldnt be considered north of Brisbane.
Many thermal rating tools, such as those used to assess NatHERS ratings, work off historical climate data. Houses built now might not be prepared for the climate change affected temperatures of the future. The publication says that it would be prudent to look at projected temperatures over the 50-80 year lifetime of a house, with CSIRO research predicting temperatures up to 4°C warmer on average by 2100.
Therefore even in southe000rn parts of Australia, homes should be properly insulated to protect both heat and cold extremes, and light roofs should be installed.
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Painting White Roofs Vs Black Roofs
Traditional roofing is typically dark colored or blackcolors that absorb heat faster than brighter colors. However, there are energy-saving benefits to painting your roof whitethis increases your roofs albedo causing more heat reflection, which keeps your small business cooler.
And if you think simply opting for a happy mediumsuch as gray, for instanceis the answer, consider that its not all about white roofs vs. black roofs. For instance, when you compare a white reflective roof with a gray roof, the white roof reflects 80 percent of incoming sunlight compared to the gray roof, which reflects only 20 percent.
The difference means the white roof will stay approximately 55 degrees Fahrenheit cooler than its gray counterpart. Ultimately, painting a roof white to save energy is a great idea, because its one of the simplest ways to increase its solar reflectance and thermal emittance properties.
White Roofs Aren’t So Cool
One of the latest trends in home energy efficiency is the white roof, as described in a recent Sun article . Indeed, they can be useful in certain climates, but I would urge careful consideration before jumping on the cool roof bandwagon in a northern city like Baltimore.
I work for a Pennsylvania-based roofing material manufacturer that produces both black and white roofing membranes, so I am offering what I believe is an informed and unbiased perspective. The idea that white roofs always save energy and reduce pollution is, unfortunately, a green myth.
“Cool,” or reflective, roofs have become very popular over the past 15 years. They are part of the prescriptive requirements of California’s Title 24 energy code, and they are mandated in some northern cities like New York, Chicago and Philadelphia.
However, the perceived benefits of reflective roofs, especially when used in colder climates, is based on sloppy science and self-serving marketing, which led to the belief that white roofs are energy-efficient and more cost-effective than dark ones. White roofs have also been portrayed as a panacea for global warming and the “heat island effect,” in which dark-colored building materials raise the ambient temperature in urban areas by a few degrees in summer.
I am not saying that white reflective roofing doesn’t have its place, it certainly does, and we sell it when the environment and conditions warrant, mostly in southern climates.
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Insulate For Added Energy Efficiency
An energy efficient roof makes a huge difference, but it cant do all the work. If the insulation in your attic or under your roof is not up to snuff, youre still likely to have a problem with unwanted heat. Installing attic fans and rafters can also encourage air circulation in a way that will bring down temperatures beneath your roof. If youre considering energy efficiency upgrades for your roof, you should think of upgrading your attic as part of a package deal.
Dont let a dark asphalt roof absorb heat into your home summer after summer. Start thinking about your new roof today, and choose energy efficient roofing materials to help it pay for itself.
Check out more home improvement and energy efficiency tips in the Live Brighter Blog!
Can Reflective Roofing Save Energy And Help The Environment
Hey Mr. Green,
I liked your article about the amount of space needed to provide solar power in the United States. I have wondered how many roofs would have to be painted white to replace the albedo of the melting ice caps. We live in Houston, Texas, and Ive often thought of painting ours white to reduce our AC bill.
Dianne in Houston
Painting rooftops white may have some promise, although there remains considerable debate about its benefits, especially given that the soot in our air has the potential to trap their reflected heat.
There are several other factors to consider. The cooling effects of the white roof could, for example, force folks to turn the furnace up in winter, thereby offsetting at least some of its benefit. As the EPA politely and rather cautiously reminds us: Please remember the energy savings that can be achieved with reflective roofing is highly dependent on facility design, insulation used, climatic conditions, building location, and building envelope efficiency.
If youd like to dig deeper into the issue and come up with a specific calculation for your location and type of dwelling, the Oak Ridge National Laboratory offers a Cool Roof Calculator. Based on evidence Ive seen, the benefits of the white roof would probably be greater in your area than in cities farther north. Good luck with your research!
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Ways To Work With Your Cool Reflective Roof
Cooling your roof is only one way to improve your small businesss energy efficiency! You can try these other strategies to help you save on your energy use:
As a final note, keep in mind that making changes to your roof or taking steps to reduce your small businesss energy expenditures may qualify you for small business energy tax credits. These incentives allow you to recoup a portion of the expense associated with transitioning to an energy-efficient workplace.
Painting the roof white, or implementing any type of cool roof, is only one step small businesses can make to conserve energy, reduce their carbon footprint and provide a more comfortable workplace for employees and clients alike.
