Curtain Wall System Types

Most architects are familiar with curtain wall systems and have probably used them extensively. But aside from a basic familiarity with the systems that they've used in the past or that their office 'typically specifies', how much do architects really know about the system they are relying on to provide the daylight, acoustic, thermal, structural and aesthetic finished product of their deisgn? Probably less than they care to think about. There are three basic types of curtain wall systems. The major divisions are stick-built, unitized, and structural glazed. Each system has its own unique attributes and design considerations, as well as potential visual differences. All these systems can be constructed of aluminum, steel, stainless steel, and occasionally wood and more recently fiberglass. The other component of all curtain wall systems that designers may also not give much thought is the glazing itself, which can have a greater impact on the overall performance of the wall system that the framing alone. Stick-built curtain wall systems are characterized by being assembled in the field. This provides opportunities for complete customization of the installation. But it also forces all the installation work to take place where the ability to control quality are the lowest - on the construction site along side every other trade. This is the most common type of system in use today.

Unitized curtain wall systems are essentially prefabricated sections of curtain wall that are delivered to the jobsite and erected section-by-section. The benefit of this system is that the panels can be fabricated in a factory to ensure quality control, and field installation can be more efficient, especially in inclement weather, tall buildings, and other challenging applications. The panelized system introduces another joint between mullions which increases the possible locations for water or air leakage, which must be carefully constructed in the field to prevent problems.

Structural glazed curtain wall systems are visually distinct from stick-built and unitized systems in that the vertical and sometimes horizontal mullions are without mullion caps, replaced with a silicone structural element that captures the concealed edges of the glazing. The back-frame of this type of system looks the same as the other curtain wall types from the interior of the building.

Curtain wall systems are constructed by attaching vertical mullions at floor levels with expansion joints at these levels to permit movement of the building structure as well as the curtain wall system itself. Horizontal rails are installed between these mullions to create openings for glazing, spandrel, or other infill panels. A structural engineer typically designs the clips back to the building structure based on the loading provided by the curtain wall manufacturer based on the specific design requirements of each project. Because of this, the archtiect should facilitate coordination between the manufacturer and the structural engineer.

With any system, care must be taken during construction to ensure longevity and performance of the components that make up the curtain wall system. This includes proper installation of framing, corner blocks, setting blocks for insulated glazed units (IGUs), pressure plates, vent and drainage holes, and sealants. If a drainage hole is blocked by excesive sealant, water build-up can degrade the seal of the IGU, and cause premature glazing unit. Even the small details are important to ensure overall system performance.

The second aspect of curtain wall performance are the glazing units. These range from single panes of glass to energy efficient triple-pane laminated glass assemblies. The units themselves can be constructed with a range of spacers and edge seals to impart particular performance. Warm-edge spacers reduce heat conduction from the exterior to the interior surfaces of the IGU by replacing the typical metal spacer with a non-metallic element, frequently fiberglass. Glass coatings also have a great impact on thermal performance as well as the aesthetics of the overall curtain wall assembly. As such, glazing characteristics should be considered carefully for each project application in consultation with a glazing representative.

AGB 6/2005