黑洞社区

01 / Introduction

For most people, the appearance of a building is the one aspect of design that really counts. Appearance is, however, more than skin deep and the facade has a critical role in determining internal comfort conditions and the performance of the building. Facade design is highly specialist, and although specialist consultants work on many large projects, the input of contractors into realising the client and design team鈥檚 aesthetic and performance expectations can be very high indeed.

The UK market for building facades is very diverse. On the one hand, London鈥檚 global status as a finance centre means that there is a concentration of schemes featuring very high quality, purpose-designed curtain walling. Diverse requirements of clients and planners outside of the City office market, combined with the ingenuity of UK designers, has encouraged the development of a wide range of glazed and solid wall solutions, many of which look bespoke but are actually based on off-the-shelf products.

This article reviews the work of specialist contractors who provide contractor-designed walling solutions ranging from totally glazed curtain walling, through precast concrete panels to the blend of solid wall construction and glazing known as the 鈥渢otal envelope solution鈥�. All of these technologies are non-loadbearing, so their design involves interface with structural engineers as well as building services engineers.

In summary, building envelope contractors design and deliver integrated solutions and manage a complex supply chain, while collaborating closely with other designers and contractors to deliver the expected performance in a cost-effective way.

Appearance and performance are the main success criteria for a walling system, but there are other issues to consider as well. Capital cost is a critical consideration as external walls and windows can account for 15-25% of the overall cost of the building. As a result, there is little margin to permit solutions that are not cost effective.

The buildability of the facade is also critical, as this will be a major factor in determining the overall procurement and construction programme. Finally procurement is also a key consideration, relating to contractor design input, lead-in times, competitive pricing and the risk management of single points of failure in the supply chain.

Effective engagement with building envelope specialists relies upon understanding how they operate and where the opportunities to add value can be found. In many cases, an approach that can secure early collaborative input from a specialist can be shown to add further value to a project.

02 / The big players

In the UK, there are three broad categories of specialist contractor-designed building envelope solutions. Beyond these groups there are also specialist contractors who install rain screen cladding and windows onto conventional in-situ substrates such as blockwork or drywall.

Curtain wall describes glazed and solid walling solutions which are hung off a building鈥檚 frame using a structure of mullions and transoms. The main distinctions in curtain walling relate to whether a solution is purpose-designed or based on a standard system, and whether a system is fully prefabricated off-site (unitised) or partially site-assembled (stick).

Purpose-designed, unitised curtain wall is the solution that has been adopted on many high-quality UK buildings. These walls are based on established technologies, but the framing selections used will be designed to meet the specific needs of the project. Most specialist contractors active in the UK are European-based, albeit with a well-established UK presence. The service offered by these contractors is the design and delivery of cost-effective solutions which meet the aesthetic, performance and buildability requirements of a specific project. This may involve the design and fabrication of one-off aluminium sections, sourcing of specialist glass units and so on.

The advantage of a purpose-designed solution is that the frame sections can be optimised for loads and so on 鈥� only using heavy sections when necessitated by high wind loads, for example. The consequence of extensive design, prototyping and testing associated with purpose-designed curtain wall in the UK is extended lead-in times and increased on-costs. As a result, these systems are only economic on large and high value projects. There are a large number of established players in the UK including the Permasteelisa Group, Seele and Lindner.

Curtain wall solutions based on the products of system providers such as Sch眉co, Kawneer or Reynaers are available from a number of UK-based curtain wall contractors such as Parry Bowen, Red Architectural or McMullen.

The service offered by these contractors is the design and installation of curtain wall that meets aesthetic and performance requirements subject to the characteristics of standard products. These contractors typically work through main contractors and tend to have less pre-contract involvement than providers of purpose-designed solutions. During the design stages, project teams typically work with system suppliers to develop the design. There is some limited scope for the modification of the appearance of the walling by use of project-specific cover profiles, although this has an effect on lead-in times and programme. Overall, lead-in times are much shorter as standard systems do not require the same amount of design, prototyping and testing as purpose-designed.

In the current cost-driven market, most systems are site installed. This has implications for the assurance of build-quality and performance.

