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Green Roofs vs. Reflective Roofs

Green Instance / January 6, 2012

There are a number of debates swirling around the roofing industry these days, possibly none more so than the green roof versus reflective white roofs debate.  Numerous factors go into the decision making process when deciding which roof best suits your needs.  Short term cost is a no-brainer discussion, but the environmentally friendliness and long-term investment debate is a bit more complex.

The environmentally friendliness debate is typically discussed within the context of energy savings and heat-island mitigation.  To accurately assess the differences in energy savings between the two roof types for a specific building a mechanical engineer, who is familiar with the thermodynamic properties of the various layers of green roofs and how they work together with the existing building, would need to be consulted.  However, to gain a general idea of what temperatures are at play, below are graphs showing the differences in temperature on various types of roofing surfaces.
temperature variation on roofs

Temperature variation on roofs

Temperature at Membrane Horizon by Roof Type. July 15-20, 2003. Source: MWH 2004
The illustrations above show that the green roof is the most even keel choice out of the group, providing heat insulation during colder temperatures and cooling the roof during the warmer periods.  As shown above, a cool roof provides more cooling effects than a regular tar roof; however, it is still considerably warmer than the green roof during the warmest times of the day.   Conversely, the cool roof is the coolest of all the roof types when the temperature dips.

You might say to yourself, “Well the cool roof may not be as effective temperature wise as a green roof but it’s cheaper.”  Wrong.  While the initial installation of a cool roof may be cheaper than that of a cool roof consider this; according to Greenroofs.com “Green Roofs vs. Cool Roofs” in March 2011, cool roofs may reflect up to 90% of the sunlight the first year however it quickly loses this capability to only about 70% from UV damage, pollution, dust, and exposure to the elements unless meticulously cared for.

The other price tag to examine is a cool roof like any other regular roof will need to be replaced after about 20 years.  A properly designed and maintained green roof should last 50 years or more.  Consider this the next time you need to re-roof.

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By Kat Harrold

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