The best way to save money, energy and CO2 on an existing building is also the cheapest and the quickest. The very first thing you should do is to make sure that your building is not wasting energy. There's little point in spending big bucks on a fancy heating system if all the heat escapes up the chimney. Buildings use energy to keep the interior at a comfortable temperature, to provide light, fresh air and to power the various appliances. Temperature control can be very expensive especially in the large houses that are common in America. Your priorities should be:-
  1. Seal all the air leaks (infiltration).
  2. Insulate roof, walls and the hot water tank (insulation)
  3. Ventilate (ventilation).
  4. Use natural light (daylighting).
  5. Turn off lights and appliances and computers (just say off!).
  6. Install efficient lights and appliances (upgrade).
  7. Use window blinds and curtains (solar gains).
  8. Heating and cooling thermostats should be set properly (thermostats).
Buildings should comply with various standards; some, like California's Title 24, are mandatory and others, like LEED, are voluntary (but a good idea as they'll save running costs and ensure a higher rental or selling price).