Heat is going to be problem #1. The foundation is drafty, the floors are drafty, the windows are drafty -- and the furnaces are broken, the ductwork largely removed (leaving big drafty registers from the basement). So after cleaning, and replumbing the shattered frozen CVPC pipes, and getting a working kitchen assembled, I will be working with my good friend Mr. Urethane Foam to start plugging up every possible gap in the foundation.
There is a missing window in the east side of the basement, which is now an opening to the crawl space under the porch (the porch is a later addition to the original house). There are actually two, but one has been bricked up. The one that hasn't been bricked up is wide open, allowing cats into the house. (Said cats have contributed greatly to the poor air quality, I can assure you.) Along with the cats, I'm pretty sure it's letting a great deal of cold air in. And that's just the biggest opening I've found so far. There will be more.
There's a lot to be said for proper insulation.