The whole house approach considers the house as a system of interdependent parts, each of which affects the performance of the entire system. The goal is to make your home as effective a barrier to the outside as possible. When you’re running your heater or air conditioner, you can think of your home as a refrigerator—any leaks or gaps will allow that air you paid to heat or cool out and makes your house work that much harder to maintain a temperature—costing you extra money.
This photo represents identical buildings running their heating systems in Germany during the cold winter. Greens, yellows, and reds represent the loss of heated air through the building envelope. The building on the right has whole-house aspects taken into consideration (improved insulation, sealed windows) and the one on the left is unimproved.