Curtain Walling and Energy Performance

Transcription

1 Curtain Walling and Energy Performance 1 Bligh Street, Sydney, Australia Dexus Fund Management Ltd DOW CORNING The thermal performance of the building envelope is characterised by the thermal transmission coefficient, the U-value (W/ m2k). The U-value is a measure of the heat loss through 1m2 of the building envelope for each degree Celsius (or Kelvin) temperature difference. The characterisation is valid for standardised environmental conditions, while actual environmental conditions may be markedly different in operation. When assessing the energy performance of a building, it is the overall (or aggregate) performance of the building envelope that determines the overall heat loss. The heat loss through a curtain wall occurs through the constituent parts, including glazing (centre pane), insulated parts (centre panel), and framing. The framing will always to some extent constitute a thermal bridge, when compared with the insulated parts and insulated glazing. The resulting heat loss and consequently the U-value depends on the detailing and the overall performance should always be assessed including the effect of the frames. Typically the overall U-value is determined by an area-weighting approach, whereby the U-values of the different parts of the facade are taken into account according to their share of the overall transmission area. As the thermal performance is enhanced for the vision area (the insulated glazing) and the insulated parts (such as the spandrel in a curtain wall), the performance of the frame becomes relatively more essential for the resulting overall performance. Significant improvements to the insulated areas will not pay off unless the frame is also improved, for instance through the introduction of high efficiency thermal breaks. intelligent glass solutions 49

2 Performance by design The path is made by walking [African Proverb] Mikkel Kragh PhD MSc (Eng) CEng MCIBSE FSFE Global Façade and Architectural Design Leader, Dow Corning, High Performance Building Solutions why so much bad design is commissioned: because it doesn t make any difference when it is totalled up in a column. Project managers get fired because buildings are late or go over budget, but rarely because a building isn t very good. [Sam Jacob, FAT, column in Dezeen Magazine] From a technological standpoint, the development of high performance solutions will empower design teams to deliver successful projects. Educated clients will know how to manage the process or employ advisors to assist them. The assessment will go beyond first costs and explore the value of the property as well as its marketability. Whereas commercial developers have created highly sophisticated business modelling methods and instruments, aspects concerning the architectural quality of buildings and their integration into their surroundings are usually more soft areas where decisions are typically made based on judgement alone. High performance buildings of architectural quality are likely to retain their value because of their perceived worth to the owner, the occupier, and society at large. This longevity may be hard to put a price on when the property is first developed, but attention to detail in the delivery and the utilisation of high performance solutions all add value in the pursuit of good design and high value architecture. subtle differences in facade detailing will not be looked at but the overall performance requirements will typically be fixed (including facade thermal transmittance, U-value). For design teams to deliver high performance buildings they need to be able to assess the options throughout the design stages, understanding the links between facade detailing and overall building performance. The thermal performance of curtain walling is an area where there is room for interpretation and there is a real risk that a compliance check will not capture the overall performance. The Performance Gap In stark contrast to the compliance checks is the performance assessment based on project-specific assumptions and detailed in put on the intended use of the building. The level of detail will depend on the nature of the project and how ambitious the targets are in terms of building energy performance and occupant comfort. Where clients are aiming high the brief may well include a target environmental rating. While the energy consumption of buildings is targeted through the building code design performance requirements, actual in-service performance is coming under scrutiny. The performance gap refers Gema, St. Petersburg, Russia; to this difference, which can be ascribed to numerous factors such as occupant behaviour, climatic conditions, and differences in detailing between the design and the built product. Green is the colour - High Performance as Good Business Environmental rating schemes are increasingly adopted by clients and design teams, in an effort to drive the delivery of projects and make responsible choices, but also to be in a position to objectively label the quality of the project. In a competitive market place, this kind of differentiation is gradually becoming a significant commercial element where prospective buyers or tenants may look for a certain rating as an integral part of their requirements. There are commercial developers who use environmental ratings prominently when take real estate to the market. In this sense, high performance can be seen to add to the commercial value of assets in terms of premiums, but also as a way of enhancing the marketability of a given project. At the other end of the spectrum energy labelling schemes are likely to influence the way property is negotiated. It is quite likely that prospective tenants will be expecting a certain performance standard The Value of Good Design The building envelope is such a fascinating construction element because of the way it needs to embody architectural qualities and performance. We all relate to buildings in no small part because of the architectural expression of the facade. When confronted with certain types building designs most people will react emotionally in some way Even people with no direct connection to the architectural field. A successful building project is a project that will meet or exceed the Client s expectations, be delivered on time and on budget. But how do you then define successful design? Paraphrasing a quote, there are only two types of design: good and bad. The beauty is in the eye of the beholder and in this context it would serve no purpose to try and define good design at least not the aesthetic aspects. It is, however, relevant to consider the value of good design and how the right combination of architectural expression and building performance can combine to enhance the value of a given project. Part of the reason it is such a tremendous challenge to deliver high performance buildings and quality architecture can no doubt be found in the way construction projects are conceived and procured. (...) we should be wary of focusing our argument on the bottom line. Architecture and design are fundamentally useless activities when viewed through the lens of a project manager s spreadsheet. That s Code Compliance versus Performance Whole building energy performance assessment is becoming the norm and design teams are increasingly sharing information early on to demonstrate compliance with the codes. Progress is being made in terms of design tools and availability of information, which facilitates fairly complex modelling as an integral part of the design process. It is possible for designers to explore options and communicate the potential benefits to the clients Potentially driving towards high performance building solutions. In this context, however, it is important to distinguish between on the one hand assessments carried out as part of code compliance checks and, on the other, project-related performance assessments. Code compliance checks are intrinsically based on a series of standard assumptions which do not necessarily relate to the actual or intended use of the building. The reason for this is simple: it would not be feasible to base compliance on specifics which may not apply throughout the service life of a building. The checks are based on assumed operation similar to what is done when the fuel consumption is declared for motor cars. Since the codes cover the entire industry, the description and the modelling of the building is fairly crude for the purpose of a mandatory compliance check. The energy performance requirement stipulated by code is a benchmark which cannot be expected to closely match the actual performance of the building. In this context 52 intelligent glass solutions

