Glazing Contractor Sees Applications of
Double-Skin Façades on the Rise
by Jeffrey Vaglio and Mic Patterson
We’ve heard about them, some of us have seen them, a lot
fewer of us have actually worked with them, but that may be about to change.
In spite of the adverse economic conditions, double-skin façade
(DSF) applications have actually increased; they’re part of the green
trend that continues to thrive in the down economy. So what are they,
what’s the point and can you expect to see one in your backyard anytime
soon? Well, it depends a little on where you live, but with recent applications
in major metropolitan areas including New York City, Boston, Chicago and
Los Angeles, the chances are that there may be one not too far from your
DSFs are simply a strategy for improving building envelope performance
through the introduction of a second glazed layer, thereby creating an
airflow cavity between the two.
The application of the technology in the United States has
been a long time coming. Although early examples of DSFs exist here—the
Occidental Chemical Center in Niagara Falls, N.Y., in 1980 is but one
example—the major development and implementation of the technology took
place in Northern Europe through the 1990s and 2000s. Numerous completed
examples of a great variety of DSFs were driven by legislative mandates
for improved energy efficiency in buildings. The impetus for the initial
development of DSFs was not only thermal comfort and energy efficiency,
however; it was also about acoustical performance, as these façades
mitigate sound transmission through the glazed building envelope. Nonetheless,
thermal performance and natural ventilation have been the more recent
drivers of this advanced façade technology.
It’s All About the Cavity
Let us provide some background first to guide glazing contractors’ entry
into DSFs. The cavity is useful for a few things. First, it acts as a
thermal and acoustical buffer between the inside and outside environments.
Second, the cavity can be employed in various ways to provide airflow
and even building ventilation. Third, the cavity provides an optimal space
for the location of shading devices: outside the inboard skin so that
solar radiation is stopped before penetrating into the building, yet shielded
from the elements by the outboard skin. If the cavity is deep enough,
it can also house mechanical equipment and maintenance platforms. It turns
out that cavity depth ranges widely from about 4 inches to 6 feet among
the various built DSFs. It should be no surprise then, that the applications
of DSFs are most often categorized by variations in cavity design and
behavior. Specifically, ventilation type, ventilation mode and cavity
partitioning are the most commonly used criteria.
The ventilation type refers to the driver of airflow within the cavity,
which can include natural, mechanical and hybrid systems. The ventilation
mode refers to the airflow pathway from intake to exhaust. The five common
ventilation modes are outdoor air curtain, indoor air curtain, air supply,
air exhaust and buffer zone. The diagrams in Figure 1 trace the pathways
characteristic of each mode. Finally, DSFs are most usefully classified
by the cavity partitioning strategy employed in any given design. The
four primary cavity configurations are box window, shaft-box, story-height
(corridor) and multi-story (see Figure 2). Each configuration possesses
unique attributes of design, performance and application. The multi-story
types tend to be the deep cavity systems, while the other configurations
typically utilize much shallower cavities.
In a recent evaluation of 23 existing applications, the most common DSF
cavity partition configuration in the United States is the multi-story
(70 percent) and the most common ventilation mode is the outdoor air curtain
(74 percent). The multi-story DSF cavity has no horizontal or vertical
divisions, and may encompass an entire elevation of a building façade.
Intake air openings are placed at the bottom of the cavity, with exhaust
openings at the top. Ventilation of the cavity can be induced naturally
through the stack effect (as the cavity air warms it rises and is exhausted
through the top vent, in turn drawing air into the cavity through the
bottom vent) or mechanically assisted as required to prevent overheating
of the cavity air. The more advanced designs utilize this cavity behavior
to provide ventilation to the building.
The evolution of DSFs in the United States exhibits other emerging trends.
An alternative to the multi-story system is the increasingly popular box-window
type, with a cavity depth at the shallow end of the spectrum, typically
in the range of 4 to 8 inches. This DSF type is easily configured as a
modular, prefabricated unitized curtainwall system appropriate for application
on high-rise buildings. In addition to high-performance unitized curtainwall
systems capable of cladding an entire building, box-window configurations
can be developed as discrete window or window wall units, and have been
used as a façade component in office, residential and hospital
projects where the floorplan is subdivided into many repeating units (offices,
condos or patient rooms).
Assembly and installation issues with DSFs range as widely as the system
variations. Unitized double-skin curtainwall systems can be complicated
by the need for panel operability to accommodate maintenance needs. Prefabrication
may include the installation of shading devices and controllers as part
of the unit assembly process. Once the units are assembled, installation
proceeds much the same as with conventional units. An exception is that
the units are typically heavier, which may preclude lifting several simultaneously.
