I am a new member and I am planning an addition in which I have the need for a finished basement below a screened porch. For the porch floor (and basement ceiling), I have been planning to use sloping conventional wood joist framing with  batt insulation, plywood deck, rubber membrane, sleepers, and trex decking.  Are there any other approaches that are more sustainable? more cost effective? thinner in section? Someone suggested concrete to me but I was concerned about cost, structure, pouring the slab, insulation, and its ability to remain waterproof. Does anyone have any suggestions? Thanks!

Check out a product called LockDry. (www.lockdry.com).
We have used it on one exterior decking project, and have it speced on an upcoming roof patio. So far I like what I have seen. In short, it is an interlocking deck material that forms a waterproof deck.