I have a basement floor in an old house - circa 1950's. I have to cut
open the floor to move some plumbing around for a new bathroom and I'm
researching different methods / techniques to use when pouring the new
floor (I'll be laying down ceramic tile eventually). My current plan is
as follows:

1) Lay down a layer of gravel over the existing tampered excavated
area.
2) Lay down a polystrene foam insulation.
3) Pour concrete with a rebar mesh to re-inforce (I'll also need to
paint a bonding agent down on the old concrete floor edges to ensure
the new floor bonds to the old).

Is some of this overkill ? Although the rest of the concrete slab is
still the original - is there any side-effect of using the foam
insulation on only a portion of the floor ? Or is it useless unless it
spans the entire area of the houses concrete slab ?

thanks in advance for any answers.