Greenhouses are sometimes built with small openings in the roof to allow air to vent, which is critical if you're using supplemental heat from an oil heater or wood fired stove, for example. For geothermal greenhouses, that kind of venting is unnecessary and wasteful. After unnecessary gaps, the next greatest area for heat loss is the cover. Generally made of rigid plastic panels or flexible film, greenhouse covers are far better at insulating than they used to be, but these panels can cover 80% or more of your structure and even small amounts of heat lost can add up.
Choose high quality materials with high insulating values, use double walls with an air gap between, use IR reflective films, and in especially cold conditions, consider using thermal curtains at nighttime - sort of a cozy insulated blanket you can deploy over your greenhouse to minimize heat loss at night.
