So many of us preach to the choir that we forget some people don’t know how to sing. Most people’s cognitive map of a house puts the basement in a “safe” area. Safe from storms and thus the outside world. But basements leak energy for the same reason the rest of the house does, because it’s confronted with an ever changing temperature. Admittedly at about 10 ft. below the ground the temerature starts to stabilize so that geothermal heat pumps start to work but you really need to go deeper to take advantage of a constant 50 degrees farenheit. So you insulate your basement for the same reason you insulate the rest of the house – to save energy and to save money.