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Building Science Digest 018

The Building Enclosure

2006-08-01 by John Straube

Abstract:

That part of any building that physically separates the exterior environment from the interior
environment(s) is called the building enclosure or building envelope. Environmental separator is
another term used to describe the enclosure, but note that this generic term also applies to separators of
two different interior environments. The term building enclosure is preferred to the term building
envelope largely because it is considered both more general and more precise. Also note that the
building enclosure may contain, but is not the same as, the so-called thermal envelope, a term that is
used to refer to the thermal insulation within the enclosure.

The enclosure, the loadings it must resist, and its functions are addressed in this digest.

The Nature of the Building Enclosure

Both the above-grade and the below-grade portions of the building enclosure are part
of a physical system involving three interactive components: the exterior
environment(s), the enclosure system, and the interior environment(s). The exterior
environment above grade is very different from that below grade, and within any
building there can be numerous interior environments. The nature of the building
enclosure and its spatial relationship with the other parts of the building are shown in
Figure 1.

1
2 Building Science Digest 018

Figure 1: The building as a set of separated spaces and the as-built separators

As suggested by Figure 1, a building in general consists of a collection of spaces

bounded by a set of spatial separators. There are separators between interior
environments as well as separators of an interior environment and the exterior
environment; collectively, the latter constitute the building enclosure. In many
respects, the functions of a separator of internal environments within a building (floors
and interior walls) are very similar to those of the building enclosure. Usually,
however, the performance requirements of internal separators are fewer and much less
onerous.
The Building Enclosure 3

Enclosure Components

The primary function of the enclosure is to separate the interior environment from the
exterior environment to which it is exposed. Physically, the typical building enclosure
usually consists of the following components:

• the roof system(s)

• the above-grade wall system(s) including windows (fenestration) and

doors

• the below-grade wall system(s), and

• the base floor system(s).

The building enclosure should not be thought of as a combination of numerous one-

dimensional or even two-dimensional planar components. Each enclosure component
is a three-dimensional, multi-layer, multi-material assembly that extends from the
inside face of the innermost interior layer (e.g., the paint or wallpaper) to the outside
face of the outermost layer (e.g., paint or roof shingles). The overall enclosure is made
up of all the contiguous enclosure sub-assemblies.

Each enclosure component is an assemblage of layers of specified products (such as

gypsum board or wallpaper) or materials (such as paint or wood). For instance, the
thermal insulation could consist of a layer of blown-in glass fiber filaments. A
deliberate air space or cavity is also a considered to be a layer. Consider, for example,
the sloped roof system shown in Figure 1; this constitutes an enclosure assembly
consisting of all the contiguous layers between the finish on the ceiling and the outer
face of the roof shingles. Furthermore, the compacted gravel under the basement
floor (or base floor) system and the backfill to the outside of the foundation or
basement wall system are each specified layers within their as–built assemblies. As
such, they can be considered to be an integral part of their respective building
enclosure components. By common definition each enclosure sub-assembly
incorporates all the contiguous (in-contact) layers.

The climate-related loadings that a building and its enclosure actually experience are
modified versions of the local climate. These microclimates are modified by adjacent
buildings, the landscaping (especially trees), and other parts of the enclosure. Roof
overhangs, for instance, are climate modifiers to the walls below as well as integral
parts of the roof system.

The above definition of the enclosure has some limitations. First, the physical
difference between site microclimate modifiers and the enclosure is not that
straightforward. Second, although one can define the enclosure as the building
component that separates interior from exterior environments, it is not always easy to
4 Building Science Digest 018

precisely define “interior” and “exterior.” For example, the backfill around a basement
wall or ivy growing on a masonry wall are, by definition, considered to be part of the
enclosure. They are also part of the site. What cannot be ignored is the significant
impact of this soil or ivy on moisture and thermal loadings.

Consider also an attached garage. The space in the garage does not experience
outdoor conditions, nor is it conditioned like the interior of the building. The
enclosure could be considered to incorporate both the exterior wall of the house and
the garage wall, with the space between (containing the car) being an integral part of
the enclosure. Alternatively and preferably, the enclosure could be considered to be
the garage wall, and the intermediate wall could be viewed as an interior separator with
significantly different conditions on each side.

