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Dual-Skin Façades

Look twice at dual-skin façade emergence and benefits

Ma Special Feature Oct17 2
A dual-skin façade is one of the best options to manage the interaction between a building’s exterior and internal spaces. This building envelope consists of two glass layers with an insulating space in between them. The second glass façade mounted on the external side of the thermal building partition can efficiently control the solar energy into the building. Air flows in the intermediate cavity, and the ventilation can be natural, fan supported or mechanical.

Dual-skin façades are more common in high-end European and Pacific Rim architecture, but are gaining acceptance more in the United States. One reason is because dual-skin façades are being touted as a powerful green building strategy; their thermal performance capabilities can reduce building heating, cooling and ventilation costs. The internal façade can even protect against precipitation, wind and dust. What follows are some of their other benefits and considerations.

DUAL-SKIN BENEFITS

“Dual-skin façades are complicated, and their use and function affect different parameters [such as] heating, cooling, acoustic, sun control, daylight, etc., of the building,” says Vaidas Kazlauskas, chief technology officer, STATICUS, Vilnius, Lithuania. “The design of the system is important for the performance of the building. Dual-skin façades can provide both improved indoor climate and reduced energy use at the same time if designed properly. Dual-skin façades are flexible enough to meet climatic changes for most types of building use.”

An important dual-skin façade attribute, and one that Alloy Kemp, senior project engineer of façade engineering atThornton Tomasetti’s Chicago office, says is responsible for the majority of its application, is its active shading system. “An active shading system will change its performance in response to outside conditions (sun position, solar irradiance, wind, etc.), but if not protected by a second layer of glass, can suffer from exposure to the elements, resulting in premature failure,” she says. “By using a double layer of glass with an active shading system within, the glass itself can be relatively clear, even in hot climates, when compared to a standard response of very dark or reflective glass to accomplish the same solar goals.”

When designed correctly, Kemp claims a double-skin façade can reflect up to 88 percent of the excessive solar energy in hot climates, while still allowing views out through the façade. “Dynamic systems that adapt to the season, time of day or external weather conditions will have higher energy savings than a static (unmoving) system,” she adds. “Most static systems will favor one season over the other. For example, if designed for a summer condition, a static system may result in a darker space than preferred in the winter.”

The addition of an external skin, combined with the cavity space, increases external heat transfer resistance. The cavity space acts as a buffer zone that is warmer than the exterior, thus reducing the rate of heat transfer at the exterior glass. “The main benefit of double-skin façades in colder climates is their ability to harness the solar heat gain,” says Jeffrey Vaglio, Ph.D., PE, AIA, vice president of the Advanced Technology Studio of Enclos Corp., Los Angeles. “The air located between the two glass skins inside the air cavity acts as a thermal buffer. This buffer façade insulates the building interior from losing heat and significantly improves the U-value of the building. Lower heat losses mean a reduced heating consumption profile. The use of low-E glass in double-skin façades permits solar heat gain and daylight to penetrate the envelope while simultaneously preventing heat loss from the space.”

Sound insulation is another double-skin façade benefit. According to Mic Patterson, director of strategic development at the Virtual Construction Lab, New York City, part of Schüco USA, Newington, Conn., and ambassador of innovation and collaboration at the Façade Tectonics Institute, Los Angeles, reducing outdoor-indoor sound transmission, in general, is dual-skin façades’ greatest strength. Because they can suppress exterior sound, they are often mounted in buildings near noisy streets. Depending on the dual-skin configuration, even internal room-to-room sound transmission can be prevented.

Vaglio believes using double-skin façades in retrofit projects is an effective and economical alternative to new construction. This approach has many benefits including preservation of existing building stock, an aesthetic upgrade (modernization) and the perception of sustainable initiative. He feels a dual-skin façade is reasonable considering the alternative of teardown plus new construction, which may come with a significant loss of operations and/or considerable temporary displacement of building function. Buildings that fit the profile for a façade retrofit are typically structures that remain in the possession of one entity for an extended period of time, such as government, institutional or historical landmarks.

During the summer, reduced energy demand can be achieved by night cooling a building. This process of ventilating the building, and pre-cooling for the next work day reduces indoor temperatures during the early morning. A second skin allows dualskin façade systems to be night ventilated while preserving a layer of security in the outer skin.

