When I see the temps dip below zero I tend to run my oil boiler a bit more . . . partly since the pipes run through my crawl space/basement (technically it'a s basement, but not a full basement -- I guess you could call it a a hunched over space or a short wall basement) and running the boiler will heat things up . . . of course I have also insulated the rim joists and staple plastic on the windows down there every Fall.