THE ENVELOPE OF ANY BUILDING continuously responds to the changes in indoor and outdoor temperature, pressure, and humidity conditions. This results in an exchange of energy and mass (air as well as moisture) between the indoor and outdoor environments through the envelope. Building physicists refer to this phenomenon as heat, air, and moisture transport through building materials and components. Designers and builders are always interested in knowing the long-term performance of the building envelope, as subjected to the transport processes. But the global differences in construction practices, building materials, weather conditions, and indoor climate are so large that it is impractical to develop this knowledge only through experimental investigations. However, such knowledge, as and when required, can be generated through calculations. Building physicists, over the past four or five decades, have been attempting to develop reliable calculation methods for this purpose. These attempts, especially with the advance in computer technology, have resulted in a diverse set of procedures and computer models [1]. All such procedures, in spite of any diversity, make use of the following two axioms: 1. The transport of any entity B through a medium is given by the expression Rate of transport of B = A property of the medium x, a driving force responsible for the transport 2. During the transport of B, in any given volume, V, of the medium, the following balance exists: Rate of storage of B in V = Rate of B entering V through its bounding surfaces + rate of generation of B in V