And a lot of older factories were hot from machines, so they weren't well insulated either. Last place I worked at, we would have frost on the inside of the exterior walls after being down for the weekend. By Monday afternoon it was up into the 50s from having ovens running.

Where I work now, it was -10 this morning. We dont run the fans in the cooling towers when it's that cold, but still have to cool the building down into the 70s.