Anyone that has ever been on a roof in the summer has most likely experienced a significant increase in temperature. This is also known as a heat island. Originally roofs were designed to keep out the rain or snow, but they also absorb heat. This works well for colder climates most of the year. However, during the summer months that heat is transferred to the attic and eventually the living space. In hotter climate zones like the southwest a dark roof can become a real liability.
By utilizing cool roof concepts, the roof literally stays cooler and can significantly reduce a building’s solar heat gain. This reduction decreases the demand on the heating, ventilating and air conditioning (HVAC) equipment. This advantage is most notable during the hottest parts of the day, which is typically during summer afternoons. This technology reduces demand on the electrical grid and lessens the possibility of brownouts and blackouts.
A cool roof delivers these benefits by reflecting visible light (solar reflectance) as well as ultra violet waves (thermal emittance). For the occupants or building owner, an additional benefit of cool roofs is enhanced roof durability. On a larger scale, cool roofs help to reduce the urban heat island effect, exponentially lowering electricity needs and indirectly reducing smog and GHGE.
The Department of Energy (DOE) states, “Choosing a cool roof instead of a standard roof can slightly increase the need for heating energy in winter. However, winter penalties are often much smaller than summer savings even in cold climates because the northern mainland U.S. (latitude ≥ 40°N) receives about 3 to 5 times as much daily sunlight in summer as in winter.”
Implementing cool roof technology can significantly increase occupant comfort levels by working with the HVAC system to reduce the cooling load and temperature fluctuations. These savings are commonly between 7-15% of annual cooling costs. This Department of Energy calculator will help to run the numbers for specific applications.
The benefits of cool roofs can be appreciated in both factory coated and field applied roofing materials. Factory applied coatings are generally found in new construction or roof replacements. Field applied coatings are commonly used on existing roofs.
Vegetated Roofs
An emerging trend in the urban environment is vegetated roofs. City dwellers are reclaiming rooftops to grow their own plants and vegetables. This is a great way to utilize accessible flat surface roofs. These green roofs are not technically considered cool roofs though these garden rooftops can provide real energy savings. They help to offset urban heat island effect and offer the added benefit of absorbing and reducing stormwater runoff.
If considering a new or replacement roof, be sure to select a lighter color if possible. If a roof replacement is not needed yet, aftermarket cool roof coatings are viable options with minimal additional cost. There is currently a plethora of options on the market today. Finding the right fit for a particular situation requires an assessment of factors such as slope and climate zone. In certain situations, the payback period for cool roof investments can be relatively minimal.
Here is a video that may help to further explain the concept