UK-based specialists have also developed a capacity for unitised solutions during the recent commercial cycle, providing benefits of consistent quality, accelerated site installation and reduced dependence on site labour.

Precast concrete panel specialists design and manufacture concrete cladding panels which can either be pre-finished or can act as a substrate on which an outer rain screen such as hand-set stone can be applied. Precast panels may form part of a wider precast structural solution, where the frame is an integral part of the visual intent.

Monolithically applied finishes include reconstituted stone, brick, terracotta or natural stone. Panels can be pre-insulated, or the insulation can be installed as a separate, site-fixed lining. As the next iteration of Part L increases the importance of solid, highly insulated areas into a facade, off-site fabricated concrete panels may be more widely adopted, especially as investment in pre-cast plant is making it more cost effective to cast more complex units. UK and European-based specialists include Techrete, Trent, Decomo or Loveld.

Total envelope solution providers offer a range of system-based solutions including curtain wall, solid rain-screen/substrate and windows/window-wall 鈥� and may also provide the roofing as part of a wider package. Total envelope businesses are typically system integrators and do not manufacture or install systems themselves. Their role is to manage a supply chain to deliver a fully designed and managed solution to meet the client鈥檚 aesthetic and performance requirements. This will include the design of interfaces between different systems, site quality control and so on. Total envelope solution providers tend to work as specialist subcontractors and include UK players such as Lakesmere and Astec Projects, as well as some of the UK curtain-wall contractors.

03 / Market conditions

Workload for players in the building envelope sector has been disproportionately affected by the downturn. Commercial and residential work 鈥� both major sources of workload 鈥� have fallen in volume terms by by over 25% since peaking in 2008. Higher education has been a good source of workload for UK-based contractors, but is also coming under pressure. Currently active sectors such as schools have provided more opportunities for window manufacturers than curtain wall specialists. Overall, workload has fallen by some 40% in the UK. Trading conditions in Europe and the Middle East are also poor, so there has been little opportunity for specialists to diversify into other markets. However, with forecasters such as the Construction Products Association anticipating growth in the commercial and residential sectors, curtain wall specialists in particular may see their prospects improving.

Looking forward to recovery, specialists will be challenged to provide cost-effective solutions which meet significantly enhanced performance requirements related not only to Part L, but also to the 2008 update of a future revisions of BREEAM and future revisions.

There has been very little consolidation in the building envelope supply chain so far in the downturn. Companies have tended to downsize rather than chase turnover at unsustainable price levels. However, prices have fallen substantially through a combination of value engineering, reduced margins and the passing of cost reductions through the supply chain. Current prices are unlikely to be sustained in the longer term.

There is a very high proportion of value-added manufacture in curtain wall which limits the impact of raw material price increases on final costs. Aluminium billet spot prices have increased by 50% since last summer but account for no more than 5% of the total cost of a purpose-designed, unitised system. However, cost increases will affect lower cost, system-based products more than high-end curtain wall, as raw materials represent a higher proportion of the final cost. Other areas of potential cost increase include highly consolidated supply chains such as glass manufacture.

Foreign exchange fluctuation is also a concern for clients, typically as the pound to euro exchange rate does not get fixed until a contract is signed. The recent trends of the weakening of the euro against the dollar, and the pound against the euro will not help UK projects, albeit that competition has forced contractors and suppliers to absorb these increases.

One advantage of system-based curtain wall is that all transactions are in pounds, eliminating this aspect of cost uncertainty.

04 / Value drivers for curtain wall and building envelope

The key value drivers for building envelope solutions relate to appearance, performance and buildability. However, budget, which determines the solutions available to the designer, and programme/procurement, which dictates the degree of specialist input into the design, are also critically important. In broad terms, the available budget will determine the type of contractor who is likely to be involved and whether a single solution can be adopted. Tighter budgets may dictate that money is focused on a small proportion of feature facade 鈥� an approach that suits the skills of a total envelope contractor.

The building type will inevitably have an impact on the solution. Large towers, for example, will dictate a unitised solution and may also introduce buildability constraints related to unit size and weight, dictated by the main contractor鈥檚 working method. By contrast, smaller, low-rise buildings are likely to favour a site-installed option. From a performance perspective, control of heat transmittance is much more important in heated buildings such as residential schemes than office buildings, where control of cooling loads at the perimeter is more of a challenge.

