Georgia asks: I'm interested in treating my roof with either a vegetated or white surface... I've read about green roofs but I've also heard that painting a roof white does the same thing. What is the cost? What do I need to know? Thanks in advance. G

Answer: Hi Georgia:

Traditional roof surfaces (i.e. those with an untreated, non-vegetated dark surface, such as asphalt tile or shingles) can absorb up to 83% of the sun's heat and UV rays causing significant heat buildup. As anyone who has worn a black t-shirt on a sunny day knows, black surfaces can become up to 70°F (40°C) hotter than the most reflective or white surfaces. If that dark surface is a roof, the heat collected by the roof is transferred to the inside of the building resulting in higher HVAC/energy costs to cool it. In dense urban areas, traditional dark roofs also contribute to the heat island effect.

While a roof coated with a reflective coating can reflect up to 88% of the sun's heat and UV rays, significantly reducing heat buildup and ambient temperature increases, a vegetated roof can absorb significant amounts of heat while providing a cooling effect to the surrounding area as well.

The big difference between green roofs and cool roofs is in terms of aesthetics and cost. When considering both materials and labor, green roofs can cost $15 to $35 a square foot whereas cool roof coatings vary from $0.75 per square foot for reflective, acrylic paints to over $3 per square foot for PVC single-ply membranes. The payback for a green roof may be much longer than the payback for a cool roof. Also, the maintenance associated with a green roof, as well as the potential headaches associated with any leaks that may occur underneath the surface of the green roof or its membrane, can add cost. In fact, reflective roof coatings usually pay for themselves in a couple of years.

The Heat Island Group, an organization that concentrates on the study and development of more reflective surfaces for roadways and buildings, has monitored buildings in Sacramento with lightly colored, more reflective roofs. The group found that these buildings used up to 40% less energy for cooling than buildings with darker roofs. The Florida Solar Energy Center performed a similar study, also showing up to 40% cooling energy savings.

Hope this helps!
Green-Buildings.com

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