If you have a wall. It can be made up of several different items from bricks to wood to drywall. The exterior can also be subject to radiation from the sun and winds. The inside would likely no wind. The result is heat transfer from radiation, conduction, forced convection and natural convection. If you add all of these factors up for the wind and radiation, bricks. wood, dry wall and natural convection, you will get an overall heat transfer coefficient. The tubes in a heat exchanger have similar considerations as shown below.
