Double Envelope

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”


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.


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.


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.


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.


• 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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Double Envelope