3 spandrel panels coupled with a variety of design options, with particular regard to surface and material combinations to meet the needs of architects and building owners. Modules are preassembled in a controlled environment which provides for a high degree of quality control and eliminates potential issues of workmanship on site. They can be manufactured in a wide variety of architectural finishes such as screen and digital prints and combined with the full range of material inlays available from Okalux, such as glass fibre, wire mesh and wood, in line with contemporary architectural requirements. Additionally, Dow Corning has developed a process which will allow the easy configuration of modules to achieve specific combinations thermal performance and design requirements for any given project. These modules are constructed through the integration of a fumed silica vacuum insulation panel (VIP) within the sealed cavity of a standard spandrel insulating glass unit to create a compact and robust component. Fumed silica vacuum insulation technology is the most effective VIP core material as it has extremely low thermal conductivity due to a combined reduction of all three modes of energy transfer; conduction, radiation and convection. This allows much higher insulation values to be achieved and walls to become thinner. Vacuum insulation panels are made to fit architectural details. Panels are assembled using a gas-tight foil membrane wall, which not only provides physical protection, but is used to prevent air from getting into the core from which it has already been evacuated. By removing air from the foam core material, vacuum insulation panels are able to achieve high thermal performance at a fraction of the thickness of cut-to-fit insulation materials. In the centre-of-panel, the thermal conductivity is around 0.005W/mK. However, for thermal calculations, a design value of 0.007WmK is used in order to take continued on page 56 >>>>>> Public Policy Center, Annenberg, USA; Seewürfel, Zürich, Switzerland; and they will be referring to the operational ratings of buildings as part of their strategy. High performance buildings will be seen as more attractive while comparatively lower performance is likely to attract a penalty. All other things equal, these aspects will drive towards more energy efficient buildings even in situations where the operational cost is perhaps not the highest priority on the agenda it is simply good business. Custom Design and High Performance Architectural Insulation Modules allow architects to create a thin building envelope, which is often critical in building renovation where space limitations and technical constraints may prevail. Depending on project-specific conditions, the thin building envelope may offer gains in usable (i.e. lettable) floor space for a given footprint, thus adding value to real estate. Furthermore, the continued use of glass throughout the building facade facilitates infinite choice for the attainment of building performance and allows the architect a significant degree of design versatility, particularly for the non-vision elements which are gaining prominence as an alternative aesthetic for energy efficient buildings. Seewürfel, Zürich, Switzerland; Gema, St. Petersburg, Russia; For the European market, Dow Corning has entered into a strategic partnership with architectural glass specialists Okalux for the manufacture and assembly of Architectural Insulation Modules. Okalux bring their proven capability in terms of design and fabrication of the architectural facade elements which successfully complements Dow Corning s state-of-the-art, slender, vacuum insulation technology. This co-operation delivers high quality insulating glass and facade products which combine innovation and customisation capabilities for facade intelligent glass solutions 53

4 continued from page 54 >>>>>> thermal edge losses and aging into consideration. This is in line with accelerated aging according to DIN If the panel is punctured, the thermal conductivity increases to 0.02W/mK according to DIBt certification, which is still lower significantly lower than the 0.04W/mK of mineral wool as an example of conventional thermal insulation materials. The slim line construction of the fumed silica vacuum insulation panels overcomes the age old problem of thick insulation which has historically been required to attain the required U-value, at approximately equivalent to 0.2W/m2K for roofs and walls. To assist in the facade design process, Dow Corning is capable of offering thermal modelling which takes into account the entire facade, whereby the losses through transoms and mullions are considered to define the average U-value of the facade. Dow Corning has developed guidance including wind load calculations to determine the required glass specifications during concept and schematic design stages, based upon country and project specific requirements that ensure full stability and durability. The vacuum insulation panel itself is DIBt compliant and has a compressive strength at 160kPa at 10% compression when measured to DIN EN 826. Architectural Insulation Modules have been extensively tested via accelerated aging that correlates to normal life in unison with strong quality control processes in place at manufacturing. Typically performance is declared in terms of design values based on a standard 30 service life, while the modules are generally expected to perform beyond the service life of a standard curtain wall. Suitable for installation in stick or unitised curtain wall systems, Dow Corning has already worked with several customers and projects to demonstrate the potential of this technology to achieve both extremely low U-values and design freedom. The continuing increase in the use of glass in world-wide construction demonstrates an inherent desire to open up the commercial and residential buildings in which we spend so much time. The uninterrupted transmission of light through the extensive use of glass enhances the warmth and depth of the construction s internal features whilst reacting sympathetically with the environment in which it is situated. Through enhanced functionality of insulated areas offsets the performance of the vision area, geometrically increased transmission areas, and linear thermal losses. High performance Architectural Insulation Modules allow for an increase in the overall percentage of vision area whilst facilitating additional exciting design possibilities. Able to integrate seamlessly and conveniently within standard curtain wall systems and able to cater for bespoke design, concepts such as Architectural Insulation Modules will play a major role in the future, allowing less restricted use of vision glass and the possibility for buildings to become ever more distinctive and comfortable whilst meeting the requirements for energy saving and sustainability. Dow Corning will be participating at the CTBUH Conference in London & GPD Conference in Finland. We warmly welcome you onto our booth at one if not both of these events. 56 intelligent glass solutions

5