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Overview
Overview Enclosure design strategies require
a phenomenological approach if they are to be effective. These phenomena,
which are more fully discussed in the March 2002 contribution to
Architectural Science Forum, involve site and occupancy influences
on the enclosure. There are also effects which arise from building-as-a-system
behaviour where problems such as indoor air quality or discomfort
may result from interactions between the site, occupants, enclosure
and environmental control systems.
The phenomenological approach to architectural design may be equally
applied to firmness, commodity and delight, recognizing that in
the case of delight, the psychological and cultural factors influencing
the aesthetic experience represent phenomena which are subjective
and difficult to predict at the design stage.
The phenomenological approach may also be extended to human factors
and random error. When reviewing enclosure design strategies, there
are several practical considerations that prove critical to enclosure
performance.
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Simplified schema of phemonenological approach to building
system behaviour.
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The phenomenological approach to architectural design
may be equally applied to firmness, commodity and delight, recognizing
that in the case of delight, the psychological and cultural factors
influencing the aesthetic experience represent phenomena which are
subjective and difficult to predict at the design stage.
The phenomenological approach may also be extended to human factors
and random error. 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 a certainty 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 which only
approximately fulfill their design intent.
Management
approaches with respect to physical phenomena affecting
the building envelope system have shown through experience to be
superior 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 a barrier
approach.
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,
the other control requirements are either coincidentally satisfied,
or more easily satisfied than if moisture migration is not addressed
at the outset.
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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 have accounted 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.
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| Extreme values of rainfall phenomena are
statistically estimated according to intensity, duration and
frequency parameters. Similar data may be assembled for a variety
of phenomena related to site and occupancy influences. |
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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 the designer 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 instead realizes that the results are
often only accurate to within one order of magnitude. 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 experientially dependent fields
such as engineering and architecture.
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| The degree of risk for building failure that
society is willing to accept varies with the consequences in
terms of loss of life and property damage. For many phenomena,
such as tornadoes and earthquakes, it is either not possible
or affordable to construct buildings that can withstand extreme
events. |
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Redundancy and Multi-Functionality
Given the practical considerations noted earlier, and the incomplete
state of architectural science knowledge regarding enclosure performance,
it is prudent to apply a factor of safety (or more appropriately,
a factor of ignorance) to enclosure designs. 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(s),
(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 fundamental considerations may be summarized
as:
Take a phenomenological approach
to design which fully considers site and occupancy influences, and
building-as-a-system interactions.
Assess the intensity, duration and
frequency of the phenomena affecting the enclosure.
Reconcile these with the risk
and consequences of premature deterioration or failure of
the enclosure.
Consider how to effectively balance redundancy
and multi-functionality to achieve a design which is forgiving
of imperfect workmanship and materials.
The next section on Moisture
Rules takes a closer look at moisture management and its pivotal
role in cold climate enclosure design.
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