Similarly, two adjacent internal spaces can also have quite different environmental
conditions. For example, consider the not uncommon scenario of a hotel with a warm
humid swimming pool next to an air-conditioned exercise room or a community
center containing both a swimming pool and ice arena. In such cases the loadings
and performance of the interior separator are more like those of a building enclosure
component. In this type of condition enclosure design principles should be applied to
the interior separator.

The influence of climate modifiers, such as trees, garages, overhangs, and the soil on
the loadings experienced by the enclosure is not always appreciated. Especially in the
case of low-rise buildings, the effect on the enclosure from the exterior environment
can be intentionally reduced or moderated by the use of microclimate modifiers such
as plantings, fountains, building overhangs and windbreaks. For example, a roof
overhang (an integral part of the roofing system) will moderate the amount of rain on
the wall below and will therefore be a microclimate moderator for the vertical portion
of the building enclosure. With taller buildings it is more difficult to modify the
external microclimate.

The definition of where the enclosure begins and exterior environment stops can
sometimes be confusing, especially in the case of buffer spaces such as garages,
screened porches, vented and storage attics, or vented crawlspaces.

The exterior environment could be considered to be a three-dimensional space with

randomly varying mass and energy properties. The local climate and weather provide
the major, but not the only, exterior loadings. Local climatic records ⎯ for instance,
average and extreme values ⎯ are commonly available but these data are usually based
on open-field, weather-station records. The building may be some distance from the
weather station, and the terrain (hills, other buildings, etc.), the landscaping (trees,
shrubs, etc.) and the building itself (overhangs, protrusions, etc.) can moderate the
weather that each enclosure component is actually subjected to. As a matter of
convention, that portion of the exterior environment that is close to, and affected by,
The Building Enclosure 5

the building is called the external microclimate. In fact, different parts of the enclosure
are subjected to different exterior microclimates.

The interior environment is usually specified in relation to the physical needs of people
and are defined in terms of temperature, relative humidity, airflow rate, and air quality.
However, sensitive equipment, materials, or processes (e.g., computers, archived paper,
electronics factories) may require different, often more stringent, conditioning
requirements than those required by (adaptable) human occupants. Special uses, such
as swimming pools, pharmaceutical plants, ice arenas, etc have significantly different
conditions than normal. Because the interior spaces are usually conditioned, the state
of the interior environment is often assumed to be constant, but in practice, significant
variations occur over the day, between different spaces and over the seasons.

The service systems (e.g., lights, motors, stoves) and the contents of a space have a
strong and time-dependent influence on the interior environment. The level of the
influence depends on the controls, output capacity, and distribution effectiveness.

Figure 2 indicates that, while the building enclosure separates the interior and exterior
environments, it really experiences several microclimates. The enclosure interacts with
both environments and, in turn, affects both environments. This interaction is usually
dependent on the time of day, the day of the week, and the season. There is thus a
cyclical (diurnal, workday/non-workday, weekly and seasonal) aspect to the time-
dependent response of the enclosure as it separates and, to some degree, modifies the
influence of both environments.

Figure 2: Contributors to environmental modification

6 Building Science Digest 018

While the enclosure may be the largest modifier, the difference or the modification
between the ambient and interface conditions can also be significant. Note that the
scale of Figure 2 will be different for different parameters. Moreover, this simple
representation of variation over one cycle of time for one parameter does not
accurately or fully represent the complex behavior and interactions that occur.

The properties of exterior environments and interior environments are separate and
major fields of study. Each environment is discussed in other Building Science
Digests, Primers, and Design Guides.

