There are several areas in a house where energy is commonly lost, including:

1. Windows: Single-paned windows can be a major source of heat loss in the winter and heat gain in the summer, leading to higher energy bills.
2. Doors: Drafty doors can also contribute to energy loss and higher energy bills.
3. Attic: Insufficient insulation in the attic can allow heat to escape in the winter and enter in the summer, leading to higher energy bills.
4. Walls: Insufficient insulation in walls can also contribute to energy loss.
5. Heating and cooling systems: An inefficient heating and cooling system can use more energy and drive up energy bills.
6. Appliances: Old or inefficient appliances can use more energy and contribute to higher energy bills.

By addressing these areas and making energy-efficient upgrades, you can reduce energy loss and lower your energy bills.