Insulating your basement, while a bit lower down on the ladder of efficiency measure effectiveness, is still something to consider if you live in a cold climate and the basement is used as a living space.
http://www.nesea.org/uncategorized/heat-loss-to-the-ground-part-2/
Let’s assume the temperature below the slab is 50˚F and the average temperature against the foundation wall is 35˚F. Our basement is 24 ft x 36 ft. Doing the math, we get a heat loss of 2,161 BTU/hr…
There’s a lot more in the full article.