Curtain walling

Aside from silicone glazed systems, most cladding and window installations rely upon the proper performance of gaskets to maintain water and airtightness. To achieve a seal, the lip of the gasket must exert a pressure on the glass (lip pressure), usually circa 0.720N/mm to 1.760N/mm. (Nicastro, D., Glazing system leaks, article in The Construction Specifier, January 1996 issue, The Construction Specifications Institute, USA, 1996)

Gaskets are usually made from elastomeric materials such as butyl rubber which, although durable, degrade over time as a result of exposure to UV radiation, direct and reflected sunlight, and ozone. Volatile compounds in the gasket are driven off (out-gassing) by these mechanisms with the result that they eventually suffer shrinkage in volume and or brittleness. Often gasket materials are stretched as they are installed, with the result that they gradually shrink back into shape, opening corner or butt joints in the process.

It is better to anticipate some water leakage through the outer gasket system and to take steps to drain it away harmlessly to the outside. To an extent therefore, loss of lip pressure can be partly accommodated. However, if the inner or air seal gasket is also defective, there is a risk that water leakage can take place.

Replacement of the outer gaskets may be possible, but replacement of the air seals will probably involve deglazing (unless they are harder wedge gaskets - used when the glass in windows is bead glazed). An alternative to gasket replacement in these circumstances is the provision of a wet seal or top cap over the lip of the gasket.

Given accurate and careful workmanship a wet seal can be a useful expedient, but it should only be considered if the glazing system and its weaknesses have been properly understood. Unfortunately, the application of mastic is often seen as a quick fix, carried out by inexpert applicators. For success, Nicastro recommends:

• Use only silicone sealant that can be shown to be compatible with the original gasket.
• Use black sealant and ensure that it is applied by trained applicators.
• Prior to application, clean all contact surfaces with solvent cleaner.
• Maintain a minimum cross section of fillet of 6mm and a contact area on each face of 6mm.

Wet sealing can make a marginal difference to the shading of the perimeter of the glass and carries a slight, possibly theoretical, risk of increasing thermal stress between the centre of a pane and its perimeter.