I’m in the process of adding an ICF porch to my ICF basement foundation. The porch foundation will extend about four feet below grade.

The ICF manufacturer’s tech support suggested that I cut away the foam where the porch foundation will meet the basement wall. If the foundation depth between the two structures matched, I would be able to cut the dimpled drainage membrane and the adhered waterproofing membrane and wrap a new piece neatly around the corner. Since I won’t be escavating to the full basement depth, I would have to notch or split the membrane horizontally, which seems like an invitation for water infiltration.

Do I really need to have concrete-to-concrete contact?

To avoid water intrusion, I would prefer to dowel the porch foundation into the basement wall with 12 pieces of #6 rebar, and place concrete directly against the drainage membrane. Since the foam is only 2.5 inches thick, I think the rebar could still serve the purpose of preventing separation or differential settling. The walls will be well reinforced, and they will be tied together with a slab on composite steel decking. I think I’ll probably turn the rebar from the walls into the elevated slab.

I would appreciate your opinions.