The science and art of curtain walling is a highly specialised area. A whole-life cost and performance assessment provides a rational framework within which to assess the many component options.
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&#; Stick systems are site assembled and carry a higher risk of workmanship non&#;conformities.
&#; Unitised systems are factory manufactured units comprising framework, glazing and infill panels. This reduces the time on site and can cut workmanship defects.
Framing materials include: aluminium, low carbon steel, stainless steel, plastics or timber as well as composites.
Aluminium extrusions are the most common option, usually based on alloy to BS EN 755&#;1. Aluminium would have a service life of more than 60years. The type of finish dictates life to first maintenance or replacement:
&#; Anodising to BS , minimum 25 microns thick, 30 years or more.
&#; Polyester powder coating to BS , minimum 40 microns thick, 15&#;25 years
&#; Polyvinylidene&#;fluoride (PVDF or PVF2) minimum 25 microns thick, 20 years or more.
Fixings should be stainless steel or other non&#;corrosive material, fitted to avoid bimetallic corrosion.
Glass has an expected service life well in excess of 60 years. Insulated glass has shorter expectancies: edge seal failures and infiltration of water vapour into the space between the panes are the main causes of failure. Insulating glass units to BE EN should be specified to give an expected service life of 25 years. Safety glass should comply with BS .
The configuration of the glazing system influences the thermal characteristics of the façade which is calculated to ISO &#;1. The less heat is lost through the glazing, the lower the U-value. Measures to lower U-values include:
&#; Metal frames with thermal breaks to reduce linear thermal transmittance.
&#; Spacing frames further apart reduces U&#;values by increasing the insulating cavity. But frame cross&#;sections, glazing thickness and hardware strength may have to be increased at higher capital cost.
Glazing U&#;values are calculated to EN 673, values as lower than 1.0W/m2 k are achievable. Factors which affect glazing U-values include double or triple glazing, low-emissivity coatings, or uncoated glass.
Choices for the gas to fill the gap between panes include air, argon or sulfur hexafluoride and krypton.
Gaskets and sealants are often the shortest-life components. Non&#;cellular EPDM gaskets to BS have an expected service life 15 &#;30 years or more.
Sealants should comply with ISO , with evidence from manufacturers for durability. Neutral cure type silicone sealant has an expected service life of 5-20 years.
A whole building approach, started at outline design process, is most likely to provide an energy-efficient solution. Issues to take into account include building orientation, form, aspect, lighting, internal layout and materials used internally.
A suite of European standards offers assurance of curtain wall performance in different locations:
&#; Watertightness: BS EN defines five classes based on variable air pressures and a water spray rate of 2 l/min/m2 .
&#; Air permeability: BS EN tests confirm air permeability for overall area is no more than 1.5 m3/m2 hour for five air pressure classes from 150 &#; 600 Pa or more.
&#; Resistance to wind load: BS EN confirms performance of deflection and fixings.
&#; Site test of watertightness: BS EN may be used to check for water leakage for installed curtain walling by water spray with or without applied air pressure.
Glazed curtain walling g systems Capital cost
&#; A discount rate of 3.5% is used to calculate net present values.
&#; Curtain walling framing based on nominal 100mm x 50mm sections at horizontal centres, 3m storey height, double-glazing plain, toughened 6-16-6mm, unless otherwise stated. Window and doors included.
&#; Costs include cleaning, maintenance, recoating replacements and an allowance for repairs. Cost and work frequencies are generic, based on a simple m2 of glazed cladding. The model assumes the framing cladding is not replaced during the 60 years.
&#; Energy costs excluded.
&#; Major refurbishment period is 25 &#; 35 years and includes replacement of insulating glass units, gaskets and capping to frames as necessary.
First published in Building
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Photo: Ed Wonsek; courtesy of The Architectural Team
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The Raffles Back Bay Hotel and Residences in Boston use a unitized curtain-wall system with an estimated lifespan of 60 to 70 years.
