Not inefficiency as such. Many houses were empty because the occupants were forcefully evicted.
"Another feature of the political violence was the involuntary or forced displacement of both Catholics and Protestants from formerly mixed residential areas. For example, in Belfast, Protestants were forced out of Lenadoon, and Catholics were driven out of the Rathcoole estate and the Westvale neighbourhood. In Derry city, almost all the Protestants fled to the predominantly loyalist Fountain Estate and Waterside areas."
A potent reminder of the kind of glaringly painful inefficiencies that come out of this kind of thing.