A typical building fabric consists of many components of different thermal conductances arranged variously in series and parallel. For example, the side of a house may have some regions of cavity brick wall into which are inset some metal window frames fitted with single sheets of glass. Thus, there will be some parts of the room behind that are separated from the outside by only one layer of glass, in parallel with some parts that have two layers of thin metal (the window frame), in parallel with some parts that have a layer of brick in series with an air gap, another layer of brick and a coating of plaster. The overall thermal performance of the wall will be a function of all of these.
Fortunately, enough is known about various materials to enable the calculation of an overall thermal character for most common building fabrics so that an overall conductance (or resistance) can be derived. Such values can be calculated for single glazed and double glazed windows, concrete slab floors, suspended wooden floors, walls and so on. These characteristics are usually written as an R-value or a U-Value for each of the various forms of construction and/or structural elements. More complex simulation techniques add a lag and decrement value or a set of response factors to describe the dynamic thermal behavior of the element.
Therefore, the knowledge of the various properties of the building fabric enables mathematical modeling of a whole building by taking all the various components and their areas into account and subjecting the hypothetical building to a dynamic regime of internal energy inputs, external solar loads, outside air temperatures, wind velocities, etc. Thus, if all the fundamental sources of heat loss and heat gain in a building are properly considered, it is possible to determine quite accurately the resulting internal conditions within it and, more importantly, how comfortable it is likely to be or how much air-conditioning energy will be required to make it so.