When an interior window is closed with draperies but not sealed at the top and bottom, warm air moves between the window treatment and the glass creating a wind tunnel effect. The warm air enters the top, cools as it falls between the glass and the window treatment and then leaves through the bottom as cold air. The closed-top cornice helps reduce these convection currents. The simple addition of a liner of tightly woven fabric can aid insulation by forming an air pocket between the liner and curtain. Many renters develop a short-timer mentality and don't worry much about energy savings.
Hawks offers the following guidelines for shopping for energy efficient rental housing.
If you want exact figures you need to get written permission from the previous occupant, owner or apartment manager to ask the utility companies for past records. These records are good guidelines, but realize that your lifestyle and energy use may differ greatly from previous tenants. Gas is generally cheaper and you can save money if you are able to adjust your water heater to fit your own needs. South facing windows can be an asset during Utah winters because they let in maximum warming sunlight. On the other hand, north facing windows can contribute to heat loss.