To enable a well sealed and insulated building envelope you must expose any existing crawl spaces and basement and then insulate/seal those areas. For a crawl space under a new addition use rigid foam to insulate the sides and under the slab rather than insulating between the floor joists above. Air flow between existing and new crawl spaces allows warmer air to circulate resulting in warmer floors above.
Next step is to seal those sill plates so that air does not enter the insulated crawl space. We chose to use a gasket and acoustical sealant - both will remain soft and flexible when dry.
Recycling as much existing building material as possible goes a long way to reducing environmental impact. Separating out different materials such as concrete and metal enables is the first step. When I asked about recycling the old HVAC ducting I was told it no longer made financial sense to do that with commodity prices so low. I was dismayed but figured we would find a way to do it when the time came. Later I noticed that metal was not making its way into the dumpster but instead was leaving the site piece by piece under the guidance of one of the subs who decided to take on the metal recycling job himself ! I like a guy that does the right thing without being asked!