The sale of German insulation firm Armacell by its Bahrain-based private equity owner Investcorp has attracted six bidders as interest grows in companies making energy-efficient products, two people familiar with the transaction said.

Private equity investors Charterhouse, Pamplona, Equistone, HgCapital, Goldman Sachs Private Equity as well as a U.S.-based producer of building materials have placed tentative bids, the people said, adding final bids are due in mid-March.

The offers value Armacell, which makes insulation materials, at 400 to 500 million euros ($538-$673 million), a discount to the valuations of peers, the people said.

Last year, Armacell Chief Executive Patrick Mathieu had told Reuters: "Our peers trade at (EBITDA) multiples of 7-9. We see ourselves at the higher end of this range."

If it was valued in line with peers, Armacell would fetch a price tag of 490 to 630 million euros as it expects to post earnings before interest, tax, depreciation and amortization (EBITDA) of 70 million euros this year.

The seller may, however, still book a gain on the sale. According to media reports, Investcorp bought Armacell for 350-400 million euros in 2006.

Germany-headquartered Armacell says it is the world's largest maker of flexible insulation, claiming 40 percent of the market, with its products used to lag pipes and ductwork in large buildings such as factories and schools.

The company has annual sales of about 440 million euros, 60 percent of which stem from outside of Europe.

Investcorp declined to comment, while the bidders were not immediately available for comment. – Reuters