Yeah, it's pretty cool. I used to be all fanatical about that stuff back in the 90s. The idea with the thermal mass is that the sun comes in through the windows and heats up the walls, and then, at night, the walls radiate the heat back into the house. It's like a thermal storage battery. If you want it cooler, you draw the curtains and open the roof vent. You have buried air pipes at floor level that go outside, so the hot air escaping from the roof vent draws in cool air via the floor vents, with the earth around the pipe also cooling the intake air. Works really well in desert environments, where the days are hot, and the nights are cold, but I've read about people building them in places like northern Ontario too, where the winters are -40C. The biggest issue with the winter climate ones actually seems to be too much solar gain, instead of not enough, and also controlling the humidity. If you're hard core about living off the grid, it's viable, and as a bonus, your house will look like something out of a '70s scifi flick.