All living organisms have a preferred environment. Whenever the environment is not within the preferred parameters, the organism is stressed. Stress can reduce productivity, increase the incidents of illness, shorten the storage live of products and cause many other problems. In the natural environment, organism modify their environment by moving from one location to another. Once animals and other organisms are confined, they cannot roam, and the environment must be modified to meet their needs. Two ways environments are modified are heating and cooling. Heat is a form of energy. It is transmitted in three different ways:

1. Radiation—the exchange of thermal energy, heat, between objects by electromagnetic waves.

2. Convection—the transfer of heat from or to an object by a gas or a liquid by movement of the gas or liquid.

3. Conduction—the exchange of heat between contacting bodies that are at different temperatures.

The movement of heat by radiation and convection is more difficult to calculate than conduction. Conduct heat losses will be explained in more detail to illustrate the factors that influence the amount and rate of heat movement. The amount of and rate of conduction heat movement is controlled by the area through which the heat passes, the difference in temperatures, and the thermal resistance of the materials. These concepts and others will be discussed in this chapter. Latter sections will show that the greater the area, the greater the heat flow. A similar relationship exists for temperature. The greater the temperature difference the greater the heat flow. The thermal resistance of materials is primarily dependent on the density of the material. A material with many small, trapped air spaces will have higher thermal resistance. A material with high thermal resistance is called insulation.

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