The most energy-efficient home improvements are those done to your heating, cooling, and ventilation systems. These include:
Insulating your walls and draught-proofing your windows
Upgrading your old gas boiler to an electric combi boiler or biomass  boiler
Installing solar panels to generate green electricity for your home
Replacing your old HVAC system with an air source or ground source heat pump
Installing a solar thermal panels system for domestic water heating purposes