PIR insulation board (cited as the FR5000 product of Celotex, a Saint-Gobain company) was proposed to be used externally in the refurbishment of Grenfell Tower, London, with vertical and horizontal runs of 100 mm and 150 mm thickness respectively; subsequently "Ipswich firm Celotex confirmed it provided insulation materials for the refurbishment." On 14 June 2017 the block of flats, within 15 minutes, was enveloped in flames from the fourth floor to the top 24th floor. The causes of the rapid spread of fire up the outside of the building have yet to be established. Flames can occupy the cavity between the insulation material and the cladding, and be drawn upwards by convection, elongating to create secondary fires, and do so "regardless of the materials used to line the cavities".