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How reusing and recycling glass can cut a building’s carbon emissions

Words:
Graham Coult and Stéphany Le Rhun

Glass can be reused or recycled, greatly reducing emissions, but designs need to allow for greater durability and easier dismantling at end of life

Unesco V, Paris, by Zehrfuss and Prouvé, 1970; ongoing refurbishment by Patriarche Architectes. Existing facade before refurbishment
Unesco V, Paris, by Zehrfuss and Prouvé, 1970; ongoing refurbishment by Patriarche Architectes. Existing facade before refurbishment Credit: Patriarche Architectes © Nicolas Grosmond

The main markets for the glass industry are buildings, accounting for 80 per cent, and transport (15 per cent). The UK has three float glass plants producing around 750,000 tonnes annually. The EU produces around 10 million tonnes while the total global output is more than 76 million tonnes.

Glass is 100 per cent – and infinitely – recyclable yet, to date, this quality has been largely underexploited. We now need to deploy recycling on a global scale. As designers, we can support this by ensuring the collection and recycling of end-of-life glass from existing buildings, particularly during deconstruction before refurbishment.

How could we reduce the embodied carbon of glass? The predominantly virgin raw materials are heated in the batch, decarbonation occurs and CO2 is released in the air, a process responsible for up to a quarter of the float process’s total CO2e emissions. These emissions can be avoided by replacing the raw materials with cullet – recycled glass produced from production-line waste or from existing buildings, which has already undergone decarbonation.

Cullet also melts faster than raw materials, resulting in up to a 30 per cent saving in energy consumption and related emissions. It is therefore urgent to develop the cullet stream at an industrial and national scale.

Process of producing float glass from raw material in a float line factor (above) and the additional process to toughen glass through tempering (below).
Process of producing float glass from raw material in a float line factor (above) and the additional process to toughen glass through tempering (below). Credit: Tom Mcveigh of Feilden Clegg Bradley Studios

To achieve circularity, glass should be reused or, at a minimum, recycled into a material of the same quality (flat glass). Recent best practice has only resulted in 6 per cent of flat glass being effectively recycled. Most of this glass is downcycled, ie turned into lower-quality products such as glass bottles and glass wool insulation.

When flat glass is recycled back into flat glass of the same quality, it is crushed into small pieces, forming the cullet, which returns to the float line, where it is melted down with other glass-making materials. Most cullet used in today’s flat glass comes from pre-consumer waste from manufacturing or processing prior to its use on site.

For example, in 2018, less than 1 per cent of the cullet used by materials manufacturer Saint-Gobain came from post-consumer waste from renovation or demolition sites. With incentives for increased circularity, this is starting to change, particularly in France where it is driven by new anti-waste regulations (AGEC/REP). The amount of post-consumer cullet collected by Saint-Gobain increased from 30 tonnes in 2018 to 4,107 tonnes in 2023 through a new network of more than 40 glass waste processors.

Closed-loop recycling of flat glass presents the following challenges:

  • New logistical and recycling streams need to be set up locally and nationally to collect glass on site.
  • Non-glass components must be removed before cullet production to avoid contamination by other materials, which can cause issues for float lines. Disassembly can be complicated, labour-intensive and time-consuming.
  • It is sometimes necessary to transport the glass intact to the off-site cullet production facility. This involves careful dismantling, which must be anticipated in terms of logistics, planning and budget at the beginning of a refurbishment project.

Closed-loop recycling has huge benefits. Recycling one tonne of glass saves up to 300kgCO2e, 1.2 tonnes of raw materials, including 700kg of sand, and 30 per cent of energy.

To date, reuse of insulated glass units has not been widely adopted due to their short lifespan, rapid obsolescence, warranty requirements and lack of standardisation of sizes. Reuse of monolithic glass is more developed, especially for internal use.

For both circular approaches (reuse and recycling), the complexity lies in the fact that facade glazing is a composite unit that is very labour-intensive to dismantle.

Sometimes, the glass is also bonded to other parts of the facade (window and curtain wall frames). This increases the obstacles to separating the elements and integrating them into a circular process. To avoid repeating past errors, new facades should be designed as kits of parts, with demountable systems to facilitate reuse and recycling of glass.

In addition, designers have tried to combine all the functions of the facade within insulated glazing products: integrated blinds, photovoltaics, dynamic glass, etc. Although these elements can be very beneficial to reducing a building’s energy consumption, they lead to increased embodied carbon and less demountability, reuse and recycling potential. Simplicity and low-tech are often the right choice for glass.

Acceptance of defects such as bubbles could reduce glass furnace temperatures and associated emissions. We need to decide as consumers what we are demanding of glass as a product

Over the last few decades, designers and users have been demanding ever higher quality and transparency from glass, and the industry has responded. Designers may not realise that these demands also mean increased energy use.

Moves to incorporate more reused or recycled glass may affect glass quality, in terms of colour neutrality and presence of aesthetic defects, when the volumes recovered rise to substantial levels. Acceptance of defects such as bubbles – which were the norm a few decades ago – could also reduce glass furnace temperatures and associated emissions. We need to decide as consumers what we are demanding of glass as a product.

For existing façades, we must first increase their lifespan by refurbishing them. Poorly performing glass units could be extended by repair or reconditioning. We could identify points of leaks, seal them again and reinject gas. Ideally, this could be done on site, or the panels could be removed for remanufacturing: they could be disassembled, cleaned, and bonded back together again with new edge seals and replenished cavity  gas. Performance coatings can be added to bring the thermal and solar performance to current standards. These operations are not yet developed on the market. More research and development on approaches to repair and upgrade is urgently needed.

When replacement becomes necessary, we must ensure that no resources are wasted by organising the reuse and recycling of materials and glass.

We must also adopt designs that allow for full longevity of the facade, incorporating timeless design, simple dismantling, and best practice for durability.

We also need to anticipate the end of life of products to ensure that they are circular and the resources will be fully reused. An online database (www.glazingrecovery.org) will launch later this year, providing guidance on glazing replacement and refurbishment and linking providers participating in this sector.

  • Unesco’s kit of parts
    Unesco’s kit of parts Credit: Eckersley O'Callaghan
  • Unesco’s kit of parts
    Unesco’s kit of parts Credit: Eckersley O'Callaghan
12

Case study: Unesco V, Paris, by Zehrfuss and Prouvé, 1970; refurbishment by Patriarche

Unesco V is a pilot circular refurbishment project led by architecture practice Patriarche with engineer Eckersley O’Callaghan as facade consultant. It involves reuse of the cladding and recycling of the flat glass. The glass was carefully dismantled from the existing facade and transported intact to the cullet producer Ares/ The cullet was inspected and sent to the Saint-Gobain float line to become part of its low-carbon production. The flat glass was recycled and the cladding was reused in a method audited and approved by the glass manufacturer. Thanks to glass recycling, up to 30 tonnes of CO2e were saved.

Crcular deconstruction was made possible thanks to the original ‘kit of parts’ facade made with components of ‘human’ dimensions that can be easily handled. It has been part of a larger low-carbon design of the project, where the reused materials have been installed on top of a bio-based and high-performing inner skin, with high-performing glazed facades.


This article is based on the Glass chapter from Materials: An Environmental Primer, edited by Hattie Hartman and Joe Jack Williams, published by RIBA Publishing, 2024

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