Enclosure Loadings

The performance of any portion of the enclosure should be considered in relation to

the various loadings generated by i) the exterior environment, ii) the interior
environments, and iii) the enclosure itself. The generic term loading refers to any event,
phenomenon or characteristic that can affect the enclosure. Each and every loading
that impacts the building enclosure can be categorized into one of just five types,
namely:

• gravity related
• ground related
• heat related (thermal)
• moisture related
• air related

Exterior Environment Loadings

The exterior environment described above is responsible for many different loadings.
Sources of these loadings are the climate (weather over the short term), human or
human-made effects, and nature or natural phenomena. Table 1 identifies, by type and
source, those loadings due to the exterior environment that could affect the enclosure.
It should be evident from this list of loadings that the design or analysis of the
enclosure involves consideration of not only the usual structural loadings but also mass
(air, moisture, etc.) and energy (heat, light, sound, etc.) loadings. Moreover, for design
purposes, we need to have some knowledge of both average and extreme conditions
for each relevant loading.
The Building Enclosure 7

Table 1: Loadings from the exterior environment

Type
Moisture Air Ground Gravity
Heat related
related related related related
Source
Weather or Ambient RH, fog, Barometric Water,
natural climate conditions, rain, ice, pressure, snow, hail
solar snow wind
Abnormal Reflected Tornado, Tornado, Frost heave, Wind-borne
climatic effects solar, hurricane, hurricane landslide missile
lightning flooding
Natural Fire, Adfreezing, Radon, Seismic, Hydrostatic
phenomena Ground Freezing methane, land- slide, pressure,
water soil gas settlement, soil pressure
termites,
plants, etc.
Human-made Global Smog, Wind related
weather warming, Acid rain vortex/swirl
city effect
0
(2-7 C)
Human-induced Fire Fire (hoses, Smoke, Impact, wear
events sprinklers, sonic boom, and tear
etc.) sound,
explosion

Interior Environment Loadings

The interior environment consists of occupied, used and, often, conditioned spaces.
As shown in Table 2, the main sources of loading are human-induced, the result of
natural phenomena or due to conditioning. Variations in the state of the interior
“climate” and the related extreme values tend to be smaller or less than those
occurring within either the enclosure or the exterior. This occurs not only because the
building enclosure modifies the effect of exterior variations but also because the
interior environment is generally augmented by inputs of mass and energy, e.g., the
building or parts of it may be mechanically conditioned.

For a building to successfully meet its desired performance, the required state of each
of the interior environments must be ensured. The state of an interior space may
either be maintained constant or may vary in time as a function of the nature and
degree of space conditioning, internal gains (equipment, lighting, solar effects and
human activities) and the modifying influence of the enclosure.
8 Building Science Digest 018

Table 2: Loadings from the interior environment

Type
Heat Moisture Air-flow Ground Gravity
related related related related related
Source
Interior space Ambient RH, water Barometric Water
conditions, (sprinklers, pressure, wind,
solar etc.) stack, fan
induced
Natural Fire Fungal growth, Radon, Settlement,
phenomena mold methane termites,
plants, etc.
Human- Fire, People, Smoke, sound, Impact,
induced people flooding, explosion wear and
events combustion, tear, dead
equipment & live
loads

Loadings from the Enclosure

The enclosure itself is a source of loading and, as shown in Table 3, loadings arise
from the enclosure element under consideration as well as adjacent elements.

To properly design buildings and their enclosures it is necessary to have some

knowledge and understanding of the relevant environments as well as their
interdependence. A considerable body of empirical and experimental data exists on
the interaction of climate, site and building [Watson and Labs 1983, Olgyay 1963,
Brown 2001, Moore 1993]. Research, notably that by Fanger [1970] and Givoni
[1981], has identified and defined the physiological aspects of human response to both
interior and exterior climates, and these data have been distilled into convenient forms
for building designers [ASHRAE 2001].

The enclosure is only one part of the larger system that moderates or controls the
environments on both sides (especially the inside) of the enclosure at some location on
the building. The building itself and the enclosure both influence the exterior
microclimate that interfaces with the exterior surface of the building enclosure at a
specific location. Consider, for example, the local impact of wind, rain, and solar
radiation. In addition, the mass and energy inputs provided by heating, ventilation, air
conditioning equipment or other internal gains likewise modify the interior
environment.
The Building Enclosure 9

Table 3: Loadings from the enclosure

Type
Moisture Air Ground Gravity
Heat related
related related related related
Source
Element or Volume RH, built-in Off-gassing, Self
component change, moisture, air flow, weight,
being shape volume air pressure live loads
considered change, change, fungal differentials
fire growth, mold,
creep,
shrinkage, etc.
Adjacent Volume and Volume Smoke Dead
Elements shape change loads,
change, fire live loads

The Functions of the Enclosure

At the most basic level, the primary function of the building enclosure is to separate
the interior and exterior environments. In practise the building enclosure has to
provide the “skin” to the building, i.e., not just separation but also the visible façade.
Unlike the superstructure or the service systems of buildings, the enclosure is seen and
is therefore of critical importance to owners, the architect and the public. The users or
occupants are concerned with both sides of the building enclosure. The appearance
and the operation of the enclosure have an influence on the interior environment and
on factors such as productivity and satisfaction.

