Hi GBA,
I have a few questions about an insulated slab detail found in 'strategies and details" section (http://www.greenbuildingadvisor.com/green-basics/slab-foundations). What is the ideal substrate underneath the rigid insulation board and where, if necessary, should a vapor barrier be placed? In the past I have constructed in this order... slab, vapor barrier, 4" of gravel (passive radon vent integrated if EPA zone 1 or 2).

Thanks for the help!

Dave