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 Box-windows within the double skin facade plane is often drawn back into the of the GAAG Architecture Gallery in Gelsenkirchen, Germany. depth of the wall. 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 closelycoupled 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 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.

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.

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