Economy of scale is also an important consideration. Highly bespoke curtain wall can be used economically on large buildings such as those at Canary Wharf because the costs of customisation can be amortised over a large contract. However, if different wall types are required, or if variations such as solar shading are introduced, then some of this economy of scale will be lost.

The choice between unitised and stick system solutions is similarly influenced by scale 鈥� as projects need to be large enough for the full programme benefit of a unitised system to be secured.

Visual intent dictates much of the complexity of a walling solution. Variables include panel sizes, proportion of glazing, cladding material, framing section sizes, glass type and quality, glass colour, solar shading devices and other features.

In addition to determining selection of materials and so on, visual intent issues may have an impact on buildability, such as the erection sequence. Visual intent is closely aligned with planning and in many locations a good quality facade is a material planning consideration. Examples of how visual intent drives cost and value include:

• System design costs
• Panel sizes
• Detailing and interfaces 鈥� for example, parapets, corner details, frame cladding and system interfaces
• Variations in glass coatings and frit patterns used to deliver visual intent as well as envelope performance.

Envelope performance is increasingly critical, and should be considered holistically alongside the structure and the building services. Where buildings are procured on a single-stage tender or design-and-build route, it may be harder for the client鈥檚 team to obtain this joined-up input from the supply chain, particularly as it is partially dependent on the winning specialist contractors鈥� preferred solution. Key areas of performance that need to be considered include:

• Environmental performance including air infiltration, U-value, light transmittance, glare control and perimeter heat gain
• Physical performance including structural deflection, acoustic control, fire separation
• Performance in use, including anticipated durability, ease of cleaning and replacement of components.

Environmental performance criteria are inevitably dictated by elements of the visual intent. A fully glazed building represents more of a challenge with respect to Part L, for example, but more detailed aspects 鈥� narrow mullion and transom sections, for example, may make it harder to meet overall targets. Mixed envelope solutions which enable areas of high performance facade to compensate for poorer performing 鈥渇eature鈥� walling provide greater flexibility. Similarly, high-tech double wall facades with integral blinds and active control of ventilation within the cavity enable light transmittance and G-values to be optimised in use in response to prevailing conditions. Increasingly designers are introducing larger areas of solid panel into curtain wall to secure the best combination of appearance, performance and buildability.

Optimising performance does, however, require a holistic approach. For example, stick system curtain walling can accommodate much less structural deflection than allowed in a typical engineer鈥檚 specification, so this requirement needs to be taken into account in the initial structural design. Similarly, interfaces between the walling system and the BMS may be necessary to enable mixed-mode ventilation strategies such as night-time purging. Even the introduction of opening windows needs to be considered carefully, as the extra-framing will increase a panel鈥檚 U-value.

Buildability is concerned mostly with the manufacturing and installation process, which in turn is strongly influenced by visual design issues, economy of scale and programme. Most building envelope specialists will advise that off-site fabrication provides the best means of guaranteeing high quality assembling while speeding up the on-site installation sequence 鈥� simplifying the build process, shortening the critical path and reducing dependence on site labour.

Almost all purpose-designed curtain wall is unitised 鈥� installed as pre-finished composite units. During the boom, UK system specialists also developed unitised expertise. However, despite industry-wide support for off-site manufacture, UK domestic specialists must also focus on stick solutions, which involve more extensive on-site installation works. Key aspects of buildability include unit size, requirements for

on-site, pre-assembly work such as the fixing of fins or solar shading, the need for craneage, or the impact of the main contractor鈥檚 exclusion zones on the erection process. Looking forward to 鈥渋n-use鈥� issues, ease of cleaning, ability to replace glazing and other life expectancy issues should be considered as part of the design process.

05 / Procurement

黑洞社区 envelope specialists can add considerable value to the construction process 鈥� supporting the development of holistic design solutions, selecting and integrating appropriate walling systems with robust interfaces and managing a complex and very diverse supply chain.

In the current market, clients face challenges related to the cost effective achievement of new building performance standards, fast delivery of buildings to obtain 鈥渇irst mover鈥� advantage as well as mitigation of risks related to input price inflation and supply chain consolidation.

