Home is in Northern NJ and built in 1920. I'm looking at adding some rigid foam insulation between joists before next winter. The previous owner jammed a bunch of batt insulation between joists but there are plenty of gaps and it's costing a fortune to keep warm. When I pulled out the batt insulation I discovered the joists are notched over the "Sill Plate" and the exterior walls are framed directly off the the plate resting on the foundation, so the entire exterior wall is open to the basement. What problems am I looking at by creating an airtight barrier between warm basement and exterior wall? I'm looking into insulated siding in addition. I figured this was more of a framing question since it is uncommon. Thanks in advance!