Heat transfer occurs from warmer to cooler substance or material and continues until there is no temperature difference between them. It happens all around us in one of the three ways:
Conduction is the transfer of heat through matter (i.e., solids, liquids, or gases) without bulk motion of the matter.
Convection is the process of heat transfer from one location to the next by the movement of fluids or gases.
Radiation is the transfer of heat by electromagnetic waves, and it does not need any matter, gas, liquid or solid, to occur.
Energy efficiency is concerned with the heat transfer between our houses and the outside and internally within each room and between the rooms. It is also about efficiency of our appliances because in order to keep the temperature within comfortable range we must continue to run our appliances to heat or cool our house to counteract the effects of unwanted heat transfer. So the cost of running the house depends on how fast is the unwanted heat transfer and how much we are paying to run our appliances. More efficient appliances will use less energy to produce the same amount of heating or cooling, and slower heat transfer means they will have to work less to keep the house comfortable.