If you’re building a new house or making major changes to an old one, ensuring your roof is energy-efficient is essential. In simple terms, this means taking steps to make sure your new roof is “cool” by using suitable materials and methods.
Cool roofs save property owners money by reducing energy bills, make homes more comfortable by helping regulate temperature, and also help protect the environment. Ultimately, they are also likely to last longer. Furthermore, while a cool roof will keep the interior cool in hot weather, it will also keep the interior warm when conditions are cold.
Basics of Cool Roofs
The United States Department of Energy (DOE) defines a cool roof in terms of its ability to meet or exceed minimum solar reflectance and thermal emittance values or its ability to meet or exceed minimum SRI (Solar Reflectance Index) requirements.
Solar Reflectance refers to the amount (measured in fractions on a scale of 0 to 1) of sunlight that a surface reflects.
Thermal Emittance refers to the ability of a surface to cool by emitting thermal radiation. For example, aluminum foil has a low thermal emittance, while potato skin has a high thermal emittance.
SRI (Solar Reflectance Index) is used to compare how cool roof surfaces are in relation to solar reflectance and thermal emittance. A higher SRI is preferable, as it indicates the roof will be cooler when exposed to sunlight.
Most cool roofs cost about the same to install as those that are not specifically energy-efficient, but their efficiency will depend on several factors, including:
- Localized climate
- Insulation in your roof structure
- How the building is used
- Energy prices
- The type and efficiency of cooling and heating systems used within the building
In essence, a cool roof will reflect sunlight and transmit heat more effectively than a conventional roof.
Color plays a significant role. Dark-colored roofs can absorb up to 90% or more of solar energy and can reach temperatures higher than 150°F (66°C) on hot, sunny days. It’s easy to understand that higher roof temperatures will lead to an increase in heat inside the building. This, in turn, means you’ll need more air conditioning, consuming more energy to stay cool. Light-colored roofs, on the other hand, absorb less than half the solar energy, resulting in lower roof temperatures and reduced air conditioning needs.
While light-colored roof coverings are generally better, it doesn’t end there. As the United States DOE’s Guidelines for Selecting Cool Roofs (published in July 2010) state, “not all cool roofs are white.”
Colors of Cool Roofs
While white and other light-colored roofing materials tend to be good solar reflectors, there are also cool materials with high solar reflectance that come in dark colors. Many of these look nearly identical to traditional “hot dark colors” to the human eye.
According to the DOE, a conventional dark surface reflects about 20% of sunlight, while a light-colored surface reflects four times as much. However, a cool, dark surface can reflect up to 40%.
Various types of cool roofs, ironically, often involve the use of dark-colored materials.
Types of Cool Roofs
First, it’s important to understand that the uppermost surface of the roof determines whether your roof is cool or not. You may even be able to add a surface to your existing roof to make it cooler. For example, you could coat an existing roof surface with special paint containing pigments that reflect sunlight.
The design of your roof will also affect the materials and methods you can use to create a cool roof.
Low Sloping Roofs
For low-sloping roofs, there are several options, including pre-fabricated (usually white) membranes attached with adhesives and fasteners, which are held in place with gravel, pavers, or stones. Ironically, some black membranes work similarly but are coated to make the surface reflective.
Roofs can also be built up with layers of fabric reinforcement and a protective surface layer (often dark in color) made of gravel, asphalt, and other materials, including light-colored marble chips. Alternatively, layers of suitable rubber or plastic materials may be used, as well as modified bitumen that is factory-coated to make the surface “cool.”
Another option is spray polyurethane foam, which reacts and expands to form a solid roof surface with a reflective protective coating.
Steep Sloping Roofs
Most shingles can be coated either at the factory or on the roof to make them more reflective. Asphalt shingles should only be factory-coated.
Roof tiles, made of materials like natural clay and slate, vary in reflectivity, but all can be glazed to enhance reflectivity.
Metal roofing can be used for both low and steep-sloping roofs, either as sheeting or shingles. While unpainted metal is reflective, it has poor thermal emittance. Special coatings can be applied to make metal roofing more energy-efficient.
Coatings to Make Roofs Cool
Roof coatings can be sprayed, brushed, or rolled onto most roof surfaces. However, these are not the same as exterior roof paint. There are three effective types of coatings:
- Acrylic water-based coatings, which are easy to use, cure by evaporation, and are reasonably strong. These are the least expensive and most commonly used type.
- Silicone solvent-based coatings, which have weaker tensile strength but are highly water-resistant and weather well.
- Urethane solvent-based coatings, which are stronger and more energy-efficient but are expensive and difficult to work with.
ENERGY STAR Roof Products
When selecting materials for a new roof, look for the ENERGY STAR label. These products can reduce the temperature of your roof surface by as much as 100°F (37.8°C). While the energy-efficiency criteria don’t specify insulation requirements, these products can reduce the peak cooling demand for air conditioning by 10% to 15%.
ENERGY STAR-qualified roof products, based on their solar reflectance capabilities, include:
- Roof tiles
- Metal roofing products
- Asphalt shingles
- Single-ply membranes
- Modified bitumen
- Spray polyurethane foam roof systems
- Roof coating products
- Built-up roofs
