INTRODUCTION
We hear a lot about climate change and global warming. We're told that the situation has become very serious. But when it comes to how we can help, the advice seems inadequate.
And let's face it... Most people aren't willing to make an effort, particularly if it involves sacrifice. Well, it doesn't have to. Your roof can do the job for you!
That's what "cool roof" technology offers, effortlessly reducing your cooling energy use by 20-70%. (That's a number straight from the Environmental Protection Agency). When the sun's infrared radiation hits a traditional roof, it heats it up, which increases inside temperatures. To cool things down, your air conditioner works harder, driving up your electric bill.
Cool roofs do two things. They reflect incoming radiation back into the atmosphere (reflectance) and help your roof release trapped heat (emittance). This keeps your home cooler and reduces your energy use, but there's other benefits as well.
Reduced energy use means that power plants don't have to work as hard, so they produce fewer emissions. Your roof and its insulation last longer thanks to a decrease in temperature extremes and expansion/contraction cycles. This saves you money in the long term and reduces landfill waste. And with lower rooftop temperatures, the surrounding outside air becomes cooler, which helps mitigate the "heat island effect". This, in turn, reduces smog levels.
THEN WHY AREN'T ALL ROOFS "COOL"?
In part, it's because of some pervasive myths.
Myth #1: "Cool roofs are only available in white." Nowadays, you can find cool roofing materials in a wide variety of colors. There's no getting around the fact that lighter colored roofs will save you more money than dark ones, but if you're set on having a black roof, a cooler alternative is available.
Myth #2: "Cool roofs only make sense in warm climates." The assumption is that winter heat losses will outweigh summer benefits. However, in many cold climates, rooftops don't factor in much when it comes to winter heating. Snow creates a cool roof of its own. Winter days are shorter and tend to have more frequent cloud cover, decreasing infrared radiation. Lastly, the sun is lower in the sky, with rays hitting roofs at an angle rather than from overhead.
In summer, on the other hand, rooftops are a big factor. The sun is directly overhead, skies tend to be clear and days are longer and hotter. Summertime is also the time of "peak demand", when energy prices in many localities shoot up. While it's true that the payback time for a cool roof will be longer in cold climates, it still makes energy sense for most buildings that use air conditioning.
Myth #3: "You can get the same benefits from better insulation." There's no doubt that good insulation is critical to making a building energy efficient. It should go hand in hand with cool roofs wherever possible. However, insulation alone can't decrease a building's exterior temperature, meaning that it won't reduce the heat island effect. Which brings us to...
WHAT DOES THIS HAVE TO DO WITH GLOBAL WARMING?
In November of 2005, New Science magazine caused a stir when they theorized that the rise in global temperatures might be "cancelled out" if everyone embraced cool roofs. Not only would cool roofs produce an immediate drop in local temperatures, but the resulting reduction in energy use would lower greenhouse gases produced by power plants. Greenhouse gases are what drives global warming.
In April of 2006, Arthur Rosenfeld, a founding father in the field of energy efficiency, published his paper 'Cool Roofs to Save Money and Delay Global Warming'. This much more thorough work showed that even a relatively small deployment of cool roofs in major tropical and temperate cities would slow global warming's progress by one critical year.
Delaying global warming buys us time to reduce our emissions elsewhere. For example, we can create more efficient appliances, generate more of our energy sustainably and reduce vehicle emissions, all of which help counter climate change. (Speaking of cars, light colored vehicles need less air conditioning, which gives them a 2% boost in fuel efficiency.)
IS MY HOME A COOL ROOF CANDIDATE?
Cool roofs save the most money for buildings with one or more of these characteristics:
- Significant air conditioning use in summer
- Little or no insulation
- Ducts running through the attic
- Clear skies most of the summer
- No emissions, such as from chimneys
For more insight, try the DOE Cool Roof Calculator. (If your building has an enclosed attic, read on for more accurate tools.) Also try the Energy Star Roofing Calculator.
According to a review by Nexant, Inc, these calculators tend to underestimate how much money you can save. Instead, they recommend the more comprehensive analysis tools listed at the DOE's Building Energy Software Tools Directory.
HOW DO I EVALUATE COOL ROOFING PRODUCTS?
As we mentioned earlier, "reflectance" refers to how much incoming solar radiation a material reflects back into the atmosphere. "Emittance" refers to how much heat is released by a material rather than being absorbed.
