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Flammability and Multi-objective Performance of Building Façades: Towards Optimum Design

  • Bonner, Matthew (Department of Mechanical Engineering, Imperial College London) ;
  • Rein, Guillermo (Department of Mechanical Engineering, Imperial College London)
  • Published : 2018.12.01

Abstract

The façade is an important, complex, and costly part of a building, performing multiple objectives of value to the occupants, like protecting from wind, rain, sunlight, heat, cold, and sound. But the frequency of façade fires in large buildings is alarming, and has multiplied by seven times worldwide over the last three decades, to a current rate of 4.8 fires per year. High-performing polymer based materials allow for a significant improvement across several objectives of a facade (e.g., thermal insulation, weight, and construction time) thereby increasing the quality of a building. However, all polymers are flammable to some degree. If this safety problem is to be tackled effectively, then it is essential to understand how different materials, and the façade as a whole, perform in the event of a fire. This paper discusses the drivers for flammability in facades, the interaction of facade materials, and current gaps in knowledge. In doing so, it aims to provide an introduction to the field of façade fires, and to show that because of the drive for thermal efficiency and sustainability, façade systems have become more complex over time, and they have also become more flammable. We discuss the importance of quantifying the flammability of different façade systems, but highlight that it is currently impossible to do so, which hinders research progress. We finish by putting forward an integral framework of design that uses multi-objective optimization to ensure that flammability is minimized while considering other objectives, such as maximizing thermal performance or minimizing weight.

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Figure 1. Data showing the frequency of large façade fires worldwide from 1990 to present day. Data found from news articles online.

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Figure 2. Images demonstrating the different levels of analysis when considering a façade. Currently, fire research focuses mainly on individual components, while large scale fire testing is used to assess façade systems.

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Figure 3. Maximum U-Values allowed for external walls in progressive editions of the UK building code. The U-Value of a wall quantifies its overall thermal resistance - a lower value indicates a better insulating performance (HM Government, 2016).

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Figure 4. Market share of German insulation products from 1989-2004 (Bozsaky, 2010). The market is divided between non-combustible mineral wool and more thermally efficient polymeric insulation.

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Figure 5. Simplified sections of common façade systems: (a) Monolithic Façade, (b) Filled Cavity Façade, (c) External Thermal Insulating Composite System (ETICS) Façade, (d) Sandwich Panel (or Metal Insulated Panel), (e) Rainscreen Façade. Note: The vapor control layer and weather resistant barrier in (e) are shown on the warm side of the insulation, for a climate that has an annual desire for vapor to flow from inside to outside.

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Figure 6. Plots illustrating how cavity width could hypothetically affect: (a) the heat flux on the cavity walls (radiation enhancement), (b) the heat released into the cavity (chimney effect), and (c) the total flammability (combination). The “flammability index” plotted in (c) is a hypothetical variable that quantifies the flammability of a facade. Currently, no such variable exists, but our research aims to create one.

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Figure 7. Example of a Pareto front, minimizing flammability vs. U-Value. Each solution represents a particular façade system design. Crosses represent solutions that are feasible, but not optimal. Labels show where hypothetical systems using polymer foam or mineral wool insulation might fall. The “flammability index” plotted in (c) is a hypothetical variable that quantifies the flammability of a facade. Currently, no such variable exists, but our research aims to create one.

Table 1. Objectives in façade design (Herzog et al., 2017). Objectives are ordered by whether they are intended to be minimized or maximized, and from those that ensure the building is safe, to those that ensure the building is comfortable

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Acknowledgement

Supported by : Engineering and Physical Sciences Research Council (EPSRC)

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