How do you 'frame' a zero clearance/prefab fireplace when using ICF?
I would want the firebox outside of the main structure.  Would this reduce the energy efficiency
of the ICF?

thanks

Know your specs on the particular fireplace you plan to install, there are no second chances.  Frame an opening in the ICF per manufacturers specs.  Once the walls are poured, build your chase on the exterior.  The only forethought in the process is knowing the size of opening you need.  If you are using a lightweight siding or faux stone on the chase, attach the chase directly to the ICF screw strips.