The economy of energy is becoming a very important aspect of living. Those in the lower income brackets tend to waste more energy and spend more on energy per square foot then do those who can afford better built, energy efficient homes. Old homes and apartment buildings for example.  Especially brick and cement structures with air flow through windows, little or no insulation in the walls and attic. 

There are aso many small areas of concern which add to the disproportionate air exchange and draft which defines the equation of energy efficiency.  If one were to add up all of the energy wasted through unoticed crevices, it might be as large as a 3ftX3ft hole in the wall. Let me explain. There are spaces in walls and floors for the passage of pipes, wires and outlet or switch boxes. There are drafty basement windows and doors.  Even if you can't afford to insulate the old walls, there are inexpensive products like expandable foam and foil-backed (celulite) insulation foam board, which can be applied to these draft vents, which extent from basement to attic.  One huge disadvantage would be the sill plate resting atop the cement basement wall.  The floor joist spaces there can be filled or stuffed with cut sections of R-22 fiberglass. That's a big difference in many cases.  Stuffing between the joists that rest upon the sill plate and then covering basement windows and weatherizing any doors to the outside, would drastically increase the ambient temperature in the heating system chamber (if furnace is located in basement). This in turn would allow heat to build more quickly in the plenum. It means that in a warmer basement, much of the heat won't be wasted through the furnace flu and leaks through the outside, through the sill pate. So, doing this and using fiberglass, expansion foam and foam board to prevent updraft through the floors and walls, would significantly increase the energy eficiency and the cost. An adiitional benefit of furnace efficiency would be derived from glueing (...and taping) sections of heat resistant insulation sheild, to rectangle ducts and plenum up to the basement ceiling.  DO NOT USE urethane foam boards, as the heat from plenum and ducting would create an environmental hazard of formaldehyde gas release, which is linked with cancer.  This has been a huge complaint of FEMA trailers, used in the New Orleans disaster relief. 

By doing the above tasks, annual savings can be applied to other areas of energy loss, like replacement windows and attic insulation..etc..