Curtain wall design should start with the assumption that external glazing seals, perimeter sealant joints and corten steel curtain wall sills will leak. The following summarizes recommended features:
Select frames with wept glazing and pocket sills sloped to the exterior to collect water that penetrates the glazing and drain it to the exterior. Do not use vertical mullions as drain conductors. Each glazing pocket should be fully isolated from adjacent glazing pockets. Provide a sill flashing with end dams and with an upturned back leg turned up into the glazing pocket at the base of the curtain wall to collect and drain curtain wall sill leakage; provide jamb flashings to direct perimeter leakage down to the sill flashing.
Key frame drainage features include slope to the exterior at surfaces that collect water (slope top of exposed horizontal mullion surfaces, slope at flashings), large (3/8 inch diameter or a slot 5/16" x 3/8" minimum) weep holes closely spaced (three weep holes per each section of horizontal mullion between vertical mullions, typically), and drainage at every horizontal frame (do not use vertical frames to drain past horizontal frames). Use as many 1/4-inch by 2-inch slots as required for pressure-equalized systems. Design the drainage system to handle condensation as well as rain.
Curtain wall perimeters should have flashings (sill, jambs and head) that are sealed to the air and water barrier at adjacent walls. Slope head and sill flashings to the exterior to promote drainage. Integrate curtain wall sill flashings with sill flashings or base of wall flashings of adjacent walls. Curtain wall should have a primary air/water seal between the shoulder of the tube at the plane of the glazing pocket and the air barrier of the adjacent construction.
Perimeter sealants are useful as a rainscreen for limiting air and water penetration through the outermost plane of the wall, but should not be relied upon as the sole air/water penetration barrier.
Coordinate placement of setting blocks with weep holes to avoid blocking drainage paths.