Curtain Wall Fundamentals
Concepts, considerations and execution
|The CRL-U.S. Aluminum Series 3250 Curtain Wall System encloses the entrance of a Crate & Barrel retail store.|
Understanding the basic concepts of curtain walls, the important considerations of the varying curtain wall types, and the performance specifications for curtain wall systems is critical for architects, project managers and installers in the field. This article looks at the various classifications of curtain wall systems, along with considerations for each.
Stick built v. unitized
Curtain walls are classified by their method of fabrication and installation in two categories: stick built and unitized systems. Unitized curtain wall systems are comprised of large units that are assembled and glazed in the factory. They are then shipped to the job site and erected on the building façade. The vertical and horizontal modules mate and stack together to create a complete system. Cranes are most often used to install these systems as modules can be one story tall and five to six feet wide.
Stick systems consist of the curtain wall frame verticals (mullions) and glass or opaque panels that are installed and connected piece by piece. These parts are usually fabricated and shipped KD (knocked down) to the job site for installation. In stick-framed aluminum curtain walls, vertical mullions typically run past two floors, with a combined gravity/lateral anchor on one floor (wind load anchor), and another lateral anchor (dead load anchor) on the other. Splices between the mullions allow vertical movement while providing lateral resistance. In larger areas of stick-framed curtain walls, split vertical mullions are sometimes used to allow for thermal movement, which can slightly distort the anchors. In this case, glass units must accommodate movement of the surrounding aluminum frame by sliding along glazing gaskets. This movement within the frame and in the anchors tends to induce additional stress on stick built systems.
All curtain wall systems—through their floor structure or immediate framing—must transfer and uphold its own dead load and live loads, including positive and negative wind loads, snow loads at large horizontal areas, seismic loads, and maintenance loads. While curtain walls are likely to demonstrate movement caused by perpetual thermal changes, it is important to ensure that the connections that anchor the curtain wall are engineered to allow differential movement while resisting applied loads and pressures.
Interior v. exterior glazing
Both unitized and stick systems can be either interior or exterior glazed. Both types offer glazing contractors various advantages and disadvantages during the installation process. Interior glazed systems allow for glass or spandrel installation into curtain wall openings from the interior of the building. Interior glazed systems are often specified for low-rise buildings, or applications with limited interior obstructions that allow for easy interior access.
With exterior glazed systems, glass and spandrel components are installed from the exterior side of the curtain wall. They require swing stage, scaffolding, or a man-lift to install and glaze.
Water-managed or pressure-equalized
Curtain walls can be further classified as either water-managed or pressure-equalized systems. Pressure-equalized curtain walls provide the highest level of resistance to air and water infiltration, while water-managed systems come in a close second.
Pressure-equalized systems function by blocking all external forces that can drive water across a barrier. Where the inside face of the glass and the inside face of the glazing pocket meet are interconnecting gaskets or wet seals that serve as airtight barriers. The outside face of the glass, exterior glazing materials, and the outer exposed face of the aluminum framing function as a rain screen that directs water away. Situated between the exterior rain screen and the interior air barrier, a pressure-equalization chamber in the glazing pocket reduces water penetration by eliminating (equalizing) the pressure difference across the rain screen. These are most often referred to zone-glazed systems.
On the other hand, water-managed systems incorporate drains and weeps from the glazing pocket instead of zone-glazing, which allows more water to be forced into the system that must be weeped away. Since no air barrier exists, the pressure difference between the glazing pocket and the interior may be strong enough to force water vertically higher than interior gaskets and cause leaks.
Weep holes in a water-managed system function largely to drain water that enters the glazing pocket, while weep holes in a pressure-equalized system primarily allow air movement between the exterior and glazing pocket. Weeping of water is only a secondary function.
The easiest way to recognize a pressure-equalized curtain wall system is to examine if the glazing pocket around each glass unit is isolated from adjacent units with air-tight plugs or seals between screw splines at the mullion intersections. Detailing of the spandrels, shadow boxes and interface with adjacent construction must remain consistent with the air barrier and curtain wall to function properly with pressure-equalized systems.
Most curtain wall systems utilize pressure bars (also referred to as pressure plates) that are fastened to the outside of the mullions to secure the glass. These systems frequently include gaskets that are placed between the pressure bar and mullions to function as thermal breaks, and additionally help with acoustic isolation. These systems require special attention to detail during the design and construction phases to ensure the gaskets at horizontal and vertical transitions are consistent. These gaskets are typically sealed together at their intersection in order to cushion the glass on the interior and exterior. Although gaskets tend to stretch during installation, they will shrink back to their original length in a short time; they are also designed to shrink with age and exposure to ultraviolet radiation from the sun at the corners. With a well-designed curtain wall, the water that enters the system at the gasket corners will ultimately weep out through the snap cover weep holes. To mitigate gasket shrinkage, it is recommended to use sealed vulcanized corners and diagonally cut splices.
Protection in cold environments
In colder climates, the use of back pans is required. Back pans are metal sheets, usually crafted from aluminum or galvanized steel that are attached and sealed to the curtain wall framing around the perimeter, and behind spandrel areas. In cold environments insulation should be installed between the back pan and exterior cladding in order to maintain the dew point outboard so the back pan can act as an air and vapor barrier. They provide a second line of defense against water infiltration for concealed areas of the curtain wall that are difficult to access, including in spandrel areas that can cause significant damage before even being detected.
Standards for architectural design
Curtain wall performance is the driving factor in the design of the majority of buildings today. It is important to understand the architectural needs of the curtain wall for the project to ensure optimal performance and execution. Glazing firms should be familiar with the commonly specified codes standards to use as a guide when designing a curtain wall system.