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Specifiers’ guide to glass skylights

23 October 2017
Despite the rise of modern thermoplastic materials such as polycarbonate and acrylics, glass continues to be a mainstay in building construction and design. It’s easy to see why.

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Glass skylights provide properties with unrestricted views of the outdoors and fill rooms with natural light, boasting structural properties that ensure longevity. It’s unlikely they’re going away any time soon.

When selecting glazing for skylight or rooflight projects, there are multiple factors that building specifiers should keep in mind. This guide aims to provide the vital information on key issues for glass specifiers including:

  • Common glazing types
  • Safety
  • Heat loss
  • Solar heat gain
  • Framing systems
  • Walk-on skylights

Common types of glass sklights

Annealed Glass

Representing the initial product created in the glassmaking process, annealed glass is often processed further in order to create the glazing materials listed below. Straightforwardly sourced and relatively easy to cut into shape, it is the least expensive on the market.

Annealed glass is also the weakest form of glass infill you can find. Typically possessing a bending strength of 45 N/mm2, it breaks easily and shatters into large, dangerous shards of glass when it does. Consequently, annealed glass is never acceptable for use as ‘safety glass’.

Heat Strengthened Glass

Through the application of a controlled heating and cooling process, combined with the careful application of compressive and tensile stresses on the body of glass during manufacturing, annealed glass is made into heat strengthened glass. This strengthening process provides significant gains over annealed glass in terms of bending strength – up from 45 to 70 N/mm2.

Despite its relative sturdiness, when heat strengthened glass breaks it exhibits the same hazardous properties as annealed glass, with a high potential for shattering into sharp pieces. As such, it also falls short of the acceptable standards for classification as ‘safety glass’.

Toughened Glass

Not unlike heat strengthened glass, toughened glass is created through managed heating, cooling and the application of even greater compressive stress. This provides an impressive bending strength of 120 N/mm2.

Unlike the previous items on this list, toughened glass meets the definition of ‘safety glass’. That’s because toughened glass tends to break into ‘dice’ – small and rounded fragments of glass – which are far less likely to cause injury.

However, the potential remains for the ‘dice’ to clump together and fall as a large section of glass. The presence of nickel sulphide (NiS) in toughened glass can also cause it to shatter spontaneously. As a result, it’s important to undertake a risk assessment of any toughened glass used in inner panes. Specifiers can also reduce associated risks further by opting for heat soaked toughened glass.

Heat Soaked Toughened Glass

When toughened glass is exposed to a sustained high temperature during manufacturing, problematic nickel sulphide inclusions are returned to a more stable state. Unfortunately, this process can also cause glass to break as it undergoes the treatment, leading to potentially increased manufacturing times. The units which make it to market, however, should be afforded much greater resilience and longevity.

Heat soaked toughened glass shows similar breakage behaviour to plain toughened glass, fracturing into ‘dice’.

Laminated Glass

Laminated panes separate two or more sheets of glass, typically annealed or heat strengthened glass, using an interlayer. The interlayer is essentially a film or an extra sheet of material which, by catching falling shards of glass, can prevent injuries to those below a shattering skylight. With sufficient thickness, size and material selection, the interlayer can also provide load-bearing capability to prevent other objects – or even people – from falling through.

Laminated glass represents the best available choices for overhead or walk-on glazing applications.

Specifiying for rooflight safety

Every rooflight specification should take two principal groups into account: those who will be above the glass – such as those performing maintenance – as well as those everyday users below who may be at risk.

Ensuring skylight safety for building users

The selection of laminated glass is often the specifier’s first stop when it comes to minimising risks associated with skylight breakage, such as falling glass or injury from falling.

British Standards recommend laminated glass inner panes in instances where skylights are intended to be more than 5m above floor level or a body of water (such as swimming pools). It is worth noting that in special circumstances this minimum recommendation increases to 13m.

Certain scenarios are also outlined demonstrating cases where merely a toughened inner pane can be utilised. However, unless a very thorough and conclusive risk assessment is undertaken, laminated glass is almost always the preferable option. If, after assessment, a toughened inner pane remains under consideration for your project, you should make sure it has been heat soak tested.

Where proposals suggest use of an annealed inner laminated pane, careful consideration should be given to the chance of thermal heat stress failure. This risk can be avoided by providing extra resilience with a heat strengthened or toughened laminated pane.

Achieving non-fragility in skylights

Non-fragility is an essential quality for any glazing products specified for walk-on use. It is vital to ensure that anyone walking on a rooflight, accidentally or otherwise, will not fall through – even when the glass or the rooflight product is broken.

It’s important to note, therefore, that the specification of a laminated inner pane does not qualify a rooflight for ‘non-fragile’ status by itself. When required, non-fragility must be specified separately.

CDM Regulations insist that risk assessments must be carried out in every construction project to define necessary measures for preventing falling through rooflights. Only when the assessment is absolutely satisfied that no risk of falling through the skylight is present can a fragile product be specified. In every other instance, non-fragile should be the standard classification.

