A cool roof, simply put, is a roof that’s designed to keep temperatures low even on the sunniest days. “Roofs and pavement can easily reach temperatures that are 50 to 90 degrees Fahrenheit higher than the air temperature around them,” says Christian Kaltreider, a building science research engineer at Pacific Northwest National Laboratory. Installing a cool roof will lower the amount of heat concentrating on the roof, thereby decreasing the need for energy-intensive cooling methods like air conditioning.
While there is no common standard for how well a roof has to perform in order to be called a “cool” roof, all of them will to some degree do two things: reflect the light and heat from sunlight, and more readily release the heat that it has absorbed compared to other roofs.
What makes a roof cool?
Sunlight contains both visible light and infrared energy. Both are either absorbed or reflected by roofs; when it is absorbed, it makes the roof hot. So a cool roof must be a reflective roof. There are two main factors that can contribute to how reflective a roof is: color and material.
Color is the easier of the two to wrap your head around. Lighter colors are more reflective and therefore cooler than darker colors, which soak up light. Just as wearing a white top in the sun will feel cooler than wearing a dark t-shirt, a lighter roof will be cooler than a darker one. This is because the color of a surface dictates what portion of light it reflects back—e.g., a red wall will reflect red light and absorb other parts of the visible light spectrum. The color white includes the whole visible light spectrum, and so all of that visible light will be bounced back out. Whereas the color black reflects very little, and so black objects will absorb most of that light. Hence, blacker surfaces will be hotter. That said, “cool” roof materials can come in a whole variety of colors, not just white.
Different materials also have different reflective properties. Slate tiles, for example, are a naturally very hot material. Wood shingles, on the other hand, are naturally “cool.” Fortunately, many popular roofing materials that are not naturally “cool” can be treated with either slurry or polymer coatings that will make them more reflective. These can be brushed or sprayed onto your roofs, as easily as a coat of paint.
How can energy-efficient roofing save on costs?
According to the Department of Energy, a conventional roof in the sunny summer afternoon sun could reach temperatures of 150 degrees Fahrenheit or more, but under those same conditions, a reflective roof could stay more than 50 degrees cooler. Having a cool roof will lower the temperature in your house and reduce cooling needs inside your house—though how many degrees cooler the interior of your house will be will depend on things like house size and the materials inside your house. But overall, having less need for cooling methods like air conditioning will reduce those associated costs.
Just how much benefit you see from a cool roof will depend on other factors in your home. A cool roof will have a higher benefit on a house with poor insulation, whereas a house with very good insulation may only see mild benefits, says Kaltreider. A house with an unvented attic will also see bigger benefits from a cool roof than a house with a vented attic. A cool roof will have the biggest cooling effect on the air directly below that roof—when an attic is sealed and unvented, that cool air can more effectively help cool the rest of the house. In a house with a vented attic, much of that cool air might be lost to the outside.
How your air ducts are laid out will also have an impact. “In a lot of areas in the U.S., it’s very common to have ductwork and HVAC equipment in the attic,” says Kaltreider. “And in that case, a cool roof can have a pretty big effect because that ductwork is picking up a whole lot of heat from that attic, so if you cool down the attic by having a cool roof, then the energy savings can be pretty significant.”
Some states and cities also have incentive or rebate programs to encourage people to build cool roofs. The Cool Roof Rating Council maintains a database of programs here.
What are the environmental benefits of energy-efficient roofing?
Beyond cooling your personal home, cool roofs can bring benefits to whole neighborhoods, “because they lower the urban heat island effect,” says Holly Samuelson, an associate professor of architectural technology at Harvard University Graduate School of Design. Urban heat islands form because of the abundance of materials like concrete that concentrate and trap heat into a dense space. And the more cool roofs in a neighborhood, the more they combat this effect. A recent study in Geophysical Research Letters found that if cool roofs were adopted widely across London (a city well known for its urban heat island effect), outdoor temperatures would drop about 1.2 degrees Celsius on average, and up to 2 degrees in some locations.1 A city-wide scale adoption of cool roofs in Sydney, Australia could help drop temperatures by 2.1°C – 2.5°C, according to research from the University of New South Wales.2
How can I install a cool roof?
If you’re building a new home or getting a new roof anyway, you might as well put in a cool roof, says Samuelson. There are no upcharges attached to the material cost, installation, or maintenance processes for cool roofs compared to other materials.
When looking to create a new cool roof, the best thing to look out for is the Solar Reflectance Index (SRI) of different roofing materials. SRI values for a material typically fall between 0 and 100 (alternatively, the SR, or solar reflectance, will be a decimal between zero and one), and that number effectively tells you how “cool” it is. “The higher the number, the cooler the roof,” says Kaltreider. The SRIs for white-colored roofs are above 60 and up to 90 for tile roofs, as high as 85 or 90 for metal roofs, and well into the 90s for membrane roofs (mostly flat roofs covered with weather-proofing material). SRIs for dark-colored roofs only reach about 35 or 40 for tile roofs, above 30 for metal roofs, and about 5 to 20 for membrane roofs.
If you have no plans to redo your roof but would like it to be cooler, there are coatings you can put over your existing roof that will improve its SRI without changing its look, though the exact coating you choose will depend on what material you have on your roof. These are often found at home improvement stores like Home Depot or Lowes.
Some reflective coatings are also marketed to be waterproofing. Depending on how many functions they promise and the degree of reflectivity they aim to provide, the costs for these coatings can vary widely, anywhere from $60 to a few hundred dollars for a 5 gallon bucket. There’s lots of affordable options on the market, which means this can be an easy fix for lowering that summer energy bill just a smidge.
More ways to make your home more efficient from the outside-in
Cool roofing is just one step in keeping heat from seeping into your home on those sweltering summer days. Plants, windows, and insulation also have their role to play in keeping your home comfortable no matter what happens outside. Here’s how to get started.
Energy-efficient landscaping
Landscaping can help cool a home through two mechanisms–shade and transpiration. The effect of shading and evapotranspiration from properly placed trees combined can reduce surrounding air temperatures by as much as 6 degrees Fahrenheit–so if you’re looking to cool your home using your yard, consider reaching out to a landscaper.
Smart glass windows
Windows found in most residential and commercial buildings are “static,” which essentially means that the pane can’t change in color or tint. “Smart glass” technology or “smart glass windows” are panels made of glass that can change how much solar energy is allowed to pass through—or how much gets reflected away. Heat gain and loss via windows are responsible for up to 30% of residential heating and cooling energy use—so checking out smart glass tech could help slim down your electricity bill significantly.
Energy-efficient insulation and air sealing
The best cooling systems available still don’t work well for a home with a poorly insulated exterior—what contractors call the “building envelope”—or one that leaks a lot of air. The Department of Energy (DOE) estimates that the average home can save up to 20% on heating and cooling costs by beefing up the insulation in attics, floors, crawl spaces, and basement rim joists, and by addressing air leaks–so investing in insulation and air sealing is crucial for keeping it a comfortable temperature without wasting electricity.
- Cool Roofs Could Be Most Effective at Reducing Outdoor Urban Temperatures in London (United Kingdom) Compared With Other Roof Top and Vegetation Interventions: A Mesoscale Urban Climate Modeling Study, Geophysical Research Letters, Jul. 2024 ↩︎
- Study on the Cool Roofs Mitigation Potential in Australia, UNSW ↩︎