The dining room addition gets no heat in winter and is unusuable. Also, there is no access to the crawlspace under the dining room floor, so I suspect it has been covered up.
The styrofoam smeared with concrete stucco looks suspicious and sounds hollow, so I decide to break off a piece to investigate.
There are cinder blocks behind the styrofoam, and the bottom ones have been properly sealed with tar.
With this information, I suspect that the crawlspace access is probably in the finished basement, covered with drywall.