Non-fragility of glass skylights should be defined with reference to documents from the Advisory Committee for Roof Safety (ACR) as well as The Centre for Window and Cladding Technology (CWCT). These include:

  • “Red Book” ACR[M]001 (ACR)
  • Technical Note 66 – Safety and Fragility of Glazed Roofing: guidance on specification (CWCT)
  • Technical Note 67 – Testing and Assessment (CWCT)
  • Technical Note 92 – Simplified Method for Assessing Glazing in Class 2 Roofs (CWCT)

These documents refer to the relevant tests for ensuring that a broken skylight unit can hold the weight of those above the glass, comprising the ACR[M001 soft body impact test, a glass-specific hard body impact test as well as a static load test. Criteria for meeting standards are the prevention of either soft bodies or dangerous glass fragments falling through the broken rooflight pane.

It’s vital to remember that any non-laminated glazing should always be considered fragile – inherently incapable of holding a body once broken.

It’s also key to remember that many skylights with laminated glass specified may remain ‘fragile’ in classification. Ensuring non-fragility may require enhanced specification, as well as an investigation into the framing system’s individual capabilities.

Satisfactory standards according to CWCT TN 66, 67 and 92 require a minimum laminated glass inner pane thickness of 9.5mm or 11.5mm (depending on the unit’s size). The unit should also be made up of two sheets of annealed or heat strengthened glass, utilising a 1.5mm interlayer of polyvinyl butryal (PVB) or ionomer supported by a rebated frame with 15mm edge cover on all four sides.

Reducing heat loss with skylight specification

There are a number of ways a specifier can limit heat loss through skylight selection.

Low-emissivity / Low E glass

Low E glass functions by using a transparent film of metal oxide in the cavity between the two ordinary panes of a double-glazed unit, thereby reflecting heat generated within the building back into the interior. At the same time, external radiation is allowed to pass from outside into the building, providing solar heat gain. Through this process of re-using internal heat and encouraging external heat to remain, products with Low E coatings are shown to offer insulation up to three times greater than units without.

Spacer bars

Most double-glazed units are separated or held apart by a frame, called a spacer bar, around the edge. Frequently this bar is constructed of aluminium, visible as a bright edge between the two panes. Due to its metallic nature, this bar can inadvertently act as a ‘thermal bridge’ between outdoors and indoors. Specification of a ‘warm edge’ spacer bar, made of a thermally non-conductive material, can help to improve the energy efficiency of the product while simultaneously reducing risks of condensation.

Gas filled cavities

By filling the cavity between the two panes in a double-glazed unit with dried air or inert gas and hermetically sealing it, a significant extra barrier against heat transfer is provided. The gas is selected specifically for its low conductivity properties, usually an inert gas such as argon, or krypton and xenon less commonly. Gas-filled double glazing combined with a low E coating can provide excellent protection against heat loss, with centre pane ‘U’ values up to 1.1 W/m²/°K.

Limiting unwanted solar heat gain

Although solar heat gain is generally advantageous during cold winter months, too much heat gain during summer can make buildings uncomfortably warm, bordering on unliveable.

Heat transfer from outside can be limited through specification of solar controlled glass, fitted as the outer pane of an insulated glass unit. Solar control usually involves the application of a metallic coating, tinting of the glass, or a combination of the above. However, it’s important to note that the level of radiated heat reduction correlates with the amount of light transmission allowed. Glass panes with higher solar (G) values – reflecting more solar energy – will be associated with a lower quantity of light entering the property.

Those who wish to benefit from solar gain during the winter without suffering excessive heat in winter often achieve the right balance through brise soleil (sun-deflecting architectural features) or clever deciduous landscaping to provide natural shade.

Rooflight framing systems

Rooflight design today is practically unlimited, with structures of any shape able to be manufactured from combinations of aluminium or steel bars with glass or polycarbonate glazing.

The framing system used can be pyramidal, rounded, pitched, vertical and more. Special-case systems for internal water management, custom flashings or tailored ventilation are routinely offered to suit multiple applications.

Note: each type of frame specified can make a major difference in the safety of the unit. It is not acceptable to deem a rooflight as either ‘safe’ or non-fragile based solely on its glass specification. The framing system itself, and the way it interacts with or retains the glass, is just as vital for a specifier to consider as the type of glass itself.

Special advice on walk-on rooflights

As mentioned previously, glass units used structurally as walkable floors necessitate the specification of strong laminated and toughened glass.

Laid out in BS EN 1991-1-1:2002, walk-on units such as these require a typical loading of 1.5kN/min domestic floor applications, increasing up to 2.0kN for concentrated loads. Specification of alternative glass types is required for higher loadings in non-domestic buildings.

Careful attention should be paid to the unit’s aspect ratio and framing support system as well as load conditions at the building site. Applying an anti-slip treatment should also be also considered to prevent unfortunate incidents as a result of slippery conditions on the final product.

Thanks for reading this guide to glass rooflight specification. We hope it has provided some valuable insights in advance of your next project – from safety and non-fragility, to balancing heat loss with heat gain, to special-case considerations.

If there’s something about skylights we can help you with, whether as a specifier, supplier or a home user, don’t hesitate to get in touch.

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