A majority of the energy cost in my home is due to heating in the winter and cooling in the summer. I would assume that a massive amount of federal money goes to heating and cooling office space.
Current dress codes and customs suggest that a person should dress exactly the same in both winter and summer. This requires more heating/cooling costs for buildings as they try to reach 70 degrees for every work day. Instead, if people dressed appropriately, thermostats could be given a wider temperature zone of 60 to 80 as people would dress more warmly in the winter and lighter in the summer. The British military looked into this 2 centuries ago and developed what is now considered Bermuda attire. Dress shorts and button down short sleeves.
Furthermore, if dressed appropriately federal employees would be more likely to walk longer distances to work/public transportation rather than getting into their air conditioned vehicles which would save more gas/energy. Private industry (that is those companies that have not already adopted casual dress) would follow and the energy needs for the nation as a whole would decrease seriously over time.