Earth shelter design saves energy & temperature fluctuations will be smaller. Having earth on your roof and walls act as a thermal storage, it will get charged in the summer sun while keeping the interior cooler. In winter heat stored in the earth can be released and the indoor temperature will be more stable. Here is a look on how earth shelter can be more efficient by creating a layered structure with a central living space around a mass storage wood-stove.
The stove in the pictures is a modular wood-stove based on the rocket mass heater.