Paul Jackson, technical director at Rodeca, explains why translucent polycarbonate panels are now being specified as rainscreen systems.
It is a popular misconception that rainscreen cladding systems are something of a modern-day wonder product but they actually date back hundreds of years. Their design principles can be found in the timber weather boarded houses of Essex and clay tile or wooden-shingle cladded properties of Kent and Sussex, to name just a couple.
A wide range of rainscreen cladding systems is now available in the UK, from aluminium and aluminium composite materials or steel cassettes to fibre cement and glass fibre reinforced polyester composite panels. These are complemented by an equally wide choice of fixing systems, ranging from wood and aluminium to steel stud.
Whatever the cladding or fixing system, the principles of ventilated rainscreen systems are the same, with the cladding fixed back to the main support structure, forming a relatively lightweight, colourful weather-resistant overcoat. The benefit of this system is that any moisture, either ingress or humidity, is ventilated out of the cavity, ensuring the insulation and the inner leaf of the building are not affected by condensation.
Over the decades architectural trends have changed, with traditional brick facades losing out to an ever-growing palette of contemporary rainscreen finishes. And translucent polycarbonate, which has long been specified for its ability to allow natural daylight (up to 66 per cent transmittance) into a building through the walls or roof, is now making its mark in this type of application. So why this evolvement?
Well, translucent buildings naturally look very different by day and by night, particularly with innovative use of back lighting within the rainscreen cavity. They are also available in almost any colour and translucency level, from opal finishes through to colours throughout the panel and different colours in the front or rear of the panel to achieve a 3D-type effect.
In line with the current trend for all things industrial, they can be used to express or conceal the structure, enabling designers to show what is happening behind. In addition, polycarbonate-glazed openings, sitting behind the exterior double-wall polycarbonate rainscreen, allow designers to introduce daylight into areas without breaks in the external facade. At night this can work in reverse, changing the building’s appearance externally from the ambient light within, allowing designers to experiment with coloured panels and/or lighting such as LED.
In addition, unlike traditional rainscreen finishes such as rigid boards or sheet materials which tend to have a black shadow line effect around the panel edges, polycarbonate materials can achieve large seamless facades with just a perimeter frame only. Panels are possible up to 23m in height/length with the only limitation on facade length being the building’s expansion joints.
The material’s light weight – approx 8kg/m2 – means the structure can be kept lean and save costs.
Panels can be lifted and installed by hand from access platforms without the aid of cranes or lifting plant and through their tongue and groove system they are quickly secret fixed as standard rather than reliant on complex mechanically-undercut anchors or structurally adhesive systems or trays.
Architects of rainscreen cladding schemes have several considerations at the design stage to ensure the system is optimised.
For example, a refurbishment project may require a fully-adjustable carrier to even out the tolerances of the existing building whereas a new-build project backed onto a lightweight steel framing system or insulated composite panels, may not have the same tolerance issues and simple top-hat arrangements that can be provided at the fastener location.
Panels can be cut on site to accommodate building irregularities and tolerances and window and door penetrations can be flush or recessed as with other rainscreen systems. A wide range of perimeter aluminium sections allows designers to overcome interfacing and building height issues such as vertical/horizontal breaks and compartmentation.
With a lighting strategy, horizontal rails may block light and therefore will need to run vertically projected via isolated top-hat sections or cavity sizes that should be increased to accommodate the lighting. Consideration also needs to be given to accessing the lighting system for maintenance.
The amount and type of insulation also requires careful consideration as its branding will be on display if the panel is fully translucent. Dark membranes and foil-faced insulations should be avoided as they reflect heat onto the rear of the panels.
Sufficient airflow from the base of the cladding out through the head should be considered, along with factored wind loads that the cladding needs to be designed to withstand. This can sometimes be optimised by the use of a thicker panel, although there is a balance to be struck between the savings of a thinner panel and the additional framework, fixings and labour it requires due to much closer fixing centres. This requires value-engineering to get the optimum structural span and costs. For example, a 10m high facade with a windload of 1.0kN/m2 and a 40mm thick panel would require five intermediate cladding rails, whereas a 50mm panel requires only three.
Through thick and thin, it seems translucent polycarbonate as a rainscreen system is here to stay!