CONNECTING SKIN TO METAL

Dual-skin façades may be comprised of any number of framing systems, and when combined with an additional layer, many different combinations. Patterson says dual-skin façades are customized on a project basis and that metal buildings present no inherent barrier to their application. A dual-skin façade’s interior layer usually has an insulated glass unit (IGU) glazed into an aluminum frame. Given the added layer, and the depth of the cavity, the external layer will try to maximize transparency.

“If the interior layer is an IGU, the exterior is often a laminated lite these placements are climate and program dependent and may be reversed in certain conditions and the exterior skin seeks to minimize the structural presence of framing,” says Vaglio. “Minimal profiles may be used. Increasingly, particularly in multistory, double-skin façades, there’s a push towards high-visibility, structural glass systems which may include but are not limited to cable mullions, cable nets, point-fixed (glass bolts), point-clamped (patch plates) and frameless details.”

“The relationship and connection of dual-skin façades to metal building components is not dissimilar to that of a single skin,” says Claudia Farabegoli, director at Thornton Tomasetti’s London office. “Frames are typically aluminum for ease of production, although timber is being used in select cases. Shading elements within the cavity can also be metallic, again, typically aluminum for its lightweight, non-corrosive nature, and its ability to be perforated, panted or finished in a multitude of ways.”

According to Farabegoli, the main difference between a single- and double-skin façade is the potential for live loads from maintenance workers in the cavity of the double skin, necessitating more robust structure and connections to the main building. “Additionally, determination of appropriate wind loads to use is less straightforward with a double-skin façade, as the internal pressure within the cavity can affect the load experienced by the inner and outer skins in different ways,” she says. “Structural design of a double-skin façade should account for these additional complexities.”

DUAL-SKIN DECISIONS

Fully integrated, dual-skin façade systems are sophisticated and, again, complicated. They can be a great solution for a project, but not be right for every project. Patterson stresses that even with their many advantages, they are expensive both economically and materially, and they add considerably to initial design and delivery, as well as operational complexity over the full building service life.

Many issues must be studied in the design process to ensure a thorough understanding of this technology’s performance. To ensure success, their design and implementation require forethought and careful planning. “Before starting their design, it’s very important to understand their performance by studying the physics inside the cavity,” Kazlauskas says. “The truth is dual-skin façades are systems that are highly dependent on outdoor conditions such as solar radiation and outdoor temperature.”

Patterson contends that many dual-skin façades are essentially “green washing,” poorly designed and poor performing. “Many have experienced problems with overheating, which can add to the building cooling load,” he adds. “They also add considerably to the embodied energy debt of a building. It can easily take decades of operational energy savings to pay that debt. The design service life of the building and façade system are critical life cycle considerations. If green building and considerations of sustainability are project goals, then a comprehensive life cycle assessment should be conducted in parallel with early design development.” He also suggests a double-skin façade program should incorporate a commissioning process to:

• Ensure appropriate design, fabrication and installation

• Provide verification that the installed work is performing as expected

• Include post-occupancy periodic evaluation to validate ongoing operational performance

Intelligent dual-skin façade design decisions can be attained via simulation and modeling throughout the design process. However, just like many buildings with single-skin façades, Vaglio cautions that double-skin façades are also susceptible to over-estimating projections for energy performance. “While energy modeling is a valuable tool in developing an approach, these systems must be validated, monitored and used to inform the next generation of double-skin façade systems,” he says. “This process includes thorough commissioning efforts, post-occupancy monitoring and evaluation, and dissemination of the findings to the architectural/engineering/construction (AEC) community so the processes of energy modeling and life cycle assessment are informed through a feedback loop. Energy models then can be calibrated based on real performance, and design and construction practices can be modified to eliminate identified shortcomings.”

Kemp says analyzing double-skin façade performance may also pose some logistical challenges, depending on the location. “United States building codes might not cover the use of double skins and the engineer may be required to consider each layer of the façade to be performing structurally separately, which is conservative,” she says. “European codes do provide load-share factors between layers based on ventilation and relative stiffness of each layer.” Ideally, a wind tunnel study considering the configuration of the façade would provide guidance.

After the recent tragic events London and the Middle East in which fire spread through the façade in buildings, particular attention should be paid to the fire compartmentation of the double-skin façade. “Depending on if the double-skin is ventilated or closed, and the internal pressure of the building, the double-skin may increase the likelihood of fire spread in the cavity,” Farabegoli cautions. “There is limited test data to provide guidance here, so the design team should engage the appropriate consultants to fully vet this issue.”