Do I need a subfloor if I put rigid foam under slab?

Hi Everybody,

I'm building a new home and the builder has an upgrade to add R10 rigid foam board under the slab. I'm planning to finish my basement. I'm still thinking about flooring options... do I need any kind of subfloor or vapor barrier... I would think not as it would create a pocket between the subfloor and rigid foam under the slab??

I might end up using engineered hardwood that can be floated on concrete slab...

Any ideas on whether I would need a sub-floor? Thanks in advance.

Cheers,
Jack.