Curtain walling

Essentially there are 3 options in curtain wall design.

Face sealed systems

Since the predominant materials of curtain walling, steel and glass, are impervious, careful joint design should be able to prevent water ingress. Early curtain walling systems tended to be face sealed and relied on a weatherproof outer seal to prevent water penetration.

The seal must remain completely free of defects to prevent leakage paths occurring. This means that workmanship must be of a high standard. Achieving watertightness during construction is one thing; maintaining it throughout the life of the cladding is something else, when the individual components are subject to temperature and structural movements, ageing of sealants, etc.

In a truly face sealed system there is no provision for drainage: any water that bypasses the outer seal could result in internal water ingress. With single glazing systems it had always been the case that glass would be fully bedded, initially with glazing putty and latterly with various synthetic glazing compounds. Early installations of insulating glass units (IGUs) followed the same route, with the edges of the units fully encapsulated with 'wet' applied glazing compounds.

The systems are now largely obsolete simply because it was very difficult to install the sealant without the creation of voids or air pockets, which, as the building aged could become filled with water. More often than not the voids would be around the edge of the IGU, thus holding moisture in contact with the edge seal and leading to premature breakdown of the unit allowing it to mist up. Furthermore, wind-induced deflections or movements in the glass could pump the water contained in the voids, forcing it towards the interior of the building.

A variation on the wet sealed system was the zipper gasket design. In this form of installation the IGUs (or single glazed panels) were fitted into neoprene rubber gaskets that completely encapsulated the edge of the glass. The gasket would be designed to hold 2 adjacent panels and contained a groove into which a rigid 'zipper' strip was pushed. The zipper would compress the gasket against the sides of the glass and, theoretically at least, provide a watertight joint.

A small number of proprietary systems employed the zipper gasket principle but they were notorious for leakage problems. Because of their flush, or nearly flush appearance, the design found favour with architects wishing to achieve a flat plane appearance in the design of a facade.

With a zipper system, the glass is restrained by the gasket; there are no additional mechanical supports. Thus, the gasket material must be rigid enough to carry the weight of the glass as well as wind loads without deformation. In practice it is easier to accomplish the structural function than the waterproofing function:

• primarily because it is impossible to install gaskets without joints; and
• secondly because of the difficulty of maintaining adequate compression on the gasket throughout its contact face and throughout its life.

To be effective at excluding water, the gasket must be held at pressure in contact with the glass or panel. The pressure of the gasket at its contact face is called lip pressure. Over time, lip pressure decreases as a result of normal hardening or weathering, compression set and or shrinkage. When installed, the gaskets are stretched; they tend to revert over time and also shrink as a result of the gassing off of volatile plasticisers (Nicastro, D., Zipper gaskets, article in The Construction Specifier, The Construction Specifications Institute, Sept. 1996). The designer of the zipper gasket system is faced with a compromise; the gasket material must be rigid enough to fulfil its structural role, yet remain flexible enough to maintain adequate lip pressure.

The zipper (sometimes termed a 'lock-strip') is usually around 10 points harder in durometer (the 'shore A' hardness reading - a measure of the resistance of plastics toward indentation) than the gasket itself and exerts a force on the centre of the gasket, which is designed to be transferred to the tips of the gasket in contact with the glass. At a corner or cruciform joint the geometry of the joint reduces the effectiveness of the zipper and hence lip pressure can be compromised. Similar problems occur at but joints.

One North American manufacturer claims to solve the joint problem by forming factory made units to fit specified openings. Corner pieces and tee joints are made by injection moulding; this injection moulding process forces uncured rubber compound into a mould that joins the gasket ends and simultaneously vulcanises the rubber splice.

A face sealed, single stage design has no back up if water bypasses the primary seal. Air pressure differentials ensure that any small leakage paths are exploited. Confronting the weather head on requires a leap of faith - faith in the engineering ability of the designer, and in the skills and knowledge of the installer. Far better to accept the fact that 100% watertightness cannot be achieved and to make allowances for small amounts of water to both leak in and drain out again.

Typical zipper gasket system

Drained systems

An improvement on the face sealed system is the drained system which contains an air cavity behind the primary outer face. While still vulnerable to leakage resulting from air pressure differentials, the cavity can at least be drained. For this arrangement to work a secondary back seal is needed.

