The 3 Lines of Defense in a Curtain Wall

One of the most common questions about curtain walls is: which areas are supposed to be wet, and which must stay dry?

The answer lies in the system’s three lines of defense against water penetration.

1. Exterior Seal – The First Line of Defense

• Location: Outboard gaskets at the glass pocket and mullion face

• Role: Stops the majority of wind-driven rain at the outer surface

• Wet Zone: Yes – this area is designed to get wet, shedding as much water as possible before it enters the system

  
  

2. Drainage & Weep System – The Second Line of Defense

• Location: Cavity and horizontal stack joints

• Role: Captures any water that bypasses the exterior seal and channels it out through weep paths

• Wet Zone: Yes – this cavity expects water, and safely drains it away from the façade

  
  

3. Interior Seal – The Final Line of Defense

• Location: Continuous gasket at the back of the mullion

• Role: Keeps the occupied space completely dry

• Wet Zone: No – if this gasket stays dry, the system is performing as designed

  
  

Putting It All Together

Here’s the big picture:

1. Water hits the exterior face → most is stopped by the outboard gasket.

2. What sneaks past drains into the cavity → then exits through weep holes.

3. The interior seal remains dry → ensuring no water ever enters the building.