About Cool Surfaces
Cool roofs are simply roofs designed to reflect more sunlight than a conventional roof. By reflecting more sunlight, these roofs absorb less heat, keeping the building and surrounding area cooler.
Cool walls, similarly, are exterior walls that efficiently reflect solar energy and radiate heat. Both cool roofs and walls come in various colors and styles – they aren’t necessarily white.
Cool surfaces are not a new concept.
Travel photos from the Mediterranean and the Middle East often show a landscape of homes with white roofs and walls. These are, in fact, cool surfaces and have been a common architectural element for thousands of years. Evidence of cool roofs and walls in vernacular design dates back to ancient Rome and Greece. Although cool roofs and walls are some of the most cost-effective ways to reduce the temperature inside houses during the hot summer, western architecture has yet to adopt them widely.
The Department of Energy (DOE) initiated research on the benefits of cool roofs for energy savings and reducing urban temperatures during the summer in the 1980s. More than twenty years later, the influence of this research is felt globally as cool roofs now dominate the commercial roofing marketplace in warm and hot climates, partly due to the actions of the state of California. Following the summer of 2001, which saw rolling blackouts, California revised its energy code to mandate cool roofing for most commercial buildings with low-sloped roofs. Today, national and international building energy standards prescribe cool surfaces for low-slope roofs, steep-slope roofs, and exterior walls in certain climates. These initiatives have not only helped public utilities reduce peak demand for electricity but also contributed to averting future power outages, marking a significant step towards global sustainability.
(Adapted from CoolCalifornia.org)
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Dark colored roofs |
Dark roofs absorb more heat from sunlight, increasing indoor temperatures and cooling costs. Cool materials can have a more significant impact on heat reduction when installed on dark roofs. |
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Roofs in direct sunlight |
Roofs that receive a lot of direct sunlight, such as south-facing roofs or roofs without significant shading from trees or other buildings, absorb more heat. |
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Flat (or low-sloped) roofs |
Flat, low-sloped roofs are ideal surfaces for applying cool roof coatings or materials. The uniform surface makes the installation process simpler and more cost-effective. |
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Buildings without AC |
Cool roofs help maintain a more comfortable indoor temperature, making living and working spaces in buildings without AC more bearable during hot weather. For buildings with cooling, cool roofs help the ACs run better. |
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Poorly insulated buildings |
Poorly insulated buildings struggle to maintain consistent indoor temperatures. Cool roofs and walls work best on buildings with little insulation. These surfaces can also improve the performance of the building's insulation. |
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Buildings with high cooling demand |
Buildings with high cooling/refrigeration needs, such as grocery stores or cold storage facilities, require a lot of energy to maintain low temperatures. Cool roofs can decrease energy demand, leading to lower operational costs. |
Cool roofs and walls come in a variety of colors and materials. While lighter-colored materials tend to be better at lowering temperatures, cool surfaces do not have to be white. There are “cooler” versions of dark-colored roof and wall products that can suit your aesthetic preference.
Darker-colored "cool" materials incorporate innovative pigments or granules that are highly reflective in the near-infrared (non-visible) portion of the solar spectrum. The image below demonstrates how dark-colored cool roof materials are “cooler” than traditional roofs. The value noted is the materials’ solar reflectance, a measure of how well a surface reflects sunlight. A higher number indicates a cooler roof. White roofs can have a solar reflectance of 0.60-0.90.
Depending on the type of roof material you are looking for, there are often multiple colors to choose from. As seen in the image below, asphalt shingles also come in a variety of colors. To explore what options may be available to you, visit the Cool Roof Rating Council's product directories.
The cost of a cool roof depends on the roof size, cool roof product (e.g., coating, membrane, shingle), installation complexity, and whether it’s a new installation or retrofit. A cool roof coating can cost between $0.50-4.50 per square foot, depending on the material, plus labor. Remember to also factor in energy savings when considering cost: a NYC study found that a reflective coating can reduce AC costs by 50% in a one-story building, 25% in a two-story building, and 10% in a five-story building.
