[JRMichler]

The earth sheltered house that I toured was a long rectangle shaped concrete box, with one exposed wall that had windows and exterior doors. This forced an inefficient floor plan with a hallway along the back wall.

The roof had about 3 feet of dirt on it. A parapet along the front kept the dirt in place. While the roof and side walls had exterior foam insulation, the parapet was bare. Concrete is thermally conductive, so the parapet acted like a big heat exchanger. That would explain the need to upgrade the heating system.

The roof had 13 penetrations, for plumbing vents, water heater exhaust, and solar tubes. I asked the builder about water leaks. His reply: "No problem, just dig it up, fix, put the dirt back".

I don't know if they insulated underneath the floor. That is also a significant heat loss.

My workshop is heavily insulated, but built on a slab without insulation underneath. I keep it 65 degrees. The highest temperature inside, during a 100 degree heat wave, was 69 degrees. The cool earth was sucking heat out as fast as it came in. And I don't get that heat back in the winter.