Shares of the St. Petersburg, Florida-based flood insurer opened at $22.50 apiece, compared with the $20 issue price. Neptune and some of its existing shareholders sold 18.4 million shares in an initial public offering on Tuesday, at the top of its target range of $18 to $20 apiece. Based on the IPO pricing, the company was initially valued at $2.76 billion. The flood insurance market remains dominated by the National Flood Insurance Program, a federal initiative that has been disrupted by the latest federal government shutdown. While such shutdowns are typically of a brief duration, programs like NFIP will not be "issuing any new policies and there may be delays in even renewing policies," Burgess said.