Curtain walling

Curtain walling may be defined as a weatherproof, non-load-bearing enclosure of windows or metal panels in a light metal framework that is fixed back to the structure of a building, usually at intermediate floor levels. There are various systems in common use ranging from simple, site-assembled 'stick systems' to larger, pre-assembled panels or unitised systems. See Introduction.

Many glass curtain walls comprise a system of mullions and transoms with the insulating glass units held in place with clamping plates. In more recent years, silicone bonded systems have become popular; in these systems either 2 or 4 edges of the glass are held in place with silicone adhesive. See Methods of preventing water ingress.

Few, if any, curtain walling systems can be made totally weathertight. It is usual to accept that some leakage will take place from the outer, weather facing seals, but to drain this water away in a controlled fashion. There are several mechanisms of water penetration into a system: force of gravity, kinetic energy, capillarity, pressure assisted capillarity, surface tension and air pressure differentials. Of these, air pressure differentials are possibly the most significant. See Weathertightness.

Wind pressure against a facade creates localised areas of high pressure. If the air pressure inside a building is lower, air leaks through, taking moisture with it. If the air in the cladding system can equalise quickly with the external air pressure, the propensity for leakage is much reduced. Drained and pressure equalised cladding systems (which are generally most common) use this principle, and divide the ventilated void in the cladding into small fields to permit rapid equalisation. See Problems with pressure plates.

Problems of leakage are possibly the most common form of defect, although defective fixing details are not unknown. Fixings must be robust and durable, as they are inaccessible and cannot be inspected or repaired easily. The fixing system must be capable of accommodating building movements due to loading, creep, thermal and moisture movements, etc. See Failure modes.

Most systems rely to a large extent upon the performance of sealants. The basic types of sealant failure are:

• adhesion failure;
• cohesion failure;
• delamination failure; and
• reversion.

See Identifying defects.