Reflectance and emittance ratings for cool roofing products generally decrease in the first year of use, then level out. This is primarily due to dirt buildup. To qualify for the Energy Star label, materials for low-slope (relatively flat) roofs must have an initial reflectance of .65 and a 3 year reflectance of .50. Materials for high-slope (steep) roofs must have an initial reflectance of .25 and a 3 year reflectance of .15.
Cool roof incentive programs tend to have stricter requirements. Typically, they look for an initial reflectance of at least .70, and an initial emittance of at least .75.
Roughly 90% of US homes are topped with asphalt shingles, the hottest roofing material available. According to the Lawrence Berkeley National Laboratory, a generic dark brown asphalt shingle has a reflectance of .08, but its "cool" counterpart has a reflectance of .27. That means that even the worst roofing choice should meet Energy Star requirements, so long as a cool version is used.
Here's how cool roofing materials rank:
- Field-Applied Coating (.92/.87)
- Factory-Applied Coating (.90/.89)
- Single-Ply-Thermoplastic (includes TPO, PVC, etc.) (.88/.87)
- Single-Ply-Thermoset (includes EPDM, Hypalon) (.85/.87)
- Built-up Roofing (incl. asphalt and coal tar pitch) (.80/.90)
- Modified Bitumen (.84/.81, .75/.93)
- Metal (.75/.86)
- Tile or Slate (.76/.84)
- Asphalt shingles or shakes (.27/.92)
There are other types of roofing materials that you may want to consider as well. Sod or "green roofs" are best for roofs that can handle extra weight. They keep infrared radiation from ever reaching the roof. In addition, they also lower the temperature of the outside air, reduce stormwater runoff, sequester carbon dioxide and replace lost green space in urban areas. On flat roofs, they can be rooftop gardens, providing food, beauty, wildlife habitat and a bird's eye view.
Then there's solar shingles. These take advantage of incoming solar energy to produce clean electricity. There are also thermal shingles, which heat water more efficiently than a hot water heater.
Both options tend to cost a lot more upfront than cool roofs, but for many, the added benefits are worth it.
Now back to our subject. When it comes to colors engineered to be cool, white is the coolest, but other colors may not rank the way you'd expect:
- White
- Yellow
- Pearlescent
- Red, orange
- Green
- Gray, blue, purple
- Brown, black
The Cool Roof Rating Council was created to provide accurate testing, evaluation and labeling of cool roofing materials. They have a searchable database online, and you can even sort your results.
Of course, there are other important considerations when choosing a roof, such as its...
- Purchase price
- Installation cost
- Reputation
- Weight (Can your building safely support the load?)
- Fire rating
- Ability to protect your building
- Maintenance requirements and costs
- Manufacturer's warranty, including exclusions and hidden costs
- Durability
- Long-term color retention
- Resistance to dirt accumulation
- Retardance of biological growth
- Coverage under your homeowner's policy
- Acceptability to your neighbors
- Overall environmental impact, including runoff and recyclability
HOW MUCH WILL IT COST?
Costs vary by material. Some cool products cost the same as their standard counterparts, while others cost more. Some even cost less, since removing an old roof costs more than simply covering or coating it.
Of course, these are only the upfront costs. Remember, most cool roofs last longer than their non-cool counterparts and save more money on electric bills. And if you ever have to replace your air conditioner, you should be able to use a smaller unit, thanks to your reduced energy use. On the other hand, some cool roofs come with extra maintenance costs.
Incentives for cool roofs range from rebates to tax deductions and loans, and come from utility companies, federal, state and local governments. Usually, these are offered through energy efficiency programs, although there are programs specifically for cool roofs. Residential roofing rebates average around .10 cents per square foot, while commercial roofing rebates average around .20 cents per square foot.
Use the DSire database to find incentives. California residents can find incentives at Flex Your Power. Whichever database you use, talk to the program manager to see if you qualify. Your roofing manufacturer may also be able to advise you on incentive programs.
Some cities provide home maintenance rebates to help seniors, single parents, low income households and disabled individuals maintain the exterior of their homes. Check to see if yours is one of them.
Once you've selected a roof, ask the manufacturer if they provide a better warranty or other incentive if you'll work with one of their preferred contractors. If not, the National Roofing Contractors Association provides guidance on choosing a installer, plus a toll-free number to call for contractors in your area. Try to find one with hands-on cool roofing experience.
So there you have it... the what, why, how and how much of cool roofs!
