Over the past few decades, rooftops have become a large contributor to excessive heat issues. As many as 90% of roofs in the United States are poorly designed and built with dark, non-reflective, heat-absorbing materials. Because of this, rooftop temperatures can rise up to 90 degrees above the prevailing air temperature. One area where this situation is commonly seen is in dense, populated areas such as cities, which have adopted the term “heat islands” due to their drastic temperature increases. Research states that average city air temperatures can be up to 5.4°F warmer than their surroundings during the day and up to 22°F warmer during the evenings! In light of this, cool roofing is one of the main strategies used to prevent future occurrences of heat island effects. Cool roofing is an emergent and powerful technology used for temperature control of buildings and areas. A cool roof prevents heat absorption by reflecting the sun’s heat and then emitting its radiation back into the atmosphere. By doing this, cool roofs allow for a more comfortable and controlled indoor environment.
Past, Present, Future
Although cool roofing is one of the most significant green technologies today, it is not a new concept. In the 1980s, research was conducted by the Departments of Energy in California and Tennessee to analyze “solar radiation control coatings” on rooftops. It was found that energy costs decreased when these coatings were used; however, this technology was not significant to designers and builders at the time. More research was also done at the same time to study the impact of light-colored coatings on rooftops. It was found that innovations such as vegetated roofs and solar-reflective roofs could lower overall air temperatures in urban areas.
After 20 years, these technologies emerged in 2001 in California. Due to excessive demand for energy and an insufficient power supply, rolling blackouts were occurring frequently. Cool roofing techniques were installed to lower both peak energy demands and electricity costs, while also avoiding future blackouts in the area.
Today, cool roofing techniques are seen in various areas throughout the world. From rural areas to urban cities and everything in between, a cool roof can be implemented to suit the needs of almost any building.
Benefits of Cool Roofs
Costs are one of the major benefits of cool roofs. Cool roofing is known to be very affordable overall, with energy savings ranging from 7%-15% of total cooling costs. Like all roofing projects, however, costs can vary depending on multiple factors: project size, project location, climate, and ease of roof access. Some projects have very low installation costs with little to no maintenance, while the associated energy savings provide a great incentive for consumers. Payback periods can be as low as six years. Other benefits of cool roofs are:
- Lower Temperatures: During hot months, internal building temperatures are much lower than those in buildings that use traditional cooling techniques.
- Reduced Maintenance Costs: Many cool roofing types require little to no maintenance. Also, cool roofs typically have a longer lifespan than conventional roofing systems, lowering overall maintenance costs and extending roof life.
- Reduced Energy Use and Cost: Since less air conditioning is needed during hot summer months, energy bills are lowered. One study estimates that buildings with cool roofs use up to 40% less cooling energy than buildings with dark-colored roofs.
- Mitigated Heat Island Index: Less heat will be maintained during the day in dense urban areas. Not only will the overall building temperature be lower, but the surrounding ambient temperature within the urban area will decrease as well.
- Reduced Air Pollutant Emissions: Since air conditioning units will be used less frequently, the subsequent decrease in energy demand will result in reduced burning of fossil fuels and, therefore, reduced emissions (including NOx) and greenhouse gases (such as CO2).
- Improved Air Quality: Reduced emissions and improved air quality go hand-in-hand. With fewer pollutants being emitted into the atmosphere, overall air quality will improve. Smog, which is produced by ground-level ozone, sulfur dioxide, nitrogen dioxide, and carbon monoxide, can be significantly lowered in urban areas and provide healthier breathing conditions for the population.
- Utility Rebates: There are multiple incentives for sustainable efforts and many pertain solely to cool roofing strategies. Please refer to GBA’s Financial Incentives packet to learn about these opportunities!
How it Works
The “coolness” of a roof can be determined by comparing two factors: solar reflectance and thermal emittance.
Solar reflectivity, or albedo, measures a roof’s ability to reflect sunlight and heat away from a building. It is rated on a scale from 0 to 1 (or 0%-100%), with higher values representing a cooler roof. The most efficient cool roof reflects more than 65% of solar energy away from the building, only absorbing a small percentage into the structure’s interior. Meanwhile, conventional roofing materials only reflect a mere 5% to 15% of this energy.
Thermal emittance refers to a roof’s ability to release absorbed heat. A high thermal emittance is preferable in warmer areas so that the heat is not held within the roof and, therefore, the building. Thermal emittance is also rated on a scale from 0 to 1 or 0%-100%.
Solar Reflectance Index (SRI) is a value that incorporates the two radiative properties above, with a single value produced to represent a material’s temperature in the sun. SRI is measured on a scale from 0-100 or 0%-100%, with a higher value representing a cooler roof. Zero refers to a temperature as hot as a black surface, while 100 refers to a temperature as cool as a white surface. SRI is very important in determining how well a material and its color will work on a roof.
Types of Cool Roofs
When considering the residential, industrial, and commercial sectors, no two buildings are the same. As a result, there are multiple types of cool roofs designed to suit various building designs and structural components. Different techniques are used for low-sloped and steep-sloped roofs.
Low-Sloped Roofs have an extremely flat roof line with a slight incline for draining needs. These types are typically found associated with industrial, institutional, and commercial buildings and are great candidates for cool roofs because of their usual large roof surface areas. Since the roof is the main source of heat entry into a building, a cool one can significantly help lower heat gain and energy costs. For low-sloped roofs, cool roof techniques generally include, but are not limited to, built-up roofing, coatings, and single-ply membranes (as described below).
