BUILT ENVIRONMENT JOURNAL

Knowing your way round glass and glazing systems

Glass and glazing systems present a number of challenges to the non-specialist charged with assessing them. The second in a series of articles on cladding offers some guidance

Author: Andrew Tee

17 August 2021

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.

Heat and moisture

Glass is an inherently long-lasting and durable material. However, risks arise when a coating is applied, where glass is heated for strengthening, or where it is used in the manufacture of insulated units.

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.

Toughened glass

One method of producing safety glass is to toughen it by heating in an oven during manufacture, meaning that when it shatters it forms dice-like fragments that are sufficiently small so as not to cause injury, if and when falling onto any persons below. Depending on the glazing support system used, the shattered glass may either remain in situ or fall from place.

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.

Overhead and vertical glazing

Toughened glass may be used at height in the outer pane of an insulated glass unit or in single-pane glazing. Its use must be subject to a risk assessment by the Centre for Window and Cladding Technology (CWCT), in order to establish the risk of any failed glass falling from height and injuring someone below.

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.

'Toughened glass may be used at height in the outer pane of an insulated glass unit or in single-pane glazing'
Spandrel glazing

Where they are used at floor level in an insulated unit, to resist heat build-up from the insulation behind, glazed spandrel panels must consist of heat-soak-tested, toughened glass.

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.

Overhead slope glazing

In atriums and rooflights, toughened glass was traditionally used in overhead slope glazing. However, this has long since been considered poor practice, given that any shattered glass will collapse and fall if it is either a single pane or the lowest in an insulated unit.

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.

Fragility

When working at height on or close to overhead glazing, a fragility assessment should be undertaken in accordance with CWCT guidelines to determine whether the glass is robust; for example, whether it is a class-2 roof or a fragile class-3 one.

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.

Summary

This guide provides a snapshot of the most common problems with glass and glazing. It is not an exhaustive list of all pitfalls but hopefully serves as a useful guide to potential problems and their remedies, and when to consult a specialist.

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

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