DOUBLE
ENVELOPE
A SUSTAINABLE
BUILDING CONCEPT
OVERVIEW
• INTRODUCTION
• SELECTION CRITERIA
• RELATIONSHIP BETWEEN GLAZING AND FAÇADE
• PERFORMANCE GOALS
• CONSTRUCTION STRATEGIES
• MAINTAINANCE REQUIREMENTS
• IMPLEMENTATION CONSIDERATIONS
• DESIGN PROCEDURE
• EXAMPLES
INTRODUCTION
• Multiple leaf wall assemblies used in the transparent or largely transparent portions of a building facade.
• Double envelopes consist of an outer facade, an intermediate space, and an inner facade.
• The outer leaf provides weather protection and a first line of acoustic Isolation.
Section through the Genzyme Center in
Cambridge, Massachusetts, illustrating the
corridor facade configuration of a double skin
facade
INTRODUCTION
• The intermediate space is used to buffer thermal impacts on the interior. Through the use of open slots and operable elements in the glass planes it is possible to ventilate the interstitial space on warm days and admit partially conditioned air to adjacent rooms on cool days.
• In most cases sunshades are placed in the intermediate zone where they can operate freely, but with reasonable access for maintenance.
• Double glazing of the inner façade provides an optimum thermal barrier (for most climates), while single glazing of the outer facade is sufficient to create the buffer space.
SELECTION CRITERIA
Double envelopes present the building designer with an
extraordinary array of options. The selection of an appropriate
system proceeds through the following considerations:
• relationship of the glazing to the overall facade
• performance objectives of the transparencies
• construction strategies, and
• maintenance requirements.
RELATIONSHIP BETWEEN GLAZING AND
FACADE
Traditional facades usually have
punched openings or horizontal bands
of glass surrounded by solid wall
elements.
Structural loads are collected in the
solid portions and the principal glass
plane is often drawn back into the
depth of the wall.
Box-windows within the double skin facade
of the GAAG Architecture Gallery in
Gelsenkirchen, Germany.
In such cases it is relatively easy to add a second glazing plane
flush with the exterior face and attached to the same structure.
Examples include the traditional storm window and its modern
counterpart, the “box window”
RELATIONSHIP BETWEEN GLAZING AND
FACADE • A second façade type consists of an
outer glazing mounted a considerable distance in front of only selected portions of the facade.
• Examples include oriel windows, glazed loggias, and attached sunspaces
• An inner leaf of glass allows the captured space to act as a double envelope and develops an intermediate zone that is large enough to be a useful space under the right conditions. Glazed balconies in Venice,
Italy.
RELATIONSHIP BETWEEN GLAZING AND
FACADE • Finally, double envelopes may consist of
an outer leaf of glass across the entire
surface of the facade,
• This general type can range from a glass
“re-wrap” of an existing structure to a
free standing glass box with on or more
buildings sheltered inside
• Included in this type are the closely-
coupled glass double facades that have
become popular in Europe since the mid
1990s. Outer glazing covers the
entire surface of the facade at
the Arup offices in London.
PERFORMANCE GOALS
• Double envelopes can be further characterized by the tasks
they are asked to perform. These requirements determine
whether ventilation openings are to be developed in one or
both glass leaves and what elements are to be placed inside
the captured space.
• Most double envelopes are designed to maximize daylight
while controlling solar gain
• The interstitial space is used first and foremost as a protected
enclosure for operable shading devices that might otherwise
suffer from wind damage and weather exposure.
PERFORMANCE GOALS
• Solar energy absorbed by the shading devices is returned to
the exterior environment by free ventilation of heated air
through paired openings in the outer leaf or by stack
ventilation of the entire facade.
• Second among typical performance attributes is acoustic
isolation in urban environments. The best examples of
successful solutions use an unbroken outer leaf of glass with
ventilation air for the cavity coming from a remote source or
through a sound-baffled inlet system.
• Double envelope installations that are considered to be
effective performers are usually motivated by one or both of
these two factors.
PERFORMANCE GOALS
PERFORMANCE GOALS
• Additional performance
benefits include the
opportunity to ventilate
occupied spaces through the
inner leaf with the buffer zone
acting to mitigate air
temperature contrasts in the
winter or adverse wind effects
in tall buildings.
• Double envelopes mitigate the surface temperature of the
interior glass, reducing the mechanical intervention
required to provide comfortable conditions under both
heating and cooling modes.
PERFORMANCE GOALS
• The interstitial space can be used as a
solar collector to warm the building
directly or to move incident energy
from a sunny exposure to a shaded
exposure.
• The space can also be used to preheat
fresh air for introduction to spaces via
the mechanical system in buildings not
ventilated directly through the skin.
• Thermal siphon effects generated in a double facade can
be used to draw air out of a building, although other
forms of stack ventilation are more cost-effective.
CONSTRUCTION STRATEGIES
• One configuration consist of a single layer of glass attached to
cantilevered edges of floor plates with a thermally insulated,
infill system
• The name “corridor facade” is given to this and any
configuration in which the intermediate space is divided floor
by floor.
• Often the outer layer is a curtain wall , while the protected
inner leaf is a much less expensive storefront system provided
by a different vendor.
