The ground temperature at -4" or greater generally stays between 40 - 60 F. It's a lot easier to heat from 40 F than from -20 F, so this saves wood. This is what I hope to do one day, but of course government frowns upon its slave building under ground and they regularly try to use scare tactics such as radon gas (which may or may not be a problem in your area). This is because gophers are exceptionally difficult to raid.