Multi-story DSFs present quite another scenario. Because of the long spans
typically involved, these applications often will have exposed structural
systems, sometimes requiring architecturally exposed structural steel
(AESS) standards. This type of work is often unfamiliar to glazing contractors
and steel fabricators alike, and is rightly regarded as a specialty item.
In fact, many of the multi-story DSFs referenced above make use of structural
glass façade technology, including the use of frameless glass systems,
as a support strategy for the exterior skin. The interior skin is often
a conventional curtainwall or storefront type system. The issue is with
the exterior skin, its means of support and the required cavity work.
The cavity often incorporates maintenance platforms, shading devices and
potentially other mechanical components such as operable vents. These
may or may not be included in the façade contractor’s scope of
work. The cavity depth is an issue of particular concern; the deeper the
cavity the easier it is for workmen to operate with all the required equipment.
Cavity depths less than 30 inches begin to seriously constrain ease of
movement for the workmen.
A particularly elegant way to support the outboard skin is with the use
of a cable net. This presents a new set of challenges to the façade
installer relating to the pre-tension requirements that must be applied
to the cable system. The magnitude of force is typically high enough that
hydraulic jacking equipment is required to achieve the required cable
prestress. Tensioning a cable net is not generally as simple as moving
from one cable to the next with a tensioning device; the progressive tensioning
tends to alter the previously tensioned cables, resulting from the residual
effects to the supporting boundary steel. Cable tensions must be confirmed
with the use of an appropriate tension metering device. The installer
should request a detailed installation method statement from the façade
designer, and carefully consider the cost impacts in the estimate of work.
Access is a consideration on any façade job. If there are maintenance
platforms in the cavity and they are installed before the outboard skin,
they can be used during installation. If not, temporary platforms may
be required within the cavity. Depending upon the glazing system design,
workers may be required on both sides of the skin.
A final consideration for the façade contractor is commissioning.
The requirement for system commissioning of advanced façade designs,
DSFs among them, is becoming increasingly common, and is something that
progressive façade contractors should prepare for. While commissioning
requirements will vary between jobs, it is essentially a process of validating
that the façade is installed and funcfunctioning as intended. With
operable and dynamic components integrated into the façade design
and critical to the intended function, commissioning processes are vital
in assuring the building owner of future performance.
Future Developments and Conclusions
Arguably, the most compelling future application of DSF technology is
in building retrofits. Realizing energy consumption and carbon emission
reduction goals established by various green platforms will require energy
retrofits to a large percentage of the current building stock. Many of
the early glass curtainwall towers built during the 1960s and 1970s were
originally constructed as single-glazed facades with low visible light
transmittance glass; they were poor energy performers from the beginning
and now are approaching something very close to old age. The addition
of a second skin may prove to be a viable approach in some, if not many
of these buildings.
DSFs are one strategy of an emerging advanced façade technology
that includes new glazing materials, improved framing systems, progressive
techniques and novel designs. That unique attribute of glass (transparency
and the manner in which it enriches our built environments with daylight
and view) assure that glass will remain a predominant material in the
Glass, however, as we well know, is a poor thermal and acoustical insulator,
and these negative attributes threaten to limit its use. It is imperative
that we, as an industry, do not adopt a defensive position in an attempt
to protect a vested interest. We must embrace the mandate for improved
energy efficiency and reduced carbon emissions in buildings, and deliver
solutions that optimize façade performance. This will assure the
benefits provided by the unrestricted, but appropriate use of glass in
the building envelope. The ultimate viability of DSF technology, and the
role it will play in future facades, is unclear. We need to make a more
aggressive and sustained effort in the attempt to better understand how
these experiments in advanced façade design are actually performing.
DSF technology, however, is but one strategy to the challenge presented
by façade performance. There are others and there will be many
more. The needed solutions will involve collaboration between the profession,
academia and industry, and will require ongoing research and development
by all stakeholders.
Jeffrey Vaglio and Mic Patterson are both
PhD candidates in the School of Architecture at the University of Southern
California. They are employed by Enclos, the national curtainwall firm
headquartered in Eagan, Minn., and work out of the firm’s Advanced Technology
Studio in Los Angeles. Their opinions are solely their own and do not
necessarily reflect the views of this magazine.
© Copyright 2011 Key Communications Inc. All rights reserved.
No reproduction of any type without expressed written permission.