INSULATION
In many homes this is the most practical and cost effective way to make a house more energy efficient. Because it keeps homes cooler in summer and warmer in winter, it is possible to save up to 40 percent in heating and cooling bills. In addition, it may reduce condensation in the home.
Here is more on the importance of insulation from the US Department of Energy site: http://energy.gov/public-services/homes/home-weatherization/