Case 2

Good Practices

Curtain wall

Design

Gravity is the most frequent cause of water seepage inside the building. To deal with such a problem, openings can be provided through which leakage can be fed outwards. If the window sill is cut short and unsealed at the end or sloped in the wrong direction, water will flow directly into the room. Flashing and sill devices should always be positively sloped more than 15°C to the outside of the building.

An air equalization chamber (VIDEO) can be provided, thus eliminating the need for the closure of all holes but rather on the pressure equalizer pressure in the cavity between the external and internal skin of the curtain wall. The design consists essentially of an outer wall (rain screen) and inner partition (air barrier) which is separated by a cavity vented to the exterior through weep holes to the atmosphere (fig 1).

Joints

Joints should be properly designed and constructed to achieve the required watertightness. Structural sealant should be used. Sealant failure is one of the major causes of water leakage through curtain walls. Due to the mismatch of physical and chemical properties (coefficient of thermal expansion, cohesion, adhesion, elasticity) of sealant and its substrates, structural sealant is not widely used to accommodate thermal movement. They have been promoted by the trend to structurally glazed curtain walls where glass is bonded to the frame with special silicones.

The effects of capillary action can be foiled by breaking possible capillary paths. This can be achieved by having grooves within the joints or by widening joint interfaces.

As joints expand and contract or experience shear movement, the shape of the sealant changes accordingly, but the volume of the sealant remains constant. As the sealant shape changes, the magnitude and type of stress also change. Thus, in designing joints, it is vital that the proper width- to-depth ration may be specified so that the width of the joint is consistent with the capability of the sealant to endure daily and seasonal changes for prolonged periods (Table 1).

Installation

The following should be taken note of:

1. Care in erecting each component, check the flatness and adherence, the right modular pitch, the plumb, level and the right joint of each weather-strip element, the glazing cap or compensation profile and perpendicular and perfect modular distance.
2. Brackets: The first phase involves the installation of universal brackets making sure that all floors are vertically aligned, adding spacers to brackets where necessary. Brackets must be properly blocked making sure that the screws or any other iron element that comes in contact with the aluminium is always protected with zinc o insulating plastic material since iron-aluminium contact must be used in the following pairs: aluminium-aluminium, aluminium-stainless steel, aluminium-plastic (teflon-ABS-nylon). Whenever possible do not use the galvanized material since it may lose the galvanized lining over time generating dangerous corrosion which also blemishes surfaces.
3. Air/Water Seal: During rainy days, a front continuously exposed to a sheet of water that constantly runs along its surface subjecting all seal elements to severe Struss over the years. Seals are particularly subject to expansions and stiffening during the winter. Therefore, great care and attention must be made when sealing Curtain Wall. Sealing: Weatherstrips must be joined with cyanoacrylic adhesive. Use silicon sealants for metal parts or between metal. Thoroughly clean surface before sealing. Dust, grease and moisture will lead to poor adhesion and subsequent loss in sealing power and serious damage over time.
4. Glazing: Glass expansion is opposite to that of Aluminium expansion and therefore remember to leave enough clearance in drawings and glass tables. Glass must always be shimmed with plastic material which is unaffected by moisture allowing it to slide. Thermal shock occurs when a portion of the interior and exterior of a piece of glass exposed to the sun, in the summer or spring, is covered and thus is heated on only one side of the surface leading to subsequent expansion. The other part in the shade, and thus not heated, does not expand and therefore the glass may break along the temperature difference axis.
5. Before installing glass, always check that the perimeter has no chips, cuts or starts of breaks. If these defects are detected, use sand paper to eliminate the continuity towards the interior of the cut.
6. Finishes: The joints between plates, brackets or upper, lower and side coverings (after allowing for the proper expansion joints), as well as any cuts made by hand at the construction site must be uniform and must be properly sealed to avoid problems and dangerous infiltrations

Quality control

It is advisable to carry out a weather performance test by constructing a mock-up of the façade and performing water, air infiltration as well as wind loading tests in a test chamber so that the effectiveness and suitability of the sealants and façade components can be determined.