Curtain Call for Curtain Wall

Text Mark Brook

Contemporary glass and metal curtain wall systems can be broadly categorized by their means of assembly and installation as either stick systems or unitized systems. Stick systems consist of fabricated tubular aluminum “sticks” which are assembled onto the building frame to create a structural grid. Vision glass and spandrel backpans, insulation and face panels are inserted into the gridwork. Unitized systems are composed of factory-assembled frames complete with infill vision and spandrel panels that are installed on the building frame in an interlocking sequential manner. Both systems have advantages and disadvantages. The traditional thought is that unitized systems are restricted to large projects incorporating great expanses of uniform wall area. Where the building demands long spans, small areas of varying wall configurations, incorporation of stone or materials other than glass and metal, is of low rise construction or is to be retrofitted, stick systems are often thought of as the system of choice. However, manufacturers are demonstrating that unitized systems can be readily adapted to meet these conditions.

Long Span Frames

Unitized curtain wall frames in North America are typically 1500 millimetres wide (to meet space planning constraints) and one storey in height. It is not unusual to see longer frames used for the first or last floor of curtain wall on a building to cover parapet and spandrel conditions. While stick framing remains the dominant system for long spans, unitized curtain wall manufacturers have demonstrated that frame size is limited only by the volume of aluminum in the billet available to the extruder. Factory glazed unitized frames over 15 metres tall have been produced on projects such as the new Terminal One at Lester B. Pearson International Airport (SOM/Adamson/Safdie) in Toronto. Frames of this size present not only technical challenges but also logistical challenges with respect to assembly, shipping and erection.

Varied Wall Types

Obviously there are economies of scale in producing large numbers of identical frames, and this is the principal market for unitized systems. However, architectural designers are constantly looking for a greater range of expression in the exterior faade. This is evidenced by both the number of different wall types on a given building or the actual configuration of the wall itself. Stick systems often appear as more appropriate–especially on smaller scale projects–for buildings with varying wall types. Unitized system manufacturers have responded with flexible manufacturing processes allowing multiple interconnecting panel types on a given faade. The Concordia University Engineering Building in Montreal and the new National Ballet School (KPMB Architects) in Toronto are prime examples of this. A notable further example of the growing design flexibility of unitized curtain wall is the new front faade of the Art Gallery of Ontario (Gehry International). This project, with its flowing curves in both plan and elevation, utilizes a large number of non-uniform curtain wall frames.

Incorporating Stone

Incorporating heavier materials such as stone into curtain wall is not new. Hand-setting stone into curtain wall frames on site or hand-setting stone to a backup wall and inserting curtain wall frames between the backup walls is not uncommon. Advantages in overall performance and quality can be obtained by incorporating the stone directly into the curtain wall frame. Stone is mounted in the frame similar to a capped glass unit, by using kerf anchors or by using proprietary anchors to the back face of the stone. Large-scale projects such as the BCE Place Towers (SOM/B+H Architects) in Toronto illustrate this concept. Other spandrel materials such as fibre-reinforced concrete, stone/composite laminates, projecting solid or composite aluminum panels, shadow boxes and photovoltaic panels are also readily incorporated into the unitized frame.


Retrofit of existing curtain wall or removal of an existing wall system and replacement with a new curtain wall is a growing segment of the construction market, given the less than graceful aging of some of our building stock. Where complete wall replacement is desired and the building is occupied, unitized systems have a particular advantage in the very fast close-in of the exterior wall after removal of the existing wall.

Double Wall

Twin shell or double faades have gained notoriety on high-profile projects in Central Europe. While actually having a long history in North America, the concept is receiving renewed attention in Canada. Many aspects of the European model are not directly transferable to North American projects. However, the use of panelized construction is ideal for this wall configuration. The sensitivity of the interstitial space between the wall layers to thermal variances and air leakage focuses attention on the compatibility and integration of the wall layers. As either two independent layers or as an interconnected assembly, panelization allows greater quality and the incorporation of other technologies such as ventilation, motors and sensors more readily than stick-designed assemblies.


Energy codes and LEED-type certification programs are pushing curtain wall technology from an energy perspective. Development of glass and edge spacers has advanced faster than frame technology. However, unitized framing techniques are improving faster than stick framing with respect to overall performance. Requirements for greater seismic resistance are also easier to incorporate into unitized systems than stick systems.

Structural Silicone Glazing

Despite the claims of some sealant suppliers and some contractors, four-sided structural silicone glazing remains completely within the realm of unitized curtain wall. The reliance on an adhesive to retain the glass on four edges demands the quality control attainable only in a plant setting. This distinction has led to the growth of unitized curtain wall on high-rise residential projects, once the exclusive domain of window wall systems. The greater span, module spacing, adaptability to silicone glazing and greater sophistication available with unitized curtain wall systems has led to its use on developments such as the Shangri-La Tower in Vancouver (James KM Cheng Architects).

While both stick and unitized curtain wall systems will remain in the construction market for some time, the historical distinction between the systems and project types they are used on is beginning to blur. Unitized systems, with the advantage of factory assembly and fast building close-in, are being employed on a much wider and diverse range of projects than ever before. Manufacturers of unitized and stick framing systems continue to vie for market share, and the designer and eventual building occupant emerge as the clear victors. CA

Mark Brook is a professional engineer and President of Brook Van Dalen & Associates Limited in Nepean, Ontario.