Designing for Building Maintainability - Effective Building Maintenance: Protection of Capital Assets - page 141

glass and the frame of the window-wall system for interior glazed
units;

• Installation of a metal back-pan sealed to the frame of the window-

wall system behind all opaque panels to act as a secondary seal;

• Installation of a waterproof membrane at the level of the floor slab

extending from the vertical leg of the mounting track down the
face of the slab and onto the deflection header.

The primary intent of these measures is to provide redundancy
within the system. The seals forming the secondary line of protection
are typically better defense from exposure and temperature extremes
than the primary exterior seals and can be expected to have a longer
service life. In the second instance, means of draining the interstitial
spaces created between the primary and secondary seals are provided
so that any water making its way past the primary exterior seal would
not accumulate within the window-wall system but would be redirect-
ed to the exterior. To reduce the risk of water accumulation within the
system, designers must ensure that each element of the window-wall is
provided with means of draining, including

• Drainage of the glazing cavity and back-pans, either directly to the

exterior by way of drain holes through the horizontal mullions or
down to the deflection header by means of drain holes in the verti-
cal mullions;

• Drainage of the cavity between the primary and secondary gaskets

on operable units;

• Drainage of the horizontal mullions directly to the exterior;

• Drainage of the window-wall system at the level of each deflection

header.

An additional advantage of this twofold approach is that it creates
a series of compartmentalized cavities that provide for some measure
of pressure moderation across the primary exterior seal. Reducing the
pressure across the exterior seal reduces the amount of water that can