Heat transfer occurs when there is temperature difference between inside of the house and outside, through walls, ceilings and floors. Openings in the building shell (like doors or windows) transfer heat at higher rate than walls do. We can slow the heat transfer by adding insulation or shading or replacing the openings with more efficient ones. Slowing the heat transfer is important because it links directly to the cost of keeping the house temperature at comfortable level. If the house is quickly getting cold in winter or hot in summer, it requires heater or air conditioner to run at higher temperature setting and/or for longer time, which means using more energy and therefore higher energy expense.

In addition to those large heat transfers, there may be many small leaks due to imperfections in building shell, insufficient insulation or other issues. Some examples of such leaks are displaced or missing insulation, door and window gaps, vents, exhaust fans, chimney stacks, and more. In addition to leaks due to heat transfer, a house may also “leak” power – through standby power or by using inefficient appliances. Those leaks, while small, can add up to a significant amount of energy being wasted and are relatively easy to fix.