Thus, the elements of the wall must be selected so that in the first instance they impart the necessary resistance to keep heat, moisture, air and other flows within acceptable limits. These provide the environment in which the various parts of the wall must continue to function, and are seldom linearly graded from, outside to inside. Interactions between the various factors involved may produce conditions within the wall that require special attention. Some understanding of the phenomena involved is therefore necessary so that they can be judged quantitatively and wall designs evaluated. They may apply to individual elements of the wall as well as to the wall itself, and even to the complete building, of which the wall is only a part.
Durability is an overriding consideration.
Taken broadly it can be measured in terms of the time until some loss of function occurs that renders the service provided unacceptable. It is more appropriate to refer to service, life, as this properly implies the effects of the particular conditions of service. The environment at any plane in the wall may be determined by the arrangement and selection of the materials used. The designer may, therefore, by judicious arrangement greatly ease the requirements for the various elements and thus broaden his choice. The real importance of durability is reflected in the cost of maintaining all the required functions of the wall over the required service life.