Designing a building with sustainability, resilience, and longevity in mind calls for a recognition of complexity and interdependence. Each component of a building contributes to its embodied and operational carbon footprint, its occupants&#; experience, its architectural expression, and its economic performance. The building envelope is a particularly powerful determinant of these outcomes, since it comprises a large volume of materials, endures climatic and atmospheric stressors, and mediates between exterior and interior environments, transmitting or consuming widely varying amounts of energy in the process. The contemporary curtain wall, a product of over a century of technical evolution, can be one of a building&#;s vulnerable points, showing its age faster than the rest of the structure does. The converse of that observation is that improving a curtain wall&#;s quality and longevity is an opportunity to realize powerful gains in the whole building&#;s performance.
The Design Challenge sponsored by Metals in Construction magazine and the Ornamental Metal Institute of New York, eliciting proposals to design the curtain wall system of a new building at least 50 stories tall for a site on Broadway in midtown Manhattan, posits at least a 75-year anticipated service life for the proposed systems. This represents a substantial extension of the longevity commonly observed and expected in contemporary practice, say several experts in sustainable envelopes.
Mic Patterson, ambassador of innovation and collaboration at the Facade Tectonics Institute (FTI) and a member of the Design Challenge jury, cites a remark by Anthony Wood, executive director of the Council for Tall Buildings and Urban Habitat, at an FTI conference. &#;He said, &#;How long should a building last? It should last until we&#;re done with it,&#; which I think is the right answer.... They should be modifiable, adaptable, and repairable as need be until we are completely done with them.&#; There is no one-size-fits-all criterion for a facade system&#;s durability, Patterson says. &#;It needs to be adaptable enough to accommodate changes in use and all of the forces of obsolescence.&#;
Patterson commonly encounters facade contractors&#; expectations of 20 to 35 years for the life of a curtain-wall system, with 50 years as the customary upper limit, and he finds these figures unnecessarily low. &#;That ignores the synchronicity that needs to exist between the aspirations for the building itself and the facade system,&#; he continues. &#;If you&#;ve got a building that is designed to last 100 years and a facade system that&#;s still designed to last 75 years, you end up needing a new facade system before the building expires. And if you put a new one on there, that&#;s good for another 75 years, then you lose 50 years of facade-system service life. And so there&#;s all kinds of wasted durability going on in buildings and facade systems just because we don&#;t pay attention to that.&#; There is no reason, he believes, that certain buildings reflecting the most advanced realistic design and construction practices&#;coordinating components&#; durability rather than leaving it to chance&#;cannot last a century, perhaps even 1,000 years.
In the U.S. curtain-wall industry, Patterson reports, it is common to market systems with a 35-year expectancy as &#;zero-maintenance systems to the building owners, which is what they want to hear. Basically, what we&#;re saying is, &#;This thing is good for 35 years, and then it&#;s done,&#; because there&#;s no way to maintain it or retrofit it.&#; With few options for replacing or upgrading a facade system, &#;the only viable economic strategy in too many cases is to just rip the entire thing off and put a new one up&#;&#;the antithesis of sustainable practices, particularly when designs unwittingly create obstacles to the disassemblability, reuse, and recycling of materials.
The concept of zero maintenance, though attractive from a short-term perspective, appears roughly as realistic as a perpetual-motion machine. Patterson and other commentators contend that more farsighted approaches are within reach, however, for professionals who take a long-range view of the material cycles involved in design and product choices.
Image courtesy of Priedemann Facade Experts
Figure 1. Diagram of major components of the R-IOT system: infill (1), spandrel (2), and terminal (3) elements.
The winning entry in this year&#;s Design Challenge is a facade system rather than a building design. The R-IOT project, revealed after jury deliberations to be the work of the Berlin-based firm Priedemann Facade Experts, combines a dismountable unitized facade system (Fig. 1) with a physical/digital interface that monitors the performance status of components via integrated sensors and a digital representation of the system. (The abbreviation, Priedemann representatives say, combines the Internet Of Things with an intentionally ambiguous initial that could designate Revolutionary, Renovation, Refurbishment, Reduce, Reuse, Recycle, and others.)
