Green roofs and urban heat island
A policy-oriented question for the physics-minded folks out there (Allison?)
I’ll try to be brief here.
The question I have here is: How does a ‘green roof’ performance towards the urban heat island effect compare to a Cool Roof in a hot/dry climate?
–Let’s stipulate that there’s no formal definition of a green roof other than some kind of membrane with soil and plants planted at the time of Final Inspection. Since most green roofs are installed on low-slope roofs, then the comparable ‘cool roof’ performance is somewhere in the ballpark of Reflectance-0.8 and Emittance of 0.8.
I’m on a green building committee that’s tasked with advising the local municipality on a new green building code that would be adopted and implemented locally. This code features mandatory measures as well as a requirement to fulfill a number of ‘electives’. One of the electives on the list is ‘green roof’, and I’m just thinking about whether or not this is actually a sensible idea to include on the list. I worry that it’s actually expensive and counterproductive, though it obviously has good green ‘optics’ from a PR perspective.
references on GBA–
There a bit of good discussion already on this forum about Green Roofs. I note that the benefits cited by advocates are usually poorly defined, or can only be ascribed to specific green roof compositions in certain climates. And there’s vigorous counterpoint about whether or not the amenities of stormwater management and energy efficiency could not be better and more cheaply provided by more appropriate materials and assemblies. (I’m largely convinced by these views).
So, the question is actually taking on the last ‘benefit’ of green roofs, and wondering if it can be shown to be better than a Cool Roof at lowering Urban Heat island. I’ve narrowed the question a bit by saying I’m thinking about a hot/dry climate (e.g. many parts of California)
GBA Detail Library
A collection of one thousand construction details organized by climate and house part