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Cool Roofs vs. Garden Roofs

August 26, 2014 | 0 comments

Garden roofs and cool roof systems both offer energy-saving advantages over traditional black roofs, which may absorb heat and contribute to high HVAC loads and energy use.

Installing a cool roof or garden roof may decrease roof temperature and building cooling loads. As a result, the lifecycle of your building’s rooftop and HVAC equipment may be prolonged. Both roof types offer points toward LEED certification as well.

Although these roofs both provide energy savings by lessening the flow of heat from the roof into the building, they do so in very different ways.

Cool Roofs
Also known as white roofs, cool roofs reflect sunlight away from the building and back into space. They are able to stay cooler in the sun by minimizing solar absorption and maximizing thermal emission. These two properties are measured on a scale from 0 to 1: the higher the number, the cooler the roof (and the more sunlight it will reflect).

Many cool roofs offer initial solar reflectance of more than 80%; however, roofs become dirty with age. On black roofs, dirt build-up may actually improve reflectance, but the effect is reversed for white roofs. Keeping cool roofs clean is key to maintaining high solar reflectance levels.

Garden Roofs
Also known as living roofs or green roofs, garden roofs are roofs with a vegetative layer grown on the roof. As a result, these roofs provide shade and evaporative cooling through rainwater. Garden roofs can also help insulate roofs in winter, reducing the amount of heat needed inside the building.

Garden roofs involve several layering materials: waterproof membrane, root repellant system, insulation layer, drainage layer, filter cloth, growing medium, and plants.

Intensive green roofs typically feature large plants or conventional grass, and are designed to offer outdoor access to tenants and occupants. They tend to be more labor-intensive, may require frequent maintenance and watering, and have increased roof load requirements. Extensive green roofs, however, are designed with self-sustaining and drought-tolerant grass, moss, and wildflowers. These green roofs aren’t generally used for recreation or regular foot traffic.

Location Matters
The impact of a cool roof or garden roof on a commercial building depends on several factors:

  • Local climate
  • HVAC equipment performance and age
  • Building type (occupancy levels, hours of operation, etc.)
  • Roof surface area
  • Building height

Because of this, cool roofs and garden roofs may impact buildings differently based on geographic location. According to the Lawrence Berkeley National Laboratory, cool roofs often perform the best in states with long cooling seasons (California, Hawaii, etc.). Garden roofs may perform best on buildings with heating and cooling requirements (Nebraska, Iowa, etc.) since they help keep roofs cool in the summer and insulate roofs in the winter.

Tax credits and incentives may be available in your area for either type of roofing system.

Do you have a cool or garden roof on your building? How did you decide which type of roofing system to use?



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