The “heating penalty” refers to a potential increase in heating bills in the winter months since the roof reflects more heat than it absorbs. While this may occur, it is not a significant issue in the contiguous United States for several reasons:
- The cost savings from cool roofs and walls in the summer months typically outweigh the penalty in the winter months.
- As the climate shifts towards warmer temperatures, winters are becoming milder, and summers are becoming longer and hotter. This trend increases demand for cooling, further enhancing the value of cool surfaces.
- The heating penalty is a minimal concern in cold climates because snow naturally reflects sunlight, acting like a cool roof.
The maintenance required for a cool roof is similar to that of a conventional roof. Precautions should be taken to limit foot traffic to avoid damage or degradation to the reflective coating or covering, which can reduce its effectiveness at reflecting heat. To ensure maximum performance and lifespan, keep the roof clear and free of debris. For roofs with equipment on them, delineate pathways on the roof to access the equipment, minimizing foot traffic and potential damage.
As an added benefit, cool roofs and walls can oftentimes reduce long-term maintenance costs as they last longer by not expanding and contracting from heat absorption. In many cases, cool roofs can extend the warranty of the roof.
Consider converting to a cool roof and/or wall if any of the following apply to you:
- If your roof is due for replacement
- If your exterior walls are in need of an upgrade
- If you're experiencing high cooling costs
- If you're planning to install solar panels
- If you're experiencing discomfort due to excessive heat indoors
Cool roofs are a smart choice for new construction or a roof replacement. It is generally not cost-effective to replace a structurally sound roof solely to enhance solar reflectance, but property owners can consider applying a reflective coating on the existing structure to achieve cool roof benefits.
Cool roofs are a great complement to rooftop solar and equipment. Studies have shown that by lowering roof temperatures, cool roofs can improve the efficiency of solar panels, helping them maintain optimal performance and extending their lifespan. Similarly, cool roofs can enhance the efficiency of HVAC systems by reducing the surrounding temperature, potentially extending their operational lifespan.
If your building already has rooftop solar, you may need to coordinate with your solar developer to temporarily remove the panels while the cool roof coating gets applied and then reinstall the panels again. Talk to your roofer and solar developer about the best approach to ensure a smooth and effective installation process.
If your building does not have rooftop solar but you are planning to install it, it is generally advised to apply/install cool roofs before solar panels so it does not interfere with the panels. Inform both your cool roof contractor and solar developer of your plans to have a cool roof-solar complement at the onset of the project. Once the cool roof is applied or installed, coordinate with the solar developer to install the solar panels.
If your building has rooftop HVAC equipment, plan to delineate pathways on the roof to reach the equipment so you can minimize foot traffic and reduce potential damage to the cool roof.
It is always important to consult the shingle manufacturer before applying any type of coating to an existing asphalt shingle roof. Coatings can be used to increase solar reflectivity, resist biological growth, or increase roof service life. However, many manufacturers do not recommend field-applied coatings due to the risk of damage to the shingles. In addition, state or local building codes may prohibit this practice, so homeowners should also consult local building and zoning departments to determine whether coatings are allowed in their area. For more information, see the Asphalt Roofing Manufacturers Association Technical Bulletin.
Building owners can explore the following other climate-smart roof solutions:
- Green roof: A roof with plant cover such as a lawn, garden, or small trees. The cooling effect comes from the evaporation of water from plant surfaces.
- Green-blue roof: A roof that retains rainwater and has a controlled drainage system that releases water over a 24-hour period. It can be connected to cisterns that store rainwater for reuse, but the roof itself is not a storage system. Can include open water surfaces or be placed beneath paving or shingles.
- Solar roof: A roof with solar panels mounted onto the roof to generate electricity and absorb heat. Solar roofs are fully compatible with cool roofs.
Compared to other climate-smart roof options, cool roofs are easier and less expensive to install. They are the fastest to install and provide immediate benefits. Also, they do not significantly affect the structural load of a building, so they can be implemented without requiring major structural modifications or reinforcements.
Albedo
Albedo, or solar reflectance, is the ability of a surface material to reflect sunlight on a scale of 0 to 1 (0 - 100%).
