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Double-Skin Facades (DSF) are well-known to boost the thermal performance of a façade: they can provide extra insulation in the wintertime and lower the solar heat gain if ventilated in the summertime. However, how much better is this design strategy compared to other façade solutions? Do any dynamic shading strategies perform better than others? What about cavity ventilation? Our paper will cover the design and performance of the double skin façade of the Wesleyan University New Science Building, beginning from the basic physical principles applied during the design, the architectural implications of each decision, as well as the façade design variables influencing its measured performance.
Several climate and simulations analyses were performed to this façade design, focused specifically on maximizing both winter and summer performance. Climate analysis through solar trajectories and exterior conditions (air temperature, humidity, and solar intensity) were used both to refine the design and operation of the dynamic shading system, as well as identify the magnitude and timing of the space's cooling and heating peaking loads to optimize HVAC system sizing.
Additionally, a series of detailed Computational Fluid Dynamics were performed to understand the key elements driving winter and summer performance, the impact of closing the cavity to maximize winter u-value, the impact of cavity shade deployment and their design in removing summer heat, the ability to reduce cavity gains by using fan-driven ventilation in the cavity, amongst others.
These detailed analyses allowed the team to move beyond the notional benefits of a double-skin façade and refine a design that will become a dynamic expression of the science building's performance and sustainability.
Double Skin Façade (DSF) Performance
Modern façade design has embraced transparency in an effort to increase occupant connection to the outdoors, through access to daylight and views. However, transparency inevitably bring along lower envelope performance: higher winter heat loss and increased unwanted solar gain in summer months. To find a balance between transparency and performance, double-skin façades, well-known to boost the visual and thermal performance of a façade (Shameri et al, 2011), have become increasingly employed across North America.
Double skin facades consist of having two main glazed “skins” such that there is a cavity between them. The cavity offers several advantages: weather protection for dynamic shading strategies, pre-heating outdoor air for natural ventilation or additional thermal insulation in cooler months, and additional solar gain rejection in the summer months.
Each double skin façade is different: The cavity can be ventilated (open-cavity double-skin) or not (closed-cavity double skin); The ventilation can occur only to the exterior (open outer skin), only to the interior (open inner skin), or a combination of both (openings on both inner and outer skin); The cavity can also either span the full height of the glazed façade or be compartmentalized at every floor. Which of these options is the best for a particular project depends highly on the project’s climate and performance needs, as well as maintenance and budget constraints.
Beyond the performative benefits and increased transparency, the tectonics of a double skin façade express a forward-looking dynamism where architecture is responsive to climate and orientation. In this way, the double skin façade can also be an educational tool about the making of green buildings and the responsibility for designing a more sustainable future.
Wesleyan’s New Science Building
Wesleyan University is in Middletown, Connecticut roughly one-hundred miles northeast of New York City. Middletown is located in Climate Zone 5A with warm summers having very few extremely hot days, dry with cold winters with prolonged periods of snow cover, and relatively mild spring and fall with warm days and cool evenings (Figure 1).
The 185,000 GSF New Science Building replaces two of Wesleyan’s outdated facilities with a unified building that integrates the life science programs under a single roof. The building aims to be an exemplar of sustainability and energy performance with an pEUI of 86 kbtu/sf/yr, 75% less than the AIA 2030 baseline building of this typology.
Conceived as a highly crafted, concise volume, the New Science Building creates a dynamic and unique environment for research and teaching at Wesleyan. In response to the challenge of fitting a very large building into the delicate fabric of the existing campus, the building’s volume/mass has been articulated into components that relate to the size of existing and familiar campus structures (Figure 2).
Assigned program spaces are defined within three stone volumes anchored in the landscape, while the upper floors’ study space—the Commons—is expressed as crisp, ephemeral glass volume that pushes outward, expressing itself between the stone volumes.
A primary driver for the placement and orientation of the new building is the creation of a new campus courtyard. The western glass Commons volume engages in a direct relationship across this space from Shanklin Hall, whose image will be clearly reflected in certain lighting conditions (Figure 3).
The west-facing orientation of the glass wall posed challenge for team in achieving in the desired visual transparency and connection between the Commons and Wesleyan Place. Each afternoon throughout the year, the highly transparent façade would be subject to direct sunlight leading to glare and radiant discomfort for occupants as well as increased peak cooling loads during the summer months (Figure 4). Additionally, the desire to reduce annual heating loads meant that any large, glazed façade had to perform substantially better thermally than a standard aluminum curtain wall system.
The initial solution to the challenge of glare and peak loads was a fixed external metal mesh which could diffuse the direct afternoon sunlight (Figures 5). Coupled with a high-performing triple-glazed curtain wall system, this approach addressed both peak load and annual heating challenges. However, the static nature of the mesh meant that transparency and visual connection between the Commons and landscape was severely compromised.
It was a desire for a dynamic shading system, one that responded to sun orientation and sky conditions, that drove the team to start looking at double skin façade systems. Exterior dynamic shading systems in New England are subject to snow and ice during the winter months, so locating the shading systems within a ventilated cavity affords protection from the elements in winter while still achieving the benefits of reduced solar heat gain in the summer. With the exterior blinds protected in a semi-conditioned cavity, the natural next step for the team was to leverage the airspace as a thermal buffer in winter and to extract heat away from the façade in summer. With buy-in from Wesleyan, the team moved ahead changing the design after the schematic design phase of the project.
The resulting design of the Wesleyan double skin façade is two stories tall by 140ft wide and is comprised of an interior triple insulated glazing unit, 3ft-6in ventilated cavity with access catwalk and laminated glass exterior. The glass is retained to a precision-fabricated steel truss through toggles on the vertical mullions and structural silicone on the horizontals (Figure 6).
The purpose of the studies presented in this paper was to quantify the performance of the DSF incorporated in the design of the future Wesleyan Science Building. The key goals
The key driver to incorporate a double skin façade into the Wesleyan project was the desire to maximize façade transparency on such a challenging orientation (West).
Dynamic shading offered the
The thermal performance of a double skin façade in winter mode is characterized by the additional insulating properties offered by the additional air cavity (closed / not ventilated) between the
The improved summer performance of a DSF depends on two key factors. First, the maximized capture of solar gains by inter-cavity solar blinds. Second, the adequate ventilation of said cavity
The overall purpose of a double skin façade is to provide weather protection to any dynamic shading elements, while keeping the cavity as insulated as possible in the winter, and
The analysis performed on the Wesleyan University New Science Building DSF is an example of how to evaluate the impact of various layers of variables used to define and quantify
CFD Geometry and Key Assumptions
The flow inside the Double Skin Façade was modeled using 3D Simscale CFD according to the geometry and boundary conditions shown in Figure 16. U-values
Alonso Dominguez, PhD, Senior Project Engineer Thornton Tomasetti
Chris O'Hara, Will Babbington, Josh Moore, Studio NYL, consulting façade engineers
vanZelm, project MEP engineers
M.A. Shameri, M.A. Alghoul, K. Sopian, M. Fauzi M. Zain, Omkalthum Elayeb, Perspectives of double skin façade systems in buildings and energy saving, renewable and Sustainable Energy Reviews,Volume 15, Issue 3, 2011, Pages 1468-1475, ISSN 1364-0321, https://doi.org/10.1016/j.rser.2010.10.016.