The price has not been disclosed and the dealis subject to approval of the Office of Fair Trading (OFT).
Celotex turned over £70m in the year to August 2011. It has 170 employees and two production lines based in Hadleigh near Ipswich. It has recently extended capacity with a new £3m distribution and innovation centre.
Saint-Gobain has an annual turnover of £34bn and 195,000 employees.
Already present in the UK insulation market with its Isover brand, Saint-Gobain will now get into new segments such as flat roofs and floors. The Celotex acquisition also strengthens its positions in the insulation for the new construction and renovation markets.
Celotex will continue to operate under its brand name and with its existing management team, Saint-Gobain said.