Enclosure Design Strategies

In the March 2002 issue of Canadian Architect, Architectural Science Forum looked at Principles of Enclosure, promising to deal with enclosure design strategies and tools in this current issue. It soon became evident that enclosure design strategies was a topic of its own–one that continues to seriously challenge practitioners and researchers alike.

Fundamental considerations for building envelopes advanced by Neil Hutcheon in 1953 are now widely accepted, as is the predominant role of moisture in building enclosure performance problems. However, this has not appreciably simplified enclosure design strategies in a world where innovative materials and assemblies are emerging at an ever-increasing rate. Heightened client expectations for distinctive innovation, combined with compressed project timelines and budgets, often limit the attention required to fully consider enclosure strategies at the schematic design stage.

Before architects can take full advantage of today’s sophisticated tools for analyzing and predicting the performance of enclosures, it is necessary to first select an appropriate strategy or concept that addresses the parameters identified in the March edition of Architectural Science Forum. Otherwise, the time and resources allocated to performance assessment may be ineffectively deployed.

Practical considerations

When reviewing enclosure design strategies, there are several practical considerations that prove critical to enclosure performance.

Workmanship and materials are imperfect. If it is likely for a set of working drawings and specifications to contain errors and omissions, then it is almost certain that the translation of these instructions to contractors and their trades will be imperfect. Inaccuracy and inconsistency of workmanship and materials, in conjunction with variable weather conditions, result in buildings that fulfil their design intents only approximately.

Experience shows that management approaches with respect to physical phenomena affecting the building envelope system are preferable to barrier approaches. Not only do they provide redundancy for critical control functions (moisture migration, heat transfer, air leakage and solar radiation), but they are more forgiving to construct, since tolerances are typically greater than those required by barrier approaches.

Enclosures must adequately address critical environmental control functions–moisture migration, heat transfer, air leakage and solar radiation. In cold climates, experience indicates that when the requirements for the control of moisture migration have been satisfied, other control requirements are either coincidentally satisfied or more easily satisfied than if moisture migration is not addressed at the outset.

Intensity, duration and frequency

All physical phenomena share intensity, duration and frequency as defining characteristics. Events such as earthquakes, tornadoes, hurricanes, and to some extent floods, are typically high-intensity phenomena of short duration and low frequency. In cold climates, vapour diffusion, heat transfer and air leakage are normally low-intensity phenomena of relatively long duration and seasonal frequency. Both types of phenomena account for considerable damage to buildings, however, the strategies for dealing with each varies not only in terms of the actual control measures, but mainly according to the associated risks and consequences of failure.

Risk and consequences

Risk of building failure involves the probability of a specified level of performance proving inadequate to resist an imposed physical phenomenon or phenomena. This is the rationale behind limit states design–to reasonably avoid the likelihood of failure.

The likelihood of failure must be reconciled with the consequences of failure in terms of safety, health, functionality, economy and aesthetics. Minimum levels of adequacy for health and safety in buildings have been established through codes and standards, and guide designers in these vital aspects of building performance. However, the risks and consequences of failure for a building requirement such as structural integrity are much better understood than for moisture protection.

For this reason, many building envelope design strategies are based less on probability and analytical models, and more on precedent, heuristics and common sense. This does not suggest that analysis is completely abandoned, but allows that the results are often only accurate to within one order of magnitude. Moreover, numerical data must be interpreted qualitatively, requiring significant experience and judgement on the part of the designer. This may help explain why there are no child prodigies in experience-dependent fields such as engineering and architecture.

Redundancy and multi-functionality

Given the practical considerations noted earlier, and the incomplete state of architectural scientific knowledge regarding enclosure performance, it is prudent to apply a factor of safety to enclosure design. Two common and effective means of ensuring reliable performance involve the concepts of redundancy and multi-functionality. These are better understood when related to the physical elements of a typical cold climate building envelope: structure, cladding, air/vapour retarder or barrier, thermal insulation, and interior finish. In most enclosures it is often the case that the constituent elements address more than one critical control function.

An enclosure component or assembly with more than one line of defence against imposed phenomena may be redundant with respect to one or more critical control functions. For example, multiple layers of insulation (cavity and exterior sheathing), or an air barrier membrane combined with a tightly sealed sheathing (tape and/or gaskets), represent redundant control measures for control of heat transfer and air leakage, respectively.

A material may be uni-functional, such as a structural element, or it may be multi-functional and address more than one required control function by resisting several physical phenomena. For example, a thermal insulation material may also provide resistance to air leakage, or assist cladding with the drainage of moisture penetration. Multi-functionality ranges from a single material that addresses all separator control functions (ideal), to a material that primarily addresses one control function (first line of defence) and contributes to another control function (second line of defence).

