Understanding Curtain Wall Systems
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.
Stick-Built vs. Unitized Systems
Curtain walls are classified by their method of fabrication and installation into two categories: stick-built and unitized systems.
Unitized Curtain Wall Systems: These systems consist of large pre-assembled and glazed units fabricated in the factory, then shipped to the job site for installation. The vertical and horizontal modules interlock to create a continuous system. These units are typically large, sometimes one story tall and five to six feet wide, requiring cranes for installation.
Stick-Built Systems: Stick systems involve assembling the curtain wall frame piece by piece at the construction site. Vertical mullions (structural members) often extend past two floors, with a combination of gravity/lateral anchors and dead load anchors for support. The system accommodates movement from thermal expansion through specialized splices between mullions.
Both systems must be engineered to withstand various loads, including dead loads, wind pressure, seismic activity, snow accumulation, and maintenance loads.
Interior vs. Exterior Glazing
Both unitized and stick-built systems can be designed for interior or exterior glazing.
Interior Glazed Systems: Glass and spandrel panels are installed from the inside of the building. This approach is preferable for low-rise buildings or locations with minimal interior obstructions that allow for easy access.
Exterior Glazed Systems: Glass is installed from the exterior, often requiring scaffolding, swing stages, or lifts. This system is common for high-rise buildings where interior access is limited.
Water-Managed vs. Pressure-Equalized Systems
Curtain walls are further classified based on their approach to water management.
Pressure-Equalized Systems: These offer the highest resistance to air and water infiltration by preventing external forces from driving water into the system. Gaskets or wet seals create airtight barriers, while a pressure-equalization chamber neutralizes pressure differences, minimizing water penetration.
Water-Managed Systems: These rely on drains and weep holes to direct water out of the system. Because they lack an air barrier, they are more prone to water intrusion if pressure differences force moisture higher than the gaskets.
Most curtain wall systems use pressure bars (pressure plates) fastened to the mullions to secure the glass. These pressure bars often include gaskets that function as thermal breaks and provide acoustic insulation. Proper detailing and installation of gaskets at transitions and corners are essential to prevent water ingress and accommodate material shrinkage over time.
Protection in Cold Environments
In colder climates, back pans are necessary for spandrel areas. These metal sheets, typically aluminum or galvanized steel, are sealed to the curtain wall frame to act as a secondary water barrier. Insulation should be placed between the back pan and exterior cladding to maintain the dew point outboard, preventing condensation issues. This additional protection ensures longevity and thermal performance.
Standards for Architectural Design
Curtain wall performance is a critical factor in modern architectural design. Understanding project-specific needs ensures optimal execution and efficiency. Glazing firms should be well-versed in industry standards and building codes to ensure compliance and high-quality results.
By considering these factors, architects, project managers, and installers can implement high-performance curtain wall systems that provide aesthetic appeal, durability, and energy efficiency in diverse environmental conditions.
