What is the Future of Solar Control in High-Performance Façades?
A blog on usglassmag.com by Helen Sanders
Recently, I have heard feedback from several European sources that closed-cavity double-skin façades (CCF) have not lived up to expectations, and the European market is moving away from them. For many years, these façades have been a benchmark for high-performance. Their adoption in Europe, where energy codes are stringent, has been significantly higher than in North America, where adoption remains low.
What is a Closed Cavity Curtainwall?
Typically, CCF systems consist of an inner skin of double-pane insulating glass (sometimes triple-pane) and a single-pane outer skin separated by a closed cavity. The cavity often contains automatically controlled venetian blinds, which enable enhanced variable solar heat gain control (SHGC) as well as high thermal performance. An example in YKK’s headquarters in Tokyo can be found at this link, and a sketch of a custom double-skin system used in a retrofit in the Cummings headquarters in Indiana is shown in the image below.
The solar heat gain control range for a CCF system is similar to that offered by electrochromic glazing, achieving a lowest SHGC of 0.10 when the blinds are closed. A continuous flow of dry air is circulated in the closed cavity to reduce condensation risk. In addition to minimizing heat loss in winter and providing dynamic solar heat gain management, acoustic insulation is also provided.
Double-skin façade retrofit in the Cummings headquarters in Columbus, Indiana. Credit: courtesy of YKK AP. Photograph by Steve Bullock.
What are the Problems Experienced?
The reasons that CCF has fallen out of favor relate to higher-than-expected upfront and operating costs, due to:
- The additional weight, which increases upfront costs;
- Higher-than-expected energy costs from air circulation;
- Higher-than-expected maintenance costs; and
- Difficulty managing maintenance continuity over the full long-term service life.
Alternative Design Strategies
If not CCF, then what? From a thermal performance perspective, the go-to solutions are aluminum fenestration with wide thermal barriers and triple-pane insulating glass, along with stringent air leakage and thermal bridging requirements at the interfaces and opaque areas, including spandrel.
For solar control, the strategy is not so obvious. The optimum SHGC depends on the climate zone, elevation, time of day and year and the building type. In some instances, higher solar heat gain is beneficial, such as in winter. In summer, lower solar heat gain is more optimal to reduce cooling load. Strategies may also differ according to elevation because of the presence or absence of direct sun.
As a result, the optimum way to reduce solar control is to have variable solar heat gain, adjustable depending on the exterior conditions. This is why automated venetian blinds in a CCF cavity and dynamic glazing can achieve optimum annual heat gain control and are part of the strategy to achieve net-zero façades.
With electrochromic glazing not gaining traction in either the European or North American market, and the European market pulling back from CCFs, what are the other options?
Vertical shading strategies for buildings. Credit: Helen Sanders
European Context
In southern Europe, the focus has been on solar control glazing and external shading systems, using a different strategy on each elevation. Horizontal and vertical shading layouts are used, as appropriate, on the south, east and west elevations. Shading systems are mostly fixed because of the operational maintenance issues for movable shading. Different glass areas are typically used on each elevation. On the west elevation, where effective fixed shading is challenging, reducing the glazed area may reduce direct low-angle solar impingement.
Design teams in Europe are targeting building service lives of 50 years and beyond, so maintainability has become a critical consideration. Designers are moving away from movable shading because of the maintenance “responsibility gap,” where maintenance is not done properly over time.
Fenestration is always thermally broken, with European center of glass U-factors of 1.0 to 1.1 W/m2K. This is a high-performing argon-filled, triple-silver low-E coated, dual-pane insulating glass unit (IGU).
Regular single-skin curtainwalls will meet code in southern European climate zones, and are still used in office towers, because of client demand. However, feedback suggests that occupants are thermally and visually uncomfortable and resort to controlling their comfort with whatever they can (shades, umbrellas, aluminum foil, paper, etc.).
In markets such as the United Kingdom, Spain, Italy and Germany, architects are using unitized pre-fabricated elements (e.g., mega panels as mentioned in last month’s blog about U.S. hospital façades), which use alternative opaque infills, such as ceramics, rather than glass spandrel. European design teams are beginning to think more carefully about how much glass to use and window positioning on each elevation to effectively control solar gain and visual comfort.
On European multifamily buildings, manual exterior shutters or awnings are often used. They are a cultural norm that is not replicated in North America. Such systems are relatively inexpensive to maintain, although the latter (awnings) are considered unattractive.
Innovative glazing solutions with shading integrated into IGUs, such as integrated venetian blinds, like these, or integrated micro-structured reflecting films, such as these, are also experiencing increased adoption to solve the heat and glare control challenges.
Solar Control Opportunities for North America
For North America, opportunities exist for tuning fenestration performance and shading strategies aligned with orientation, appropriate to climate zones. Fixed exterior shading is likely to be a more acceptable and cost-effective strategy than exterior automated movable systems. But shading must be appropriately designed for the orientation to be effective (horizontal on the south, vertical on the east and west, with sufficient depth). Systems integrated into IG systems may also be useful solutions; some have no moving parts at all, while others have moving parts protected in the sealed IGU.
Room-side automated shading systems, with fabrics designed to reflect solar heat gain out of the building, and integrated with lighting controls, may also be a key part of the full solution. The performance of these systems was demonstrated 20 years ago in the New York Times building. And since then, many automated shading installations have been installed and have had well over a decade of successful operational history. The Attachment Energy Rating Council has developed energy ratings and certifications for such systems that provide the market with confidence in their performance.
An automated interior shading system was installed at SAP America headquarters in Pennsylvania. Credit: Courtesy of Lutron Electronics Company Inc.
Independent of climate zone or elevation, thermal performance must be prioritized. This means installing fenestration and opaque areas with low U-factors, and setting stringent thermal bridge mitigation and air leakage requirements, including testing and verification.
Creating appropriate prescriptive solar control strategies for above-code, high-performance façades will be critical to support capacity building in the North American market and, in so doing, encourage base energy code development for better envelope solar control.