Knowing your way round glass and glazing systems

When it comes to cladding, there are probably more issues relating to glass and glazing than there are to any other matter, apart from fire safety.

If surveyors have to carry out assessments on framing systems without a cladding consultant, the same could be true for glass and glazing.

For instance, annealed and heat-strengthened glass cannot be classified as safety glass, because on breaking these will form large shards unless an anti-shatter film is applied.

Insulated glass units, meanwhile, are typically trouble-free, but after 25 or 30 years the seals at the edge tend to fail due to increased porosity. Any moisture that enters the framing system and sits in the glazing table below the units can then be drawn up through the seal, and under certain environmental conditions this causes misting of the glazing airspace.

It is the presence of misting in the airspace rather than edge seal failure alone that is the test for tenants having breached their covenant to repair, although this may be contentious in dilapidations disputes. Unfortunately, misted units with edge seal failure cannot be repaired in situ and therefore must be replaced as a whole new unit. They can be removed to a workshop for reconstruction by stripping the glass off the edge seal and applying a new seal, however, this is never cost effective and rarely undertaken.

The integrity of the edge seals can be determined by the dew-point test, which more broadly can help assess the remaining service life of insulated glass units. The dew point test is an in situ assessment of the edge seal where dry ice in a cylinder is placed on the internal surface of the glass to reduce the dew point and hence mimic natural environmental conditions. The result is a binary pass or fail and does not in itself offer any prediction of remaining service life.

When glass is bonded to either an edge seal during manufacture of a double glazed unit or where structurally silicone bonded to a frame, it is imperative that any solar control or low-E thermal coatings on the glass are 'edge deleted' as otherwise the adhesive bond to the glass will be reduced and retention of the glass compromised.

Such glass can spontaneously shatter in service as a consequence of nickel sulphide that is a contaminant present variably to all float glass. The timescale for breakage varies according to the heat build-up from solar gain, particularly on south facing elevations. The peak age for failure is typically four to six years after manufacture, although it is not unknown for breakages to occur more than 20 years after the glass was manufactured.

The presence of nickel sulphide cannot be determined by visual inspection, and so the likelihood of failure cannot be predicted with accuracy. But if the nickel sulphide inclusion itself – present as a black dot – at the centre of any failure can be retrieved, it can be tested to determine whether the substance is present.

The likelihood of failure can be significantly reduced, but not totally eliminated, by carrying out heat soaking testing (HST) at the time of glass manufacture. HST is routinely undertaken on high-profile and institutional standard projects; however, its use is by no means universal, particularly where used for glass in low-rise buildings. All toughened glass should be stamped with a BSI Kitemark and also HST confirmation if undertaken, but again this is not always the case.

HST should be undertaken in accordance with BS EN 14179: 2016. However, glass specified as being heat-soak tested can fail due to poor practice, albeit rarely. You should consult operation and maintenance manuals to determine whether glass was specified as heat-soak tested during manufacture, as well as the HST records and the calibration certificate for the oven used. In practice, however, such information is not always available.

It cannot be assumed that the cause of failure is nickel sulphide inclusion, as toughened glass may also shatter as a result of thermal stress associated with an edge defect, vandalism or impact from a window cleaning cradle.

Be aware that that there is presently a body of opinion against using toughened glass in the outer pane of an insulated glass unit at height, where there is a zero-risk policy in respect of failure. The current standard specification for safety glass in a floor-to-ceiling height double-glazed unit in a commercial development is laminated or heat-strengthened glass externally and laminated glass internally; when it cracks, laminated glass remains in place.

The use of either annealed or heat-strengthened glass in the outer pane of an insulated glass unit at height is also permissible even though neither is a safety glass, provided that the risk of failure is properly assessed by either a façade engineer or the specialist cladding sub-contractor.

The opaque coating is typically applied to the rear face of the glazing, namely surface 2 of a single-pane spandrel or surface 4 on an insulated glass unit. The superior durability of a ceramic coating is preferred to a back-painted one, which can deteriorate from heat build-up.

This risk can be mitigated quite simply by applying an anti-shatter film internally to the underside of the glass, which will keep any shattered toughened glass in place. Nevertheless, perimeter edge retention using structural silicone or a plastic bead is almost always required; otherwise the entire pane of shattered glass with anti-shatter film could fall to the ground. Laminated glass is therefore preferred as the lowest pane in an insulated unit or in single-pane glass as, on failing, the cracked glass remains in situ until it can be safely replaced.

A class-3 roof, typically up to 10mm thick, cannot support the impact load from maintenance workers should they inadvertently fall on to it, with a consequent risk of them falling to floor-level below. Conversely, a class-2 roof is sufficiently robust to withstand the accidental impact loads without risk of falling through.

Andrew Tee is the owner and founder of cladding consultants and curtain walling specialists Tee Technical
Contact Andrew: Email

Related competencies include: Building pathology, Construction technology and environmental services, Inspection