Curtain Wall Is One of the Most Detail-Intensive Facade Systems - Getting It Right in CDs Requires Understanding What the System Must Do
Curtain wall is a non-structural exterior cladding system that spans between floor levels and carries only its own weight plus wind load - it does not carry floor or roof gravity loads from the building structure. Aluminum stick-built curtain wall systems (assembled piece-by-piece in the field from extruded aluminum mullions, transoms, and glass) and unitized curtain wall systems (pre-assembled in the factory as large floor-to-floor panels installed as complete units in the field) are the dominant facade systems for contemporary commercial buildings. The architect's curtain wall details in the construction documents must convey how the system anchors to the building structure, how thermal performance is achieved, how water infiltration is managed, and how the system accommodates structural movement and differential deflection. The ARE PDD exam tests curtain wall detailing as a complex construction document coordination skill.
Anchor Types
Curtain wall systems must be connected to the building structure to resist wind load and to carry the system's self-weight to the primary structure. Standard anchor types: Two-point anchor (stack anchor): Dead load and wind load are both transferred at the same anchor point per floor; the anchor connects the curtain wall mullion to the slab edge or spandrel beam. Floating anchor systems: Dead load is transferred at one point (a fixed support) while wind load is transferred at multiple points (sliding connections); this accommodates differential vertical movement between the building structure and the facade. For high-rise buildings with significant structural movement, the anchor system must accommodate 1-3 inches of total differential deflection between adjacent floors without racking the curtain wall frame or cracking the glass. This movement accommodation is achieved through slotted connections that allow the anchor to slide without transferring moment to the frame.
Thermal Break Requirements
Aluminum is highly thermally conductive (k ≈ 116 Btu·in/(hr·ft²·°F)). Without thermal breaks, aluminum curtain wall frames conduct heat from interior to exterior at a rate that creates interior condensation on cold days, significant heat loss, and energy code non-compliance. Thermally broken curtain wall uses polyamide thermal break elements (low-conductivity plastic struts that connect the interior and exterior aluminum extrusions without providing a conductive path) to create an effective barrier to heat flow through the frame. Thermally broken aluminum frames with high-performance glazing achieve frame U-values of 0.30-0.50, compared to non-thermally-broken frames at 0.70-1.20. ASHRAE 90.1 requires thermally broken frames for most commercial curtain wall applications in climate zones 3 and above.
Glass Lite Specification
The glass in curtain wall is specified by: Type: Insulating glass unit (IGU) with two or three panes; typical commercial: 1-inch IGU (two 1/4-inch lites + 1/2-inch argon-filled space) or 1-3/16 inch triple IGU for high-performance applications. Coating: Low-E coating (metallic oxide coating applied to an interior glass surface of the IGU) reflects long-wave infrared radiation, reducing heat loss in winter and solar heat gain in summer; specific coating type (hard coat vs. soft coat, high vs. low solar heat gain) selected based on climate and facade orientation. Performance metrics: U-value, SHGC, and VT as specified for the facade orientation and ASHRAE 90.1 climate zone requirements.
Key Exam Points
- Curtain wall: non-structural; carries own weight + wind load only; does not carry floor/roof gravity loads.
- Anchors: connect to slab edge or spandrel beam; must accommodate differential deflection (slotted connections).
- Thermal break: polyamide strut separating interior and exterior aluminum extrusions; required in most climate zones per ASHRAE 90.1.
- IGU: insulating glass unit; two or three panes; argon fill; Low-E coating for thermal performance.
- Differential deflection: building structure moves independently of facade; anchor system must accommodate without racking.
AREprep's PDD construction documents content covers curtain wall system types, anchor design concepts, thermal break requirements, glass specification, and how curtain wall details are produced and coordinated with structural and architectural drawings - giving ARE PDD candidates the curtain wall knowledge the exam tests in building envelope and construction document production questions where understanding the system's performance requirements and detailing implications is essential.
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