In general the physical function of separation of the building enclosure may be

grouped into three sub-categories, as follows:

1. Support functions, i.e., to support, resist, transfer and otherwise

accommodate all the structural forms of loading imposed by the
interior and exterior environments, by the enclosure, and by the
building itself. The enclosure or portions of it can be an integral part
of the building superstructure ⎯ usually by design but sometimes
not.

2. Control functions, i.e., to control, regulate and/or moderate all the

loadings due to the separation of the interior and exterior
environments ⎯ largely the flow of mass (air, moisture, etc.) and
energy (heat, sound, etc.).

3. Finish functions, i.e., to finish the enclosure surfaces⎯the interfaces

of the envelope with the interior and exterior environments. Each of
the two interfaces must meet the relevant visual, esthetic, wear and
tear and other performance requirements.
10 Building Science Digest 018

A fourth building-related category of functions can also be imposed on the enclosure,

namely:

4. Distribute functions, i.e., to distribute services or utilities such as

power, communication, water in its various forms, gas, and
conditioned air, to, from, and within the enclosure itself.

Figure 3 illustrates a representative portion of a building enclosure, its functions and

the nature of the loadings. The enclosure may also serve other, usually non-physical,
purposes such as advertising or as a symbol or image, e.g., as a statement about power
or security or wealth, etc., but these are not considered here.

Each and every part of the enclosure must satisfy the relevant support, control, finish
and distribution functions. Only the support and control functions are needed
everywhere. The finish functions may not be needed in some areas (such as above
suspended ceilings, in service rooms). Moreover the distribution functions, which
largely service the adjacent interior spaces, need to be met only where there is a service
or utility to be distributed – large segments of most enclosures do not need to fulfill
this function.

To further demonstrate the relationship between category of function and loading,

consider Table 4. All the possibly relevant loadings are listed and their functional
relevance is identified. This table is important because it clearly shows that the
number of loadings is considerable and that each loading affects at least one functional
sub-category for the enclosure and often more than one.

Figure 3: The enclosure and its functions

The Building Enclosure 11

Table 4: General category of loadings and related functions

Category of
functions

Support

Exterior
Control
Interior
Specific loadings

Gravity – Dead (assembly, etc.) z

Gravity – Live (people, snow, etc.) z
Wind z {
Essentially
structural

Ground Movement (seismic, settlement, etc.) z

Explosion z
Causal phenomenon or loading

Rheological (creep, shrinkage, etc.) z {

Impact (vehicles, missiles, people, etc.) z
Fire z
Heat (thermal, etc.) { z
environmental

Air (pressure, movement, leakage, etc.) { z

Essentially

Moisture (built-in, precipitation, etc) { z {

Smoke z
Solar radiation (incident, reflected, etc.) z {
Chemical attack/atmospheric (acid rain, etc.) z {
Particulate matter (dust, VOC’s, etc.) z
People (wear & tear, etc.) { z
Insects, birds, animals, (termites, rodents, etc.) z
Essentially

z
perceptual

Light (natural, incandescent, fluorescent, etc.)

Sound { z {
Visual – local z z
Visual – contextual z z

Primary significance z

Secondary significance {

Tertiary significance •
12 Building Science Digest 018

Enclosure Performance Attributes

In any built facility the building enclosure must perform satisfactorily, i.e., each and
every loading must be controlled or resisted (i.e., managed). There are the physical
functions (previously categorized as support, control, finish and distribute functions)
as well as qualitative functions or attributes. Ideally all of the attributes listed below
need to be satisfied:
• constructability (or buildability)
• economic viability or cost over both the short and long term
• viewability including aesthetic, cultural and other visual expectations
• utility
• sustainability (renewable, non-toxic, etc)
• serviceability (perform in service)
• safety in relation to life, health, injury, property and economic
enterprise
• productivity (in construction and installation)
• operability (for users)
• maintainability (for operators)
• repairability (for operators)
• durability
• convertibility in relation to modification, extension or conversion
• disposability.

