Green roofs are a neat idea and make environmental sense, at least in the city where they soften the environment and help retain stormwater. I went on a tour of three roofs in Philadelphia last week (A tour arranged by our excellent Delaware Valley Green Building Council). This picture is from one using modular trays, the easiest and cheapest solution for a extensive roof. Extensive means that the planting medium, what would be the soil on the ground is shallow; the favored type of plants are sedums, as in this example – they can survive a dry spell and are not damaged by the wind. Intensive roofs are those that have more than 6″ of soil and can sustain more varied vegetation.
At least one of the roofs was making the old claim that vegetated roofs save energy. They can cool down the air around the roof and thereby save airconditioning in adjacent buildings. But they do nothing much to insulate the building – insulation can do that much much cheaper. Their real Green (ie sustainable) benefits are in fact fairly limited but they are better than looking at expanses of bare roof, for sure.
Here is an old post I cam across where someone is confirming the point about thermal insulation:
“I can’t offer you peer reviewed ashrae stuff but everything I have heard/read seems to say that there is virtually no R value to a vegetated roof especially when they are doing their job and holding water- Perhaps when you get to the Chicago City Hall type roof with 4′ of dirt it is a different story but when you have a 4″ extensive type system I don’t think you get much at all.”
Dxxxx Bxxxx, AIA LEED AP