Press Information
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| Issued: September 2008 |
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The ins and outs of natural rainscreen cladding
Speed of build originally made rainscreens a contemporary construction choice and the systems continue to rise in popularity as increasingly stylish and natural finishes are launched to market. Although historically the preserve of premium developments, technological innovations are helping to bring the cost down and make these high quality exteriors more accessible. William Harwood, T.I. Dynamic Facades, discusses the various natural rainscreen systems that are available, and provides valuable insight into their relative benefits.
The primary function of a rainscreen is to protect the inner leaf of a building from wind driven rain and ultra violet light. While early façade products were selected almost exclusively on achieving this primary objective, more modern systems are chosen because they are both aesthetically pleasing and practical to use.
In the residential sector, natural materials, most notably terracotta finishes, are growing in popularity. This can be attributed to the economical, environmental and design benefits that these products provide.
Why use a natural rainscreen?
Cost is always a prevailing factor in property development. Rainscreen cladding helps to keep budgets down by making the speed of build more efficient and reducing the number of trades required for installation.
For example, the construction of a rainscreen enables one contractor to install the full building envelope. There is no need for a break in the site programme as the contractor can continue straight from the sub-wall to the outer-wall.
Natural rainscreen cladding can be used with a range of building constructions, including metal stud, steel frame, timber frame and structural insulated panels (SIPs). Where metal stud or SIPs are used, these are generally fixed from floor slab to floor slab. The carrier system can then be directly attached to the wall and the rainscreen finish mounted on top.
Of all the structural walls, SIPs represent the most economical as they create a ‘through-wall’ construction that removes the need for additional cavity insulation, as it is an inherent component of the panels. As such, it further increases the efficiency of site programmes and offers superior acoustic and thermal insulation compared with standard frame constructions. Property developer, Dandara, used this approach for its flagship St George’s Island apartment development in Manchester.
What’s available?
The two main types of natural materials that are used with rainscreen cladding are terracotta and stone. Terracotta has historically been most popular in the residential sector, because it is more cost effective than using brick and block and less expensive than a higher end stone finish.
Terracotta choices
Terracotta rainscreens are available in a choice of tile – either single or twin skin. The twin skin tile was the original terracotta rainscreen, and, over time, has built up confidence in its reliability. Launched to market more recently, the single skin terracotta rainscreen is much thinner and lighter, leading to the misconception amongst some specifiers that performance levels in terms of strength and durability will be compromised. The consequence of this is that twin skin remains the first specification choice at architect level, although it is often not the most practical solution on-site.
The most advanced single skin systems weigh an average of 10kg per square metre less than traditional offerings. As well as ensuring health and safety legislative requirements are met, this also helps to significantly increase speed of installation. Moreover, because the tiles are a lot thinner, cutting is easier, which reduces the amount of wastage through breakages.
The objection relating to compromised performance is unfounded, as the only complete terracotta rainscreen to be fully BBA (British Board of Agrement) certified, is a single skin system. This means it is proven to require little or no maintenance and its overall performance has been tested up to a height of 100 metres. Most twin skin terracotta rainscreens have only achieved elemental accreditation from the Centre for Window and Cladding Technology (CWCT), with the exception of just one, which is certified by the BRE (British Research Establishment).
Residential developers of all sizes are recognising the benefit of using a single skin terracotta rainscreen. For example, Broadbase Limited requested the architect review a twin skin specification for its Glasgow apartment development, because it knew the single skin would deliver equal performance and be easier to manage on-site.
Stone finishes
To withstand aggressive environmental conditions and deliver the required performance, stone façades have traditionally had to be approximately 40mm thick. The consequence of this is that the system generally takes longer to install, resulting in increased labour costs.
To help make stone finishes more competitive, façade innovators have developed lighter weight systems that are durable and easier to install than conventional constructions. Where stone is concerned, this has been achieved by creating a facing that is reinforced with a high strength fibre, aluminium or aluminium honeycomb backing. Stone facings are manufactured 6mm to 15mm thick and bonded to one of the backing systems, with the type of backing determined by the specification requirements of a project.
Sandstone, limestone, granite, basalt and marble can all be manufactured in this way to achieve a durable, high performance finish, without the need for a stonemason. As well as offering an on-site cost saving because it substantially reduces site programmes, it also brings the overall system cost down because less of the raw material is required per square metre.
Behind the rainscreen
Natural finish rainscreens should be used with precision calculated carrier systems. These are are available in a range of grid formats to allow complete design versatility and fast installation.
There are two types of carrier system on the market – vertical and horizontal. While horizontal carriers have been in use for some time, vertical hangings are a more recent development, but offer a number of on-site benefits.
Compared with standard horizontal subframes, vertical systems significantly reduce the number of fixings required, and the fixings used are designed to be virtually invisible. As well as reducing the complexity of the installation, this preserves the aesthetics of the exterior finish. Ease of interface with other subframes accommodates flexibility in design, as a number of different materials such as render and glazing, can be used on the same elevation.
Engineered with ease of use in mind, some vertical carriers make it impossible for a tile to be fitted incorrectly. As well as offering peace of mind to the developer in terms of site safety, this also contributes to increasing the speed of installation, because it is much simpler to find the correct place for the tile.
Once installed, a vertical carrier gives greater flexibility in terms of tile replacement and removal. Individual tile access makes the installation of service lines at the end of a site programme straightforward, as the tiles are easily removed and then replaced, until the most appropriate point of entry is identified.
Ultimately developers and main contractors want the outer leaf of a building to be wind and watertight as soon as possible, so the internal fit out can begin. Modern rainscreen systems help achieve this by increasing the efficiency of installation, and importantly, do so while maintaining a high quality finish that is affordable.
