Overview
Developing appropriate enclosure strategies is vital to the survival of every architectural practice. In today's litigious society, defects and failures of the building envelope are subject to a level of scrutiny unknown in previous generations. Engineering consulting firms have whole teams of experts specializing in building forensics. The body of published research and case studies continues to expand and practitioners can no longer claim ignorance of better practices and fairly exotic precedents. Architects are not sued for ugly buildings, and they may be shielded from sub-standard heating, ventilation and air-conditioning systems, but they remain a prime target of litigation related to enclosures. The realities of contemporary Canadian architectural practice become obvious after the working drawings and specifications are prepared and the tenders are received. This is often the time when the difference between the client's budget and the lowest bid causes major revisions to the building elements. Many months of thoughtful design are given the "shake and bake" treatment in a manner of several weeks. A common victim of this process is the enclosure - not the part that people see, but the part that actually performs environmental control functions.

Sales representatives and contractors join the fiscal frenzy to get costs in line, always having a cheaper alternative for what was originally proposed to placate the parsimonious client. None of the advising parties ever has to seal and sign their prescriptions, and eventually the architect is left to vouch for a "Frankenstein" enclosure system. Millennia of architectural evolution and innovation are forced to surrender to the myopia of first costs, seamlessly guided by cut and paste functions ubiquitous to all computer aided design programs. To survive, architects must adapt by evolving envelope strategies that can withstand the financial scalpel of a misguided client, the corner cutting of the lowest bidder and the vagaries of bad weather, labour disputes and substandard product substitutions. They must know and abide by the limit states of their enclosures.

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Storage and drying strategies underlie most traditional masonry buildings, where the mass of masonry has sufficient moisture absorption capacity during wetting periods to safely store water that is later released during drying periods. Experience has shown that in a number of historical buildings, the introduction of air-conditioning has detrimentally upset the wetting/drying balance leading to the dynamic buffer zone strategy described later on.

'Perfect' barriers are assemblies that have only one water resistant layer dedicated to water management. Examples include face-sealed systems such as window walls, most conventional roofing assemblies, and foundation waterproofing systems. All barrier strategies rely on a high and consistent quality of materials and workmanship, and favourable weather conditions during installation.

Low-slope roof assemblies rely on a perfect barrier approach and the success of these designs often rests with a continuous, impervious membrane layer. When this membrane forms the face seal, experience has shown that storage and drying also plays a role. Various layers of insulation and protection boards allow for the safe storage of moisture that penetrates flaws in the membrane. Sloped roofs with shingles, such as those covered with wood shingles or shakes, are actually drain-screen systems with the majority of drainage occurring at the surface. The overlap between courses of shingles provides a second drainage layer. If the roof sheathing is protected with a water resistant roofing paper this provides a third layer of drainage. Finally, the roof sheathing performs as the last line of defence - a drainage plane and a storage layer. Roofs are an excellent example of how a primary strategy such as the perfect barrier can be hybrid with secondary strategies such as storage and drying or drain-screens.

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Dynamic buffer zone (DBZ) strategy uses an active mechanical system to maintain the environment of existing masonry wall systems similar to that prior to retrofit. It works by introducing a layer of warm, dry, moderately pressurized air into the wall cavity to maintain a conducive wetting/drying balance. This strategy is not normally applied to new buildings where more cost effective solutions are available at the design stage, however, in existing and/or historical buildings which are usually of masonry construction, this approach can preserve the durability of the masonry after full environmental conditioning is introduced.

Drain-screens employ a secondary line of defence for water entry inboard of the exterior cladding. This secondary line of defense is called a drainage plane. A space between the cladding and the drainage plane promotes drainage and ventilation. Screen assemblies are typically used with water sensitive building materials such as wood framing, steel studs, wood based sheathings and gypsum based sheathings. Drain screens can also provide an indeterminate amount of pressure moderation.