CONSTRUCTION STRATEGIES
To maximize usable floor
area, the outer glass leaf
may be suspended beyond
the edge of the primary
structure using struts,
cables, or trusses. In its
pure form, this approach
leads to the “multistory
facade” in which the cavity
is ventilated through large
openings at the base and
the parapet.
CONSTRUCTION STRATEGIES
By adding monumental glass panels to the outside face, fixed
shades can be replaced with lighter weight operable units that
can respond to changes in sun angle and intensity without
having to resist external weather forces.
For large projects it is often desirable to prefabricate the double
envelope as a unitized curtain wall system. Complete assemblies,
with inner and outer glass leaves installed, can be lifted into
place in one step.
The units may be self-contained “box windows” with air intake
and exhaust ports for cavity ventilation. Alternatively, they may
be connected to adjacent units to reduce the number of
ventilation ports and separate the intake and exhaust locations
across the facade.
CONSTRUCTION STRATEGIES
• Typically this is a “corridor
facade” with staggered vents. A
continuous vertical cavity can be
used as a thermal chimney to
exhaust the individual units on
either side in what is called a
“shaft-box facade.”
• Any technique that joins façade modules across multiple
interior rooms may improve airflow and reduce costs, but
raises concerns about fire spread and sound transmission
from room to room through the facade cavity
MAINTENANCE REQUIREMENTS
• The ultimate configuration of a double envelope will be
greatly influenced by the need to get inside the cavity to clean
the glass surfaces and maintain ventilation controls and
shading devices.
• Large-scale corridor facades meet this requirement without
disturbing the workspaces, but at the cost of significant floor
area around the building perimeter that is likely to be
underutilized.
• Multistory facades often incorporate service walkways of
metal grating into the cantilevered structure of the interstitial
zone.
MAINTENANCE REQUIREMENTS
• In Europe, especially Germany, building codes and cultural
traditions require that a high percentage of the inner glazing
leaf be operable to allow for individual control over outdoor
air in the workspace.
• If operable glass can provide access into each facade unit, the
depth of the intermediate cavity can be reduced from a
matter of feet [meters] to a matter of inches [millimeters].
This greatly improves the material efficiency of a unitized
production system, particularly if the cost of the operable
units is offset by a reduction in mechanical plant capacity due
to increased use of natural ventilation.
KEY ARCHITECTURAL ISSUES
• The primary architectural issue related to
double envelope construction is the fact
that building appearance and thermal
and lighting performance are essentially
defined by the success of the facade.
• It is imperative that the designer have
clear design intent, explicit design
criteria, and a sense that the intended
envelope design can deliver what is
expected. Unfortunately, a double
envelope facade is a very complex
system that may not behave totally
intuitively.
IMPLEMENTATION CONSIDERATIONS
• The effectiveness of double envelope systems is widely debated and difficult to summarize. A simple comparison of facade costs has little meaning without also comparing the floor space available for use, the cost of a compatible structural system, the size and complexity of the mechanical plant, total building energy flows, and the cost of long-term maintenance.
• One must also examine the qualitative benefits to building occupants and the ecological impacts of the materials required. Some of the most effective double envelope applications are “re-wraps” of existing building envelopes that are poor energy performers.
IMPLEMENTATION CONSIDERATIONS
• Generally, double envelopes should not be the first green
strategy adopted.
• They should be considered when and if they complement
other steps taken in pursuit of overall environmental quality
and energy efficiency.
• Many of the benefits associated with double envelopes can be
achieved through means that have far less design and cost
impact.
• Openings in a facade should be designed to optimize the
harvesting of daylight and provide meaningful connections to
the outdoor environment.
DESIGN PROCEDURE
1. Develop a narrative to express design intent and related
design criteria for the building envelope that will be affected by a
double envelope facade—especially thermal and visual comfort,
energy efficiency, and climate control systems.
2. Consider the various types of double envelope systems and
construction strategies and sketch a building plan and a wall
section that has the elements necessary to deliver the intended
performance. Address issues such as whether the interstitial
space will be occupiable , whether individual control of light, air,
and view is intended , whether acoustic isolation is required.
DESIGN PROCEDURE
3. Do a reality check on the implications inherent in the above
narrative. How is day lighting performance enhanced by a
double envelope? How will ventilation air flow through a double
envelope? The purpose of this check is not to reject a double
envelope strategy, but rather to validate the assumptions
inherent in projections of system performance.
4. Reiterations of the conceptual sketches are made as models
(physical and simulation) are used to analyze various building
systems.
EXAMPLES
The double envelope facade of Bayerische
Vereinsbank building in Stuttgart, Germany is
a “re-wrap” or reconstruction of an existing
building in which an operable leaf of glass
louvers has been added in front of a system of
operable strip windows. There are shades in the
cavity.
Glass “wrap” facade (left) and close up
of glass panels (right) used as shingles
and hung from the facade of the
Kuntshaus Art Gallery in Bergenz,
Austria.
BIBLIOGRAPHY
• The Green Studio Handbook - Environmental strategies for schematic design
• www.sustainingtowers.com
• www.battlemccarthy.demon.co.uk
• www.e-architect.co.uk
• www.josef-gartner.de/fertigung/fassadene.htm
THANK YOU !
ANISH ABRAHAM CHERIAN
ARJUN NADESAN
GOURI UNNIKRISHNAN
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