Aesthetic and energy-saving requirements are resulting in increasingly sophisticated cladding specifications. Barbour Index and Scott Brownrigg dispense some words of advice
The days are long gone when the only criterion to consider when specifying cladding was whether it kept the weather out. The pressure is on for cladding systems to provide much better thermal performance, including lower U-values, reduced cold bridging and better condensation control. The sustainability agenda is calling for recyclable cladding systems that contain less embodied energy and solar control measures. If that wasn’t enough, the new, much tougher, Part L of the Building Regulations will have a dramatic effect on the way cladding is specified when it takes effect in 2006. Because of these developments, and the need to integrate systems with surrounding building elements, the cladding specifier has a lot to consider.
1. The frame
The most thermally inefficient element of cladding systems is the frame, which is usually made from aluminium – a metal noted for its ability to conduct heat. Remember that the most effective way of preventing heat transfer through the frame is the simple expedient of keeping its cross-section as small as possible.
Typically, installing thermal breaks – or resin blocks incorporated into the frame section – will cut energy transmission through the frame 50%. But systems are available that reduce energy flows to just 10%, thanks to highly efficient bond materials. You can ensure that the thermal break fulfils the demands of the required thermal standard by using computer modelling.
Heat transfer can be reduced by ensuring that the metal frame does not run from inside to out. Products with a timber, glass, or plastic outer capping are available, although be aware that some of these will require more maintenance.
Glazing is central to the cladding system’s performance and is becoming increasingly complex to specify. Consider industry standard guidelines from institutions such as the Centre for Window and Cladding Technology, and undertake a risk analysis. In terms of thermal performance, solar rejection of up to 60% can be achieved using special coatings. U-values across a double-glazed unit of less than 1.0 Wm2/K are achievable – but expensive. The performance tends to fall off at the edges of the unit unless high-performance spacers are specified and fitted into the frame using high-performance gaskets.
Filling the cavity with argon gas improves the U-value by about 10%. The cavity contains about 85% argon but leakage rates of about 5% per year are commonplace.
The newest and most expensive sealing techniques seek to reduce the leakage rate, but the relatively limited life offered by argon-filled double-glazed units makes its use on commercial projects difficult to justify at present.
Panels are the part of the cladding system where the overall thermal performance can be greatly improved. Panel technology is moving forwards rapidly and several manufacturers can now offer very high performing units.
A U-value of 0.15 Wm2/K is not uncommon.
4. Solar control
It is essential to fit shading elements to prevent excessive solar energy entering a building through the glazed elements. An increasingly wide selection of louvres and shades can be attached to brackets, which are fitted directly to the frame. When specifying external shades, consider weather conditions. All forms of sheet material that flex in the wind can be subject to damage. Sail-type shades have improved a lot, but nevertheless most will fray and stress their seams within five years.
Man-made materials are particularly vulnerable to ultraviolet rays, which will weaken the material and lead to eventual failure.
5. Woven metal
Shades can also be specified from woven metal, which has been used very successfully. Cladding incorporating woven metal has many benefits. It is very durable, reflects the sun’s energy, allows some light through and is not easily damaged. It is however, very heavy, which could damage other parts of the building if storms cause it to break free. It can also generate noise. Ensure large samples are viewed before choosing the exact type.
Louvre blades are traditionally available in metal, glass or timber. Metal louvres have been available for quite some time in an aerofoil profile in a range of sizes. However, wind can cause the blades to oscillate so much that over time the fixing suffers metal fatigue. Fixing knuckles should be designed, or at least checked, by a structural engineer.
Glass blades can be very dramatic in appearance but safety is a key element for the specifier. Use toughened laminated glass, especially if the units are above an area usually accessible to people.
7. Cleaning and maintaining external shading
Fundamental to the success of external shading systems is the ability to clean and maintain them easily and safely. Ensure there is a cleaning and maintenance strategy built into the design and the specification, as required by the CDM Regulations. Check the exact protocol for access to all parts of the cladding: it is relatively easy to use manufacturers’ data to superimpose the cleaning equipment performance envelope on to the elevation drawings. Remember some types of shading systems are harder to look after than others. For example, metal blades are easily damaged by cleaning, as they are not designed to take direct pressure from ladders or even from cleaners holding onto to them for support.
8. Double-walled systems
Up until now, multiple-walled systems have been seen as an expensive option. With the increasing emphasis on higher performance, the use of a double-walled system can be justified on many counts. The cavity can moderate the building climate by considerably reducing the building’s energy use. It makes night venting simpler, and can filter insects while maintaining security. The cavity also makes it easy to access the cladding for cleaning and maintenance.
Subject guides similar to this are available from Barbour Index as part of its Construction Expert and Specification Expert services. For further information, contact Barbour Index on 01344-899280 or visit www.barbour-index.co.uk