I have never found that heating before leaving does any good. I have had temps up to 22C before leaving, drive for 30 minutes and it is down to 0 (in -20C temps). Did a test last year and left mountains with inside temps at 20C, drove for 30 minutes with furnace on and it was 18C inside (outside was around -18C) and the furnace had run the entire time, just could not keep up. Shut furnace off and in another 30 minutes of driving was -5C inside. This was with my storm windows in. This year I want to insulate the roof vents and see if that makes a difference.
