If you’re building a new house or making major changes to an old one, it’s essential to ensure that the roof is energy efficient. In simple terms, this means that you must take 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; they make homes more comfortable by helping to regulate the temperature; and they also help to protect the environment. Ultimately, it is also likely to last longer. Furthermore, while it 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 either its ability to meet or exceed minimum solar reflectance and thermal emittance values, or its ability to meet or exceed minimum SRI 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 be able to cool by the emission of thermal radiation. For example, aluminum foil has a low thermal emittance, while potato skin has a high thermal emittance.
SRI (the solar reflectance index) is used to compare how cool roof surfaces are in relation to solar reflectance and thermal emittance. High SRI is preferable, since the roof will be cooler when it is exposed to sunshine.
Most cool roofs cost more or less the same to install as those that are not specifically energy efficient, but their efficiency will depend on a number of additional factors. These include:
- localized climate,
- insulation in your roof structure,
- how the building is used,
- the price of energy,
- type and efficiency of cooling and heating systems utilized within the building.
In essence, a cool roof will reflect sunlight and transmit heat considerably more effectively than a conventional roof.
Color has a profound effect. Dark-colored roofs can absorb as much as 90% or more from solar energy and they can reach temperatures higher than 150 °F/ 66 °C on hot, sunny days. It doesn’t take rocket science to realize that higher roof temperatures will result in an increase of heat inside the building; which in turn means you will need more air con, and therefore more energy, to keep cool. Light-colored roofs, on the other hand, absorb less than half the amount of solar energy, which results in reduced roof temperatures and decreased air conditioning needs.
So light colored roof coverings are generally better. But it doesn’t stop 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 tend to make good solar reflector, there are also cool materials – with a high solar reflectance – that are made in dark colors (and many look exactly the same as “hot dark colors” to the human eye).
According to the DOE, a conventional dark surface is likely to reflect about 20% of sunlight, while a light-colored surface will reflect four times as much. A cool, dark surface though may reflect as much as 40%.
There are various different types of cool roofs, and perhaps ironically, many involve the use of dark-colored materials.
Types of Cool Roofs
Firstly it’s important to understand that the surface that is upper-most is the one that will determine whether your roof is cool or not. You may even be able to add a surface to your existing roof that makes it cooler. For example, you might be able to coat an existing roof surface with a special paint that contains pigments that will reflect sunlight.
The design of your roof will also affect the materials and methods that may be used to create or construct a cool roof.
Low Sloping Roofs
There are several options for low sloping roofs, including pre-fabricated (usually white) membranes that are attached with adhesives and fasteners and held in place with gravel, pavers or stones. Ironically, some that work the same way are black, but these are coated to make the surface reflective.
Roofs may also be built up with layers of fabric reinforcement and a protective surface layer (which is usually dark in color) made with gravel, asphalt and various other bits and pieces including light-colored marble chips. Alternatively layers of suitable rubber or plastic materials may be used, as well as modified bitumen that may be factory-coated to make the surface “cool”.
Another option is spray polyurethane foam that reacts and expands, forming one solid roof surface with a protective coating that is reflective.
Steep Sloping Roofs
Most shingles can be coated either in the factory or on the roof to make them more reflective. Asphalt shingles should be factory coated only.
Tiles for roofs are made of a variety of materials, including natural clay and slate. Some types are more reflective than others, but all can be glazed to make them more reflective.
Metal may be used for both low and steep sloping roofs, in the form of sheeting or shingles. While unpainted metal is reflective, it has poor thermal emmitance. Special coatings may be used to make metal roofing more energy efficient.
Coatings to Make Roofs Cool
Roof coatings may be sprayed, brushed or rolled onto most roof surfaces, but these are not the same as exterior roof paint. There are three types that are effective:
- Acrylic water-based coatings that are easy to use, cure by evaporation and are reasonably strong. This is the least expensive and most commonly used type.
- Silicone solvent-based coatings that have a weaker tensile strength, but are highly water resistant, and weather well.
- Urethane solvent-based coatings that are stronger and more energy efficient, but expensive and difficult to work with.
ENERGY STAR Roof Products
When choosing materials for a new roof look for the ENERGY STAR label. These can reduce the temperature of your roof surface by as much as 100 °F/ 37.8 °C. While the energy-efficiency criteria don’t include any particular specification for roof insulation, these products can reduce the peak cooling demand of air conditioning by between 10% and 15%.
ENERGY STAR roof products that have qualified based on their solar reflectance abilities include:
- roof tiles,
- metal roofing products,
- asphalt shingles,
- single-ply membranes,
- modified bitumen,
- spray polyurethane foam roof systems,
- roof coating products,
- built-up roofs.