Asphalt Shingles
Asphalt shingles are roof shingles consisting of overlapping panels coated with asphalt (a mixture of mineral aggregate and bitumen) and granules. It is one of the most widely used roofing materials because of its relatively inexpensive cost and simple installation.
Built-up Roof
Built-up roofs (BUR) are continuous, multi-ply roof membranes consisting of fabric reinforcement layers between asphalt/coal tar bitumen. While traditionally dark-surfaced, BUR can achieve cool roof effects by installing reflective marble chips or gray slag on the surface or finishing with a mineral-surfaced sheet.
Cool Roof
Cool roofs are designed to maintain a lower temperature than traditional roofs when the sun is shining.
Green Roof
Green roofs, also known as garden or vegetated roofs, are rooftops planted with vegetation. They reduce stormwater runoff by absorbing rainwater and provide urban heat island mitigation benefits.
Heat Transfer
Heat transfer is the movement of heat from a higher-temperature region to a cooler-temperature one until two regions reach the same temperature (thermal equilibrium).
Low-slope Roof
A roof surface with a slope (rise to run) ratio of less than 2:12 (9.5 degrees from the horizontal).
Metal Roof
Metal roofs are available in various shapes, textures, and colors. They can come in natural metallic finishes, shingles, or shakes. Cool colors and coatings can be applied to metal roofs to achieve greater solar reflectance.
Single-Ply Membrane
A single-ply membrane is a roofing material consisting of a flexible synthetic polymer-based sheet applied to the roof in a single layer. It is rolled onto the roof and attached with mechanical fasteners, adhered with chemical adhesives, or held in place with ballast (gravel, stones, or pavers). Single-ply membranes come in many different colors, including white and other reflective colors.
Smog
Smog, derived from the words smoke and fog, describes air pollution containing ozone and other reactive chemical compounds formed by the reaction between sunlight, hydrocarbons, and nitrogen oxides, primarily from automobile exhaust.
Solar Reflectance
Solar reflectance, also known as albedo, is the fraction of sunlight that a surface reflects. Sunlight that is not reflected is absorbed as heat. Solar reflectance is measured on a scale of 0 to 1 (0 - 100%), where higher numbers indicate more reflective surfaces.
[Technical definition: The ratio of the reflected flux to the incident flux.]
Solar Reflectance Index
Solar reflectance index (SRI) is an alternative metric for comparing the coolness of roof surfaces. It is a temperature scale calculated from solar reflectance and thermal emittance. The higher the SRI, the cooler the roof will be in the sun. For example, a clean black roof has an SRI of 0, while a clean white roof has an SRI of 100. Many building codes and utility rebates now specify SRI.
[Technical definition: A calculated value that combines Solar Reflectance and Thermal Emittance into a single metric following Approach I from Section 6.1.1 of ASTM E1980-11 (2019) using only equations 1 and 3 and a convective coefficient of 12 W·m-2 ·K-1 (medium wind speed). Values typically range from 0 to 100, with especially dark or reflective products exceeding these bounds.]
Steep-slope Roof
A roof surface with a slope (rise to run) ratio greater than or equal to 2:12 (9.5 degrees from the horizontal).
Thermal Emittance
Thermal emittance describes how efficiently a surface cools itself by emitting thermal radiation. Thermal emittance is measured on a scale of 0 to 1 (0 – 100%), where a value of 1 indicates a perfectly efficient emitter.
[Technical definition: The ratio of the radiant heat flux emitted by a specimen to that emitted by a blackbody radiator at the same temperature]
Tile Roof
Tile roofs are typically made of concrete, clay, or slate. The color of clay and slate tiles differs depending on the earth's composition, but clay tiles can also be glazed to provide waterproofing layers and customized colors. Concrete tiles are colored throughout with a pigment additive during manufacturing or coated with a cementitious color coating.
Urban Heat Island
The urban heat island effect describes a regional elevation in air temperature resulting primarily from the replacement of trees and vegetation with buildings, roads, and other heat-absorbing infrastructure. Air temperatures in many urban and suburban areas can be 2 - 8 °F warmer than in rural surroundings.
(Technical terms adapted from CoolCalifornia.org and CRRC)