Doors and windows could have been better quality that in turn saves on the utility bill. Less heat or cool air would be lost out in to the atmosphere through your walls, during those summer and winter months. I am going to high light the insulation end of energy savings.
Heat flows to colder objects regardless if they are up, down or through the walls.
Thus if a wall has a heat loss of 1500 Btu's, it is losing heat in amount which would be equivalent to the heat required to heat 1500 lbs. Weather is another factor you need to consider. Weather helps determine how much insulation you are going to place into a structure. On the other hand if the temperature were to stay around 60-70 year round then this structure would not have to be insulated as well as the one with the drastic temperature differences. The amount of energy that can be saved with the proper use of insulation, storm sash and storm doors can best be illustrated by house when built before the pre-energy crisis.