Most cladding designers accept that it is difficult to exclude water and therefore provision for a small amount of leakage can be made within a drained system. Typically these dry glazed systems comprise an outer decorative cover plate, and an aluminium pressure plate with 2 narrow rubber oyster gaskets either side clamped against the glass or insulated infill panel. The pressure plates are screw fixed through a thermal break into the mullion or transom member. A further inner gasket between the glass and the mullion or transom provides the back seal.

In drained and ventilated systems the front gaskets provide an initial barrier. The rebates and cavities are drained and ventilated to the exterior to prevent the accumulation of any water that bypasses the outer seals. Drainage is usually via small holes or slots in the underside of transoms that drain water down through the mullions.

Some systems also incorporate a foil-faced, butyl adhesive tape applied over the transom and mullion nosings directly beneath the pressure plate to serve as a secondary line of defence. While these systems will accommodate a small quantity of water within the glazing rebates, the pressure plate needs to be fixed to the correct torque so that the outer gasket seal forms a good seal against the glass.

Another key issue is the adequacy of the joint design. In stick systems transoms are usually joined to mullions via metal stubs screwed to the mullion and sliding into the hollow transom profile. Since the glazing rebate functions as a drainage path, it is important to ensure that water cannot work its way back to the interior of the cladding system at the mullion/transom joint. Water at this point could track back as a result of pressure differentials and wind assisted capillarity. For this reason, manufacturers often specify a layer of sealant at the joint.

Another variation of joint design involves the use of rubber 'infiltration blocks', which are usually placed at cruciform joints. Being single mouldings, they can help keep water away from the corner joints, but again they invariably need to be sealed properly. A failure to seal properly often results in water leakage internally.

Water that enters the system can be drained either from the transoms via slots, or into the mullion, to be discharged at strategic points, usually with a plastic drainage shoe insert. In those systems that rely on an additional butyl foil layer beneath the outer gasket, check that the tape is cut around the drainage shoe, otherwise water can back up in the system and penetrate the building via a convenient joint.

Back ventilation is a concept mainly applied to rainscreen curtain walling systems. To deal with kinetic energy, joints are usually unsealed, but designed as a labyrinth so that drops of rain shattering on impact will not travel further into the joint. Even so, it is accepted that at times water can penetrate as a result of air pressure differentials, surface tension and gravity, so a drained and ventilated void is provided behind the outer skin. Behind the void may be located a 'technical wall' - providing insulation and a vapour barrier. To provide airtightness, joints in the technical wall would be sealed. The basic function of the rainscreen principle is to deal with all of the mechanisms of water entry.

Drained and back ventilated systems are not simply used for rainscreen cladding. Many high-profile office buildings now employ a similar concept - an outer single layer of glass as the rainscreen, a void of perhaps 500-600mm and then an inner glazing system of IGUs. With these systems, positive ventilation is achieved by means of fans and exhaust grills and the air space used either to warm or cool the interior of the building according to season. Although expensive, this method has the advantage that the weather is kept away from the IGUs and so the risk of leakage into the interior of the building is much reduced.

Despite the reservations expressed earlier, one face sealing system that appears to work is the use of structural silicone jointing - a system with a track record of more than 40 years.

Silicone glazing systems (sometimes called SSG) enable a flush elevational treatment with no exposed metal components such as pressure plates or caps. The system also has several advantages over conventional stick systems, for example:

• it provides a good thermal break;
• it reduces the amount of dirt and water retention in the glazing system;
• it reduces thermally induced stress; and
• it is easy to clean.

An example of a proprietary silicone glazed system in which the IGU is usually factory bonded to a carrier frame with silicone adhesive, in turn fixed to the mullions and transoms with aluminium fixing lugs. In this case, any water that bypasses the outer weather seal between the units can drain away via the air chamber behind.

There are 2 basic types of silicone glazing:

• 2-sided; and
• 4-sided.

Two-sided glazing provides support to the vertical edges only, while gravitational loads are supported by mechanical means - either a bolted system or a conventional fin support. Two-sided systems should not be confused with butt-joint glazing which does not provide a structural bond to the framing system. Butt-joint glazing provides a weather seal only on 2 edges of the glass.

