Watch the AT CPD webinar, in partnership with Sto, exploring how to correctly specify robust glass rainscreen facades for public spaces.
The conversation around public spaces has developed significantly since the pandemic, amplifying the importance of high-quality, accessible and inclusive designs. But how can we ensure that these important spaces remain safe when specifying glass rainscreen facades? This AT CPD webinar, in partnership with Sto, was chaired by Ruth Slavid, with expert guidance and advice supplied by Sto’s Head of Product Management, Greg Astill, and Business Development Manager – Rainscreen Systems, Darren Greenwell.
The speakers split the CPD into two parts. The first covered rainscreen principles, including the difference between glass rainscreen cladding and curtain walling, glass rainscreen build-ups, composite glass panels, and designing with glass. Part two explored performance and approvals, including moisture protection, wind loads, impact and blast performance, as well as fire safety.
Sto speakers: Greg Astill, Head of Product Management, and Darren Greenwell, Business Development Manager – Rainscreen Systems
Glass Rainscreens Part One
Rainscreen principles
In essence, a rainscreen cladding system is a form of double-wall construction where the outer layer keeps most of the rain out. A drained and ventilated cavity eliminates the remainder of the moisture by natural drainage and evaporation. The inner leaf provides thermal insulation, prevents excessive air leakage, and carries the wind load.
Glass cladding systems
There are two principal types of non-structural, architectural glass systems: curtain walling and rainscreens. Curtain walling is completely sealed against the weather, whereas rainscreen cladding has open-jointed panels, a ventilated cavity, and is mounted on a substrate that provides the primary defence against weather ingress.
Typical glass rainscreen build up
Glass rainscreen build-up
A typical build up for a rainscreen cladding system would comprise the substrate (a solid or sheathed steel frame); then mineral fibre rainscreen slab insulation (fixed to the substrate with dowels); followed by stainless steel wall brackets, vertical and horizontal aluminium profiles, and fixings to suit the substrate; and finally the outer leaf glass panels (typically 6mm thick, toughened, and bonded to a backing board).
Glass panel composition
The composite glass panel
The outer layer is a 6mm or 8mm heat-soak-tested safety glass that is coated on the rear face with a mineralic pigment. The glass is incredibly tough – given sufficient impact it will shatter into small, safe chunks and remains bonded to the backer panel. Manufactured from perlite, the backer panel also forms a stable background for the horizontal carrying profiles that are screwed through onto the rear face. Glass panel sizes typically start at 2.6 x 1.25 metres, but can go up to 4.5 x 1.25 metres. 8mm glass is used for lengths and heights over 2.8 metres.
Coloured glass rainscreen cladding at the Plymouth Life Centre in Devon
Designing with glass
Glass rainscreen panels are generally coloured and it’s important to choose a system where the colours are fused to the back of the glass and applied before the toughening process. This makes them colour-fast and permanent. By contrast, painted glass may not be as durable and should therefore be avoided. Traditional standard float glass carries a greenish hue, but it is possible to specify colourless toughened glass (optiwhite).
Screen and digital printing options add design flexibility
Screen and digital printing provide additional design possibilities. The graphic is fused to the back of the glass and bonded to stop peeling or wear during the enamelling process. Glass finishes tend to be gloss as standard, but specialist matt ones are available, together with mirrored and metallic finishes. There is also a huge range of textures available, with many suitable for use on glass rainscreen systems.
Specifiers can modulate the grid and panel sizes of bespoke glass rainscreens
Having a bespoke panelised cladding system allows the designer to control the visual interpretation of the building grid. Typically joints between panels are between 5mm and 12mm. The surface level of the cladding can also be controlled by using the wide range of bracket sizes that are available.
Glass Rainscreens Part Two
Moisture protection
Glass rainscreen systems draw internal building moisture into the cavity under a natural pressure differential, leading to a reduction or elimination of internal damp and mould. The walls can breathe and ventilation ensures quick dispersion of water vapour.
Glass rainscreens are designed to provide moisture protection
Sub-construction thermal performance
Stainless steel provides superior thermal (and structural) performance and is best used as the supporting brackets. Almost limitless U-values and insulation thicknesses are possible with brackets available up to 600mm as standard. Aluminium is ideal for the supporting profiles due to its high strength-to-weight ratio and comparative low cost.
Fixed-point brackets carry the dead weight and provide wind load resistance
Bracket layout
Fixed point brackets carry the deadweight and provide wind-load resistance. Glide point brackets allow for expansion/contraction of the T-Profile and restrain lateral movement whilst providing wind load resistance. The minimum possible configuration for a three metre length of ‘T’ profile is a single fixed point and two gliding point brackets.
Safety
With recent changes to the Building Regulations it’s important to verify that the chosen glass rainscreen system is suitable for the intended application. Insulation and cladding panels over 18-metres in height must be of limited combustibility and must not ‘appreciably accelerate’ the rate at which fire will spread up the building.
Firebreaks must be designed to work in conjunction with a particular building construction and cladding system. They must also provide a barrier to fire and close the cavity in a fire event. Fire Barriers may be required to limit the spread of fire within a cavity. For ventilated rainscreen cladding, they may comprise horizontal open-state barriers at floor levels, vertical full-fill barriers at party walls, and full-fill barriers around openings.
Wind provides both positive and negative pressure. Negative suction effects are normally the most critical
Wind loads
The largest force applied to a façade is often wind loading. Building corners attract the highest loads and are commonly known as rim zones. The number of fixings required are directly related to the wind load calculations and the substrate pull-out capacity. On metal-framed, sheathed substrates, the fixings will need to go into the metalwork – it is highly unlikely the sheathing board will be able to carry the dead load or adequately resist the wind load.
Glass rainscreen panel after an impact test, with shattered fragments still bonded to the backing board
Impact performance
When specifying a glass surface it’s important to know the performance of the system in relation to its finished location, as no significant portion of the cladding should detach and fall when subject to certain levels of impact. Public safety zones require rigorous impact and blast testing. CWCT TN 76 gives good guidance for testing regimes. StoVentec Glass A has undergone extensive bomb blast tests with impressive results.
Blast performance
There are several governmental departments responsible for the protective security of critical assets, including the CNI (Critical National Infrastructure). Strict guidelines are mandatory and cover many different areas, such as the structural integrity of the overall system, the strength of individual system components, and their behaviour in an explosion scenario.
Glass rainscreen cladding at Moorgate Station in London
System approval certification
The BBA will test all aspects of a UK rainscreen cladding system against relevant criteria and design considerations. In general, bodies such as the NHBC will not recommend a system without BBA certification. Most certificates provide an expected service life in excess of 30 years. The BBA also ensures that the system complies with relevant building standards.
Sustainability
Glass is weather-resistant, easy to clean, long-lasting, requires little maintenance, and is 100 per cent recyclable. The carrier board the glass is bonded to is manufactured from perlite, a natural ore.
Conclusion
Glass rainscreen systems are simple and fast to install, provide optimal thermal insulation and moisture control, and are tested to the extreme with regards to fire, impact and blasts. They are also tough, safe, eliminate the need for wet trades during installation, and provide multiple design options in respect of colour, surface and texture.
Contact Details
For more information, please call 0141 892 8000, email, or visit the Sto website.
Source: Architecture Today