Cellular or foamed glass (CG) is manufactured from a mixture of crushed glass and fine carbon powder which, upon heating to 1000°C, causes the carbon to oxidise, creating bubbles within the molten glass.
The glass is annealed, cooled and finally cut to size. The black material is durable, non-combustible, easily worked and has a high compressive strength. It is water resistant due to its closed-cell structure, impervious to water vapour and contains no CFCs. Cellular glass products are described in BS EN 13167: 2012.

Cellular glass slabs are appropriate for roof insulation, including green roofs and roof-top car parks owing to their high compressive strength. Tapered slabs are available to create roof falls. The slabs are usually bonded in hot bitumen to concrete screeds, profile metal decking or reinforced bitumen membrane on timber roofing. Foamed glass is suitable for floor insulation under the screed, and may be used internally,externally or within the cavity of external walls. Externally, it may be rendered or tile hung and internally finished with plasterboard or expanded metal and conventional plaster. (The thermal conductivity of cellular glass is within the range 0.037–0.055 W/m K at 10°C, depending upon the grade.)