During the jury&#;s assessment of competition entries, juror Vishwadeep Deo of Thornton Tomasetti hailed this proposal&#;s innovations: &#;keeping digital twins and machine learning and AI (artificial intelligence) to run early detection and preventative pattern recognition.&#; The jury unanimously found that R-IOT, although it did not offer a site-specific design for the Broadway address described in the competition&#;s design brief, more than made up for that aspect by proposing a modular concept that can improve the longevity and performance of any curtain-wall system.
Image courtesy of Priedemann Facade Experts
Figure 2. Monitoring system for R-IOT on building and facade levels.
Described in the competition entry as &#;Revolutionary Cyber-Physical Maintenance and Renovation Strategies to Extend the Lifespan of Facade Constructions,&#; the R-IOT system renders energy-intensive facade refurbishments unnecessary by taking what it calls a &#;precognitive&#; approach to continuous monitoring and maintenance (Fig. 2). By continually providing data on three main interdependent parameters&#;building energy efficiency, occupant comfort, and facade component conditions&#;the system identifies degradations in performance proactively rather than reactively. When expected and measured performance diverge and components approach a predefined threshold value for failure, the system gives stakeholders a warning that enables timely and appropriate interventions in the form of component removal and replacement or maintenance (Figs. 3, 4).
Image courtesy of Priedemann Facade Experts
Figure 3. Monitoring system for R-IOT on building and facade levels.
Image courtesy of Priedemann Facade Experts
Figure 4. Repair, reuse, recycling, remanufacture, and repurpose of R-IOT components after problem detection and interventions.
The infill, spandrel, and terminal elements of the R-IOT facade are all designed for deconstruction, with standardized components in replaceable cassettes accessible from the interior. Transparent, opaque, or partially opaque infill elements can include features such as openable windows, shading systems, and other mechanical elements; sealants, gaskets, and membranes between facade elements are accessible when the infill element is removed. Opaque spandrel elements, the interface between the facade system and the building structure, accommodate a range of cladding elements, potentially including photovoltaics or green wall systems; supporting terminal elements allow staged removal of the system and access to hidden components such as removable polyamide thermal breaks. Chemical bonding is minimized, used only where unavoidable, as in IGUs or laminated glass. A mobile glass-handling machine enables on-site maintenance or retrofitting operations, reducing downtime and carbon emissions from transportation and heavy machinery use.
The system can also accommodate new design elements and material technologies as they appear, allowing aesthetic upgrades in the form of exchanged cladding or insulating components. R-IOT&#;s designers are &#;pointing to the fact that there is a relationship between these things, and that the service life of an assembly is determined by its weakest link,&#; commented juror Mic Patterson of FTI. &#;Their approach is that it all needs to be replaceable.... With this kind of strategy, the service life is an irrelevant term, because what you have is a perpetual service life: as long as you can maintain the thing, you can go 1,000 years, and you may not have an original part in the assembly, but it&#;s seen continuous service.&#;
Jurors acknowledge that while the winning entry satisfies the requirements of the brief, a fully realized version of R-IOT will need to address practical questions such as specification of resilient materials and the mechanisms of sensor function. &#;Looking at U values and the glass performance,&#; observes juror Vivian Fu, an associate principal at Heintges in San Francisco, &#;the deterioration of the facade, a lot of times, is about air leaks and water leaks. Where&#;s the detection of that?&#; Stanford Chan of Socotec questions whether the system should incorporate exterior access for maintenance, considering the disruption to occupants that interior access may cause. Use of standardized components, Patterson also notes, can imply potential creative constriction: &#;The problem with modularity is, how do you give the architects freedom of expression with a modular concept?&#; That the R-IOT proposal stimulates such discussion, however, commented Jack Robbins, partner and director of urban design at FXCollaborative, is &#;a good sign that we&#;re getting into little details: how does this actually work?&#;
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