On the qualitative side of the environmental control picture is the knowledge of what makes a building envelope function and a general understanding of how suitable the materials are for a given use. However, qualitative decisions can appear somewhat arbitrary, and therefore, attempts are made to support them with a quantitative analysis. Additional effort is directed at changing material standards so that they are based less on the properties of the materials and more on field performance. Twenty years later the concerns are still valid. While many of today's achievements in building science demonstrate a successful understanding of both the scientific principles and the art of construction, there is no formal procedure underlying the process of designing for environmental control.
Discontinuity can significantly reduce the efficiency of the environmental barrier.
As discussed in Part 1 of this series, air transport represents a critical factor in environmental control. It underscores virtually all facets of environmental control, as it moves both heat and moisture through the building envelope. Differential movements in the structure also affect the continuity of air barriers. These movements develop after construction because of thermal expansion or contraction of the building elements, deflection of beams, or mortar shrinkage. While some structural movements can be predicted, in many cases designers must rely on experience and judgment to anticipate the impact of differential movements on the specific design.