The source of heating is not what's the most important (in any case an (electrical) heat pump is always going to be more energy efficient, everything else pales

The source of heating is not what's the most important (in any case an (electrical) heat pump is always going to be more energy efficient, everything else pales in comparison). Insulation is really the most important thing to reduce a building energy consumption (some buildings are even built with zero heating: https://en.wikipedia.org/wiki/Zero_heating_building https://en.wikipedia.org/wiki/Zero_heating_building), along with air-tightness and heat_recovery_ventilation.