Fully glazed facades are a popular design choice, opening up spaces and giving exterior views. But how sustainable is glass as a building material?

Embodied energy of manufacturing

Float glass is a type of clear glass most commonly used in buildings, and is made from natural and abundant raw materials. Bob van Schelt, sales director at PFG Building Glass, and a committee member of the South African Glass and Glazing Association, says the main ingredients are 60% sand, soda ash and limestone. The raw materials are mixed together and heated in a furnace at 1620°C. The melted material forms a viscous liquid.

Molten glass from the furnace passes through a bath containing molten tin in an atmosphere of hydrogen and nitrogen. The continuous ribbon of glass floating on the molten tin is formed into the required thickness. Since the surface of the molten tin is flat, the glass also becomes flat and has a uniform thickness. As the liquid cools, the viscosity increases until the glass becomes solid.

While it is fairly easy to recycle glass, compared to other building materials, the embodied energy associated with the manufacture (and recycling) is relatively high compared to bricks, cement and steel.

Reduce, re-use and recycle

Glass is a resource-efficient material – clear glass can be recycled fully and endlessly. Float lines use up to 30% of a recycled component known as cullet (broken or waste glass), in their glass mix. Cullet usage of 10% leads to a 3% reduction in furnace energy usage, says Van Schelt.

Performance glass – which can comprise body tints, interlayers and/or special coatings to provide appropriate safety and performance characteristics – is a better option for operational energy saving in buildings, however, it makes architectural glass a difficult product to recycle. “Recycling materials requires a ‘pure’ material, not a composite, and architectural glass is typically a complex composite material,” says Tessa Brunette, facade engineer at ARUP.

How the facade systems are fabricated also plays a role in recyclability through re-use, she adds. Flush-glazed systems use structural silicone to bond glass to an aluminium frame. If the glass is to be replaced or the facade dismantled, the silicone needs to be removed and reinstalled, which requires additional material. Using a standard pressure glazed system – where the glass is held to the aluminium mullions by another aluminium extrusion called a pressure plate that is visible on the external face – for instance, does not require any wet silicone to be applied and allows for the entire system to be dismantled and reassembled elsewhere, provided the modules are applicable in the new location.

But, the most effective way to reduce overall materials wastage, including glass, is to use less. Greg Borman, facade engineer at Sutherland, agrees: “Glass is one of the top three building materials with the highest embodied energy, but it is essential. The first place to start is making sure you waste very little of it.”

Impact of building design

The quantity, orientation and type of glazing all have a profound impact on any building’s performance and these must be carefully considered in the design, says Brunette. In the South African climate, heat from the sun has the biggest impact on a building’s internal micro-climate. How this solar load is controlled, and balanced with letting in natural daylight and access to views, can define the architectural design.

To create a high performance green building, the building envelope must be designed as an integrated whole to create an optimal balance between letting light in and keeping heat out. The starting point is using passive design principles such as correct orientation.

As a building design develops, potential problem areas can be identified, either through modelling or by carefully planning the spatial layout of interiors – for example positioning work stations away from the window areas – and specific interventions can be chosen to improve the performance.

“Linked to orientation is shading, which reduces the radiant heat loads on the glass and allows us to downgrade the glass envelope’s performance,” notes Borman.

Once done with modulation and having made the best use of orientation, the best performing glass possible for a project needs to be found. The performance of the glass itself can be improved through the specification of coatings or interlayers that control solar and thermal quality by improving the emissivity, solar heat gain coefficient and / or U-value properties.

There are many factors related to performance that need to be balanced when choosing the best glass for a specific project. “Cost of glass, running cost of HVAC to deal with the heat loads through the building envelope, running costs of lighting to top up the natural light coming through the facade, occupant comfort within the first 1m to 4m of the envelope – these are all factors that play a role in the glass selection,” says Borman.

Van Schelt says local knowledge and expertise is hugely beneficial when designing structures, particularly in regards to glazing.

Internal environmental quality

When considering the impact of glass on the internal environmental quality of buildings and the wellbeing of occupants, it has the potential to score well. It is an essential ingredient to making buildings healthier and more liveable for occupants, firstly by allowing a visual connection to the external environment. Besides the positive impact on health and productivity, daylight can be used to save on electrical lighting. Daylight should, however, be carefully balanced with glare reduction and solar heat reduction to avoid discomfort for occupants.

Van Schelt says advances in coating technology allow for the creative use of architectural glass in reducing the harmful effects of excessive glare and solar heat gain. Modern coatings are selective in transmitting acceptable levels of light and reducing solar heat gains in terms of optimising energy efficiency, he says.

The right balance

There are many different types of glass available in South Africa. The cheapest is single-layered clear glass, which is limited in use due to its lack of thermal and solar performance. This means to make glass a viable sustainable option, investment must be made into adding extra coatings, layers or designing double skin facades.

Werner van Antwerpen, sustainability manager at Growthpoint, sums up the central conflict at play when considering glass: “From an engineering point of view, one would like to have concrete windows and no sunlight exposure due to the complexity of sizing and operating HVAC systems. However, from a tenant point of view, one would like to have as much view exposure as one can get. It is about finding the right balance.”

By Femke van Zandvoort

See earthworks magazine issue 39 August-September 2017 for the full feature.