EPA recommends taking a staged approach to energy efficiency improvements.
First, focus on O&M
In general, the first priority should be operations and maintenance (O&M) and behavioral strategies spanning all building systems.
Next, look at building systems
Then, move on to low- and medium-cost measures, looking at each building system in phases: lighting, plug loads, building envelope, and HVAC.
HVAC comes last in this staged approach because by reducing all other system loads first, you may be able to reduce the load and operational hours of your HVAC equipment. If you have to purchase new HVAC equipment, you may be able to purchase a smaller unit which will cost less initially and also cost less to operate over time.
Read on for detailed assistance with each stage: