dear patrick,
strictly speaking and as they go thru the usual process, neither is green since both impact the environment negatively. synthetic materials are usually derived from minerals mined from the earth (iron, aluminum, petroleum), manufacturing process likewise pollutes, and some even off-gas - post maufacturing stage (v.o.c.'s). in the case of the latter, it makes no sense to re-use since it leads to poor indoor environment quality. natural building materials, if not properly managed (from sourcing to harvesting) can lead to depletion.
my reply is that it would depend on intent, management, process and quantification. some green building rating systems give credits to materials with pre and post consumer recycled content. in re-using let's say, plastics (only some types of plastics apply), this benefits waste reduction - although there's already one disaster at the north pacific gyre :o anyway, there is also a limit to v.o.c. outgassing. in using natural building materials, the key is whether it is sourced from certified forestry/timber farming management standards (harvesting timber at it's due age and only from a specific area, and then replanting). there are also rapidly renewable materials such as bamboo, cork, etc which may be used. the latter 2 are likewise given credits.
only if the conditions above are met, then both (plus the use of rapidly renewable materials) are considered accepted green/sustainable practice for materials. these days, certification of products and processes from credible and internationally recognized agencies/organizations are a must - no more 'self-claims'. that's the key since sustainability is a global problem.
hope this answers your question.
regards,
pabs
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Pablo F A Suarez
LEED AP, Green Mark Consultant, M. Design Sci - Sustainable Architecture, Registered Architect & B. S. Arch
Principal ESDConsultant/Architect - Greenarc
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