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GSBN:ducks, living roofs and CO2

Mark brought up the interesting question of the contribution of a living roof to CO2 production. I would like to hear others thoughts on whether a living roof is going to give contribute more carbon fixing than the bales in the walls. My first thought was, yes, of course it will. On further reflection, I have my doubts. Here's my thinking.

Plants pull CO2 out of the air, and combine it with water, using solar energy, to produce carbon compounds, which are stored in the plant, and oxygen, which they donate back to the atmosphere. So I'm thinkin' that on a living roof, we only get a net gain in carbon fixing, if we maintain a net gain in plant mass. If we want to fix more carbon each year, then we have to increase the net plant mass each year. We don't want the roof to get heavier and heavier. So we either reach equilibrium on the weight and give up on more carbon fixing, or we continually remove plant material from the roof, and put it somewhere else. If the somewhere else leads to rapid decomposition, then the carbon is returned to the atmosphere. So it seems that we have to harvest the roof and prevent the harvested material from decomposing, if we want to have sustained carbon fixing.

What am I missing here? If I haven't left something out of the picture, then I think few living roofs would be making much of a contribution to fixing carbon. Of course they could be contributing in other ways, as Mark mentioned, such as moderating temperature swings and shading the roof to decrease cooling requirements.


--On Sunday, February 24, 2002 5:07 PM -0500 Mark Piepkorn duckchow@... wrote:

Bale walls are
carbon sinks, but the bale walls won't last forever and eventually
that carbon will be released. In the meantime, a living roof can be
working to offset that eventuality, as well as the emissions
included in the overall embodied energy of the structure.

Derek Roff
Language Learning Center, Ortega Hall Rm 129, University of New Mexico
Albuquerque, NM 87131  505/277-7368 fax 505/277-3885
Internet: derek@...