- Coated Roofs are literally coated with a paint-like finish to help enhance a roof’s adhesion, durability, and longevity while simultaneously reducing bacterial growth. Cool coatings are best for low-sloped roofs on existing buildings and can be added to a multitude of surfaces, including asphalt cap sheet, gravel, metal, and other single-ply materials. They are not simply white paint, but can be pigmented in a variety of colors to implement different cooling technologies. Most coatings are also ENERGY STAR-rated.
- Foam Roofs are roofs topped with a foam-like material for insulation purposes. Used for over 45 years, foam roofing has been recognized as a dependable, long-lasting, and affordable cool roofing technique. The foam is generally made from two liquid chemicals that combine to form a solid, flexible, and lightweight material that attaches seamlessly and has proven itself to be sustainable by requiring minimal maintenance and creating minimal waste.
- Built-Up Roofing Systems, or BUR systems, are roofs made up of multiple layers of various materials and minerals. These various plies, when put together, help prevent solar heat from entering the building. Common BUR layers include: a base sheet, fabric reinforcement layers, and a protective surface layer. Cooling strategies for built-up roofs can vary by building type. One method embeds reflective materials into asphalt or coal tar to reflect the sun’s light. Another technique is to top the built-up roof with mineral-surfaced sheets consisting of reflective mineral granules or applied coatings.
- Modified Bitumen, widely known as “Mod-Bit,” is an asphalt-based roofing system similar to BUR systems. It is designed to accommodate both cold and warm temperatures and it is more elastic than BUR systems. It is installed in four main ways: torch-applied, hot-mopped, cold-applied, or with self-adhesives.
- Single-Ply Membranes are used for low-sloped roofs that require more extensive repair. They are prefabricated sheets that are individually applied to a rooftop. The two main types are single-ply EPDM (Ethylene Propylene Diene Monomer) and single-ply thermoplastics, both of which are further discussed in the “Cool Roofing Materials” section below.
Steep-Sloped Roofs have an inclined roof line and are generally seen in residential settings. Materials for steep-sloped cool roofs include asphalt shingles, metal roofing, tiles, and shakes. Different cool roofing techniques are used for steep-sloped roofs because of their different structure and materials. Applying coatings over existing shingles can prevent them from drying, causing water damage. Also, since steep-sloped roofs generally account for 40% of the exterior visual appearance of a home or building, more aesthetically pleasing techniques (such as shingling and tiling) are therefore used to cool them.
Cool Roofing Materials
As previously mentioned, various cool roofing materials are used for different roofing types. Because of this large variability, the U.S. Environmental Protection Agency has a list of over 3,000 ENERGY STAR®-rated cool roofing materials, which should be chosen based upon building type, roof type, and location.
- Asphalt Shingles are composed of asphalt mats made from organic felts or fiberglass. Their SRI is relatively low, as white shingles are only about 30% reflective and other color tiles even less so. They are widely used in the residential sector because they are low in cost and simple to install.
- Metal roofs, one of the most popular roofing materials used today, can achieve a solar reflectance of over 70%, allowing buildings to remain much cooler and lowering their energy costs. Metal is also extremely durable and weather-resistant, lightweight, and 100% recyclable at the end of its useful life.
Cool Roof Reflective Coatings
- White roof coatings are opaque and reflective, consisting of polymeric materials and some types of white pigment. They normally reflect 70% to 80% of the sun’s energy and, the thicker the layers of coating, the more reflective they are. They keep surface temperatures very close to ambient temperatures.
- Pigmented coatings are less efficient than white coatings. Darker shades such as red, green, and blue reflect only 20% or less of the sun’s solar rays. This technique is commonly seen in the residential sector, however, with colors being used to create a more aesthetically pleasing home or building.
- Aluminum roof coatings often consist of an asphalt-like resin containing aluminum leafing flakes. This coating provides a visual of an aluminum sheet and gives no indication of the asphalt layer underneath. Most aluminum coatings offer at least 50% solar reflectance and can exceed 70% with certain brands. It has also been found that indoor temperatures can be reduced by as much as 15°-20°F with their use.
- Roofing Membranes are made from felt, fiberglass, or polyester that is attached with flexible polymeric materials such as asphalt, synthetic rubber, or synthetic polymers such as polyvinyl chloride. All membranes are constructed to be strong, flexible, and waterproof. The top layer can be covered either with pigments that increase solar reflectance or with roofing gravel. Single-Ply EPDM (Ethylene Propylene Diene Monomer) alone accounts for 40% of the commercial low-sloped roofing market today. Its popularity comes from its flexibility among multiple climates. EPDM membranes are black or white in color and allow for easy repair.
- Single-Ply Thermoplastic membranes are flexible sheets made of plastic polymers. When heat is applied to this material, thermoplastics mold together and become seamless. Common examples include polyvinyl chloride (PVC) and thermoplastic polyolefin (TPO). Both PVC and TPO are among the fastest-growing roofing systems today. These thermoplastic single-ply membranes combine both the durability of EPDM membranes and the performance of a welded roofing system to provide a high-quality cool roof. Although they are commonly white in color, pigments can be added to accommodate the manufacturer’s color interest.
- Tiles are commonly seen in warmer climates because of their high solar reflectivity. Clay tiles are a popular cool roofing material, with an SRI of over 50% and a thermal emittance of up to 86%. They are known to be extremely durable and 100% recyclable at their end-of-life use. Colors vary as well to provide a great number of options to consumers. Highly durable concrete tiles are also popular and offered in a wide variety of colors, with the ability to perform in extreme weather conditions.