This list identifies each of the general attributes that apply not only to the building
enclosure but also to the building materials, sub-systems, building, and the building
facility. Except for the first and last two, all are attributes of in-service performance.

Design

The three primary design dimensions of any building have been identified and
discussed, i.e., the possible loadings, the relevant physical functions, and the attributes
of performance. These apply not only to the building enclosure but also to the other
physical components of the building.

Function, attribute and loading each constitute a dimension for design in the sense that
the design team needs to systematically and explicitly consider each function, each
The Building Enclosure 13

attribute and each loading and all relevant combinations of these loadings. In each
step a technical design criterion is set up in such a manner that the modeled or
computed response is demonstrably better than or satisfactory when compared to the
permissible performance threshold. In mathematical terms this process can be written
as follows:

{ }
max β
∑ ∑ c w
R ⊆R
m s
min $

For all (i.e., the sum of) relevant combinations of loadings (c) and for all relevant
loadings (w), the modeled or computed response (Rm) must be demonstrably better
(larger or smaller as the case may be) than a specific limit or threshold value (Rs) for
this particular response. This conditional criterion needs to be resolved for all relevant
situations and this has to be accomplished at minimum cost ($) and for maximum
benefit (β).

This approach has been widely used in limit states design for structural systems, but is
not yet well developed for building science applications such as building enclosure
design. Such a rational approach has not been applied to enclosure design because of a
lack of quantitative understanding (of building physics and material properties) and no
professional and research framework to create the approach for analysis, loadings, and
performance thresholds. This may change in the coming decades.

Closure

In this digest a concerted attempt has been made to develop a systematic, logical and
consistent framework or context to study, analyze and design both buildings and the
building enclosure. Numerous researchers [Markus and Morris 1980, Hutcheon 1963,
Elder 1974, Allen, 1980] have addressed this issue, most notably and comprehensively
James Marston Fitch [1960, 1975]. Fitch’s writings, particularly The Aesthetics of Form
and American Building, deserve to be much better known. Interested readers should
review these sources.

References

AJ Handbook of Building Enclosure, Ed. A.J. Elder, The Architectural Press, London,
1974.

Allen, E., How Buildings Work. Oxford University Press, Oxford and New York, 1980.

ASHRAE Handbook of Fundamentals, American Society of Heating, Refrigeration and

Air-Conditioning Engineers Inc., Atlanta, Georgia, USA, 2001.
14 Building Science Digest 018

Brown, G.Z. and DeKay, M., Sun, Wind and Light. Architectural Design Strategies, Second
Edition, Wiley, 2001.

Fanger, F.O., Thermal Comfort, Analysis, and Applications in Environmental Engineering.

McGraw-Hill Inc., New York, 1970.

Fitch, J.M., and Branch, D.P., "Primitive Architecture and Climate", Scientific American,
Vol. 203, pp. 134-145, 1960.

Fitch, James M., American Building: The Environmental Forces That Shape It. Schocken
Books, New York, 1975.

Givoni, B., Man, Climate, and Architecture. Van Nostrand Rheinhold, New York, 1981.

Hutcheon, N.B., Requirements for Exterior Walls. Canadian Building Digest 48. Division
of Building Research, National Research Council, Ottawa, 1963.

Markus, T.A. and Morris, E.N., Buildings, Climate, and Energy. Pitman Publishing,
London, 1980.

Moore, F., Environmental Control Systems. McGraw-Hill, New York, 1993.

Olgyay,V., Design with Climate. Princeton University Press, Princeton, New Jersey,
1963.

Watson, D., and Labs, K., Climatic Building Design. McGraw-Hill Inc., New York, 1983.

John Straube teaches in the Department of Civil Engineering and the School of
Architecture at the University of Waterloo. More information about John Straube
can be found at www.johnstraube.com

Direct all correspondence to: J.F. Straube, Department of Civil Engineering,

University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
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