In 4-sided construction, the glass is bonded on all 4 sides, with the dead weight of the unit being carried by a structural fin and wind loads carried by the silicone joints. Sealant manufacturer Dow Corning suggests that 4-sided glazing systems are less likely to leak due to the adhesive/sealant which blocks air and water ingress. (Structural Silicone Glazing, Dow Corning Corporation, USA, 1999)

Not only does the silicone joint carry wind loads it must also accommodate thermally induced movements between the IGU and the structural frame. The weather seal between adjacent units is not designed to be load-bearing. Its function is to exclude air and water, but it must be bonded to both the edges of the glass as well as the edge seal - for this reason the compatibility of sealants must be established, and if necessary demonstrated by test.

Rubber infiltration blocks at junction of mullion and transom

Two-sided silicone glazing - silicone to vertical joints, conventional pressure plate to transoms

Some forms of 2-sided structural silicone glazing are designed to be erected in the field. In this case, the glass is usually held in place with temporary mechanical fixings while curing of the adhesive takes place - this can be as long as 3 weeks. Once the adhesive is cured, the temporary fixings are removed and the weather seal installed. Field installation is fraught with quality control problems and within the UK, there has been a move to factory bonding.

Cleanliness and adequate cleaning with appropriate solvents is essential to avoid loss of adhesion, both in the adhesives and the weather seals. The consequences of adhesion failure of the structural bond could be severe as there are no additional mechanical fixings to hold the glass in place. Catastrophic failure of silicone glazed systems are rare, although worries over structural performance have lead to the design of structural spacers - these contain a groove that permits a mechanical clip fixing to the inner leaf.

While the adhesive properties of silicone are well known and documented, problems with compatibility can still occur. In recent years more coated glass products have been marketed in an attempt to improve thermal performance. These coatings are not always compatible with silicone sealants and careful checking is needed to ensure that a suitable bond will be achieved if the coating is to be exposed to the silicone. Similarly, some polyester powder coatings, particularly those with a high gloss, contain a wax residue that may prevent a proper bond.

Adhesion testing is usually by means of a peel test method - although this does not produce actual measurable results. More rigorous pull-off tests can be scientifically controlled and give more reliable results. Pull-off tests usually involve bonding the samples together and then pulling them apart. The sample will fail either in adhesion (one face pulls off) or preferably by cohesion (where the silicone actually remains bonded to the 2 surfaces but splits through its thickness).

Pressure equalised systems

The prime mechanism of water entry into a cladding system is a differential between high and low pressure. If the pressure in the glazing rebate can be equalised with that outside, the movement of air (and hence water) can be eliminated. In practice, there will always be a very slight lag in the equalisation of the pressures in the glazing rebate and the exterior, but the timing can be reduced so that to all intents and purposes the pressures are classed as equalised.

To achieve pressure equalisation it is necessary to:

• provide suitable slots in the pressure plates to enable air pressures to reach equilibrium very quickly;
• divide the facade into zones that are sealed from one another (a zone would normally exist around a single IGU);
• ensure that the rear face of the cladding system is properly sealed to both air and water;
• design joints so that they have a high degree of resistance to the mechanisms of water entry - kinetic energy, surface tension, gravity, wind-assisted capillarity; and
• have an effective means of draining water from the system - usually via transoms rather than mullions.

Fundamentally therefore, with pressure equalised systems the outer face is sealed as tight as possible against rainwater, while the inner face is sealed as tight as possible against air inflow. Nearly all modern curtain walling systems are designed using the principles of pressure equalisation.

The performance of the air barrier is crucial in the success or failure of the pressure equalisation system. The barrier needs to be rigid in order to provide an air cavity of constant volume. Pulses of air pressure during gusting wind would take longer to dispel if the air volume changed, and if the seal were flexing under wind pressure it could also serve as a pump. (Rousseau, M.Z., Facts and fictions of rainscreen walls, article in Construction Canada, Vol. 32, no. 2, National Research Council, Canada, 1990)

The effects of wind pressure on buildings are complex, with corners and the highest parts of the building suffering sharp pressure gradients. On the side facing the wind, positive pressures are experienced, while high negative pressures are experienced on the leeward side. Such pressure variations can cause air currents behind the outer face, with air moving from areas of high pressure to areas of low pressure. These movements run contrary to the principles of pressure equalisation and so it is important that separate zones are created and sealed from one another. This maximises the chances of attaining pressure equalisation much faster.

Method of pressure equalisation

Venting of the compartment permits the movement of air. Vents must be large enough to prevent water filming over them. The more effective the air barriers around the compartment, the smaller the area of venting. If the compartments are poorly sealed, larger amounts of air are needed, so more air must be drawn in from the outside.