Accordingly, selection of the specialist building envelope contractor, even as part of a single-stage lump-sum tender is a key decision that should be based on track record, quality of supply chain and ability to add value as well as least cost. Unfortunately, in the current market, and with development values under considerable pressure, on many projects, initial cost will continue to be the prime determinant of contractor selection.

One particular aspect of procurement of purpose-made curtain wall from European contractors is the management of currency risks, where rates are not typically fixed until a contract is signed. In rising markets, commodity price risks may also not be bought out until the contract is agreed. Other commercial matters that a client or contractor might need to consider include advance payment for design costs or for materials ahead of the completion of any work on site.

Procurement strategy can also make a major contribution to risk and value management, and many aspects of procurement will affect building envelope value drivers. In an ideal situation, most facade specialists prefer a two-stage procurement arrangement which allows their upfront involvement in the early design of the facade solution.

For bespoke unitised curtain wall, lead-in times are such that curtain wall contractors have often been appointed on the basis of a Pre-Contract Services Agreement before the main contractor. With commercial projects procured on the basis of design and build at an early stage, these specialists will still be able to contribute to value engineering and buildability, but less so to the refinement of the designers鈥� original concept.

However, for proprietary systems, programme issues are less of a problem, and for both design and build and fully designed schemes, the envelope can be procured competitively on the basis of a generic design solution, developed with the technical support of a system company.

Unfortunately, this approach to procurement, where structures, services and building envelope packages are procured in 鈥渟ilos鈥�, gives less opportunity for a specialist contractor to contribute to the development of best-value, holistic solutions, and certainly much less opportunity for accurate market testing of design solutions.

Opportunities to add value through the optimisation of the full design, challenging the specification and consideration of alternative proposals can unfortunately be lost when single-stage procurement is adopted.

The consideration of off-site fabrication is a case in point, where the benefits of a unitised system may not be properly articulated through the supply chain, and where cost consultants or main contractor may not fully reflect all cost, programme and contractor on-costs in the cost plan or bid issued to the client.

In the current cost-led market, clients and consultants have a critical role in ensuring that the specification will delivery the expected performance and that specialist contractors meet the highly technical aspects of the envelope spec.

Specialist facade consultants can make an invaluable contribution when acting as an intermediary between the rest of the design team and the complex building envelope supply chain. Furthermore, cost consultants and project managers need to understand how to engage effectively with the building envelope supply chain, understanding its structure and cost drivers, how the industry is responding to the revised Part L and what steps can be taken to mitigate current market risks.

06 / Effect of part l 2010 on building envelopes

The introduction of the revised Part L is likely to have a substantial effect on building envelope specification and the design process itself. As standards for carbon emissions reduction become more demanding and target U-values are reduced, the performance of the building envelope and the building services installation will have to be considered together far more than currently is the case. For example, adoption of natural ventilation through opening windows to reduce building services-related carbon emissions will have a significant knock-on effect on the average U-value of elevations, which in turn will require further improvements in component performance of the walling.

For total building envelope based solutions, Part L is likely to result in the use of a greater proportion of well-insulated solid construction. Build quality 鈥� ensuring that low air-infiltration rates are maintained 鈥� will place a premium on quality of training and site management of the installation.

One challenge in curtain wall design related to the new Part L concerns improvements in the thermal performance of the framing 鈥� the weak spot of all glazed systems. Some innovation is available to help improve U-values in glazed systems, including improved thermal breaks. Timber/aluminium composite units may be adopted more widely, albeit that these systems have performance issues related to gasket technologies and durability when compared to aluminium systems.

However, for office buildings, where the main energy uses are lighting and cooling, improving U-values does not provide the complete answer. Attention also has to be focused on managing heat gain and glare in the perimeter zone, while maximising day lighting throughout the year. Improved G-values needed to control solar gain in the summer can be achieved through use of solid panels, high performance glass and/or solar shading, but these will reduce day lighting at other times of the year 鈥� necessitating the greater use of artificial lighting. Double wall might be the solution, but has significant initial costs and maintenance implications. Further innovation will no doubt follow as clients and designers challenge the industry to deliver cost-effective, low-carbon solutions.