Overview
While the tectonic basis of enclosure has not changed significantly over time, our understanding of building envelope performance has made dramatic advances. Practising architects frequently remark that due to local construction conventions, many envelopes have been overbuilt, thus providing a much higher factor of safety against performance problems than is required in a particular climatic location. In other cases, evidence suggests that building envelopes have been under-designed, most frequently causing severe moisture problems compromising the integrity of the enclosure.

Trial and error methods of building construction underpin the inherited technologies we refer to as traditional or conventional building enclosures. Evolution was gradual, incremental with numerous local adaptations responding to differences in climate and weather, methods and materials. Undoubtedly, many failures preceded the refinement of durable, well-performing building enclosures, as remains the case to this day. The significant difference today is that failures, defects and performance problems, the consequences of inappropriate building enclosure design, are no longer confined to a handful of buildings. Our ability to execute large scale construction (thousands of buildings annually), introducing innovative materials and assemblies that have not enjoyed trial-and-error evolution, often exceeds our grasp of architectural science knowledge at the point of its primary user - the architect. Traditional materials and methods have been validated by past precedents, however innovation requires the application of architectural science and engineering.

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Moisture Engineering
A recent development in architectural science thinking has borrowed from structural engineering and involves the application of limit states design theory to the design of building enclosures, implying that concepts of load and load resistance are as applicable to moisture protection as they are to structural design. This is an encouraging development that may soon begin to close the gap between design ambitions and technological grasp.

Dr. Joseph Lstiburek of Building Science Corporation in Boston advocates that we should consider rain, temperature, humidity and the interior climate as environmental loads, the limit states of which are decay, mold and corrosion.

The limit states design of moisture protection for enclosures would consider the probable exposure to moisture based on the following conditions: 1) the regional climate and weather; 2) site influences, such as terrain, orientation, exposure and adjacent structures; 3) building geometry, roof and façade features; and 4) the use/occupancy of the building. Hence, it should be expected that a swimming pool enclosure in Yellowknife would differ significantly from a warehouse enclosure in Windsor, due primarily to differences in climate and use.

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Precipitation Exposure
Exposure to precipitation continues to represent the most significant potential for wetting of building enclosures. In North America, the level of exposure ranges from extreme to low. Comparing Nova Scotia to lower British Columbia, their wetting potential is similar, however, the mixed-humid climate in this part of British Columbia exhibits much lower drying potential. If the water gets in, during or after building construction, it will have a very difficult time getting out of the building enclosure.

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Interior Climate Classes
Interior climate classes are useful means of determining the potential severity of moisture problems, and how to better integrate enclosures with heating, ventilation and air-conditioning systems to minimize that potential.

Class I interior climates correspond to enclosures without intentional moderation of the indoor environment, such as unheated garages or porches, and also mediating enclosures such as atria, sunrooms, etc.

Class II interior climates represent the majority of conventional buildings where heating and cooling systems control temperature but not necessarily the relative humidity or internal pressures.

Class III interior climates correspond to spaces or facilities requiring careful control such as operating rooms, clean rooms, etc.

It is interesting to note that the idea of class struggle is not confined to the study of history and political science alone - in terms of interior climate classes, architecture is now also undergoing its own revolution.

Limit states design of building enclosures will propel moisture engineering and architectural science thinking. It is important that architectural science seeks to address issues related to the predictable durability and performance of building enclosures so that we know what to design. No less important is the need to decide what should we design? The next section on Environmental Mediation takes a look at a several perspectives on enclosures.

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