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.
