Building regulations are becoming ever stricter. That makes a smart approach is necessary. The only sustainable principle for lowering the energy level of your building is to eliminate energy loss.
Start with the building envelope
It all starts with a properly insulated building envelope* because (ideally) you only insulate your building once! Insulating the roof, walls and floors of your home with maximum efficiency increases your energy efficiency , improves your living comfort and protects the value of your home. This is a sustainable and sensible investment.
*Building envelope = The building envelope separates the living space from the outside world. It consists of the roof, facades, floors, windows and doors.