The deal values Luxembourg-based Armacell at 960 million euros (677 million pounds) and generates a 54 percent internal rate of return for investors, a source familiar with the matter said.

The transaction is subject to regulatory approvals and is expected to close in the first quarter of 2016.

Charterhouse acquired Armacell in 2013 in a 520 million euro deal. The sale is the seventh exit from the firm's ninth fund, and represents the best investment result so far, the source said.

(Reporting By Sinead Cruise; editing by Simon Jessop)