An interesting concept falling between redundancy and multi-functionality is contribution. A material may improve or enhance the performance of another material or assembly of materials without displaying multi-functionality or explicitly adding to redundancy. For example, an air barrier membrane may reduce air movement through an air-permeable insulation material, contributing to its thermal effectiveness, but not actually increasing the nominal thermal resistance of the assembly. Contribution is often difficult to quantify, but should not be overlooked when arranging materials.

The table depicted above summarizes fundamental control strategies corresponding to physical phenomena influencing enclosures. The relationships are somewhat hierarchical insofar as strategies for moisture management include strategies for control of heat flow and air leakage. Control of solar radiation is partially related to the previous three phenomena, but deals with a broader architectural perspective associated with insolation and fenestration.

Enclosure strategies

As noted earlier, most enclosure strategies predominantly address moisture management, recognizing that other phenomena such as fire and sound may also be critical.

“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.

Drain-screens employ a sec
ondary line of defence for water entry inboard of the exterior cladding. This secondary line of defence 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.

Rain-screens rely on the phenomenon of pressure equalization between outside air pressure due to wind and the pressure equalization cavity behind the cladding that also serves as a drain-screen. Rain-screens represent appropriate strategies for addressing frequent and/or intense exposure to wind- driven rain.

Storage and drying strategies underlie most traditional masonry buildings, where the 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 historic buildings, the introduction of air conditioning has detrimentally upset the wetting/drying balance leading to the novel strategy described below.

Dynamic buffer zone (DBZ) strategies use active mechanical systems to maintain the environments of historic masonry wall systems similar to those prior to retrofit. They work by introducing a layer of warm, dry, moderately pressurized air into the wall cavity to maintain a wetting/drying balance.

Graduated mediation is a complex strategy that arranges assemblies to manage moisture, heat and air movement, and often solar radiation, according to seasonal variations and occupant preferences. Double-skin facades, intelligent skins and kinetic enclosures represent contemporary approaches to graduated mediation of building environments.

The selection of an appropriate enclosure strategy, or strategies, involves consideration of physical phenomena, site conditions, occupant influences and issues associated with systems integration to arrive at a proposition with a reasonable chance of performing properly.

Precedent versus innovation

More buildings exist today than ever before in the history of humankind. Countless variations on typical enclosure strategies are available for precedent studies that are soundly based on demonstrated performance. For many conventional buildings, designers should seriously consider the rhetorical question, “why be original when you can be good?” New methods and materials can easily achieve excellent performance by intelligently manipulating successful precedents.

But not all buildings are conventional, and many aspire to a level of innovation that seriously challenges architecture and its allied disciplines. New materials and methods entice clients and architects to transcend precedents in order to discover new dimensions in design. This is the conceptual moment when feasible enclosure strategies are most critical. The challenge for architectural science is to evolve appropriate enclosure strategies that harness the potential of innovative materials and methods to fulfil their promise to contemporary architecture.

Coming in July 2002

The next edition of Architectural Science Forum will look at Enclosure Design Tools and how these may be used to both design innovative enclosures and assess the performance of existing building envelopes.

Ted Kesik is a professor in the Department of Architectural Science at Ryerson University and Visiting Associate Professor in the Faculty of Architecture, Landscape and Design at the University of Toronto. The full electronic version of this article is available on the Web at www.canadianarchitect.com under Architectural Science Forum. This forum is intended to facilitate the exchange of knowledge and information pertaining to architectural science among practitioners, researchers, academics and students of architecture and its allied disciplines. For further information on submissions to Architectural Science Forum, contact the editors.

In Canada, building rehabilitation for roofing and wall system repairs and replacement costs an estimated $7.5 billion annually. A conservative estimate of the premature failure rate is 3 to 5 per cent, or $225 to $375 million per year. Premature failure is defined as any performance condition requiring repair or replacement of the system before the benchmark date. The building envelope was identified as particularly vulnerable to durability problems.

–2001 Building Failures Study, Canada Mortgage and Housing Corporation, Technical Series 01-140.

CONTROLPHYSICALCONTROL
FUNCTIONMECHANISMSTRATEGY
Moisture MigrationBulk WaterShedding
Conveyance
Drainage
Storage & Drying
Drain-Screen
Rain-Screen
“Perfect” Barrier
Capillary WaterCapillary Barrier
Capillary Break
Vapour DiffusionVapour Barrier
Thermal Insulation
Air LeakageAir Barrier System
Thermal Insulation
Heat TransferConductionThermal Insulation
RadiationRadiation Barrier
ConvectionAir Barrier System
Air LeakageStack, Wind andAir Barrier System
Mechanical Effects
Solar RadiationHeatOrientation
Fenestration
Shading Devices
Thermal Resistance
Glazing Reflectance
and Emissivity
Visible LightOrientation
Fenestration
Shading Devices
Glazing Optical
Properties

Adapted from work by Bomberg, M.T. and Brown, W.C. (1993) “Building Envelope and Environmental Control: Part 1-Heat, Air and Moisture Interactions”, Construction Canada, 35(1), pp. 15-18.

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