PEX Pipe PEX, or cross-linked polyethylene, pipe is one of the newest and most popular pipes to hit the plumbing market. PEX is used only to supply water. PEX is a pipe that is rigid enough to withstand the pressures of water supply but flexible enough to weave throughout walls, ceilings, basements, and crawlspaces. PEX has made water-supply plumbing easier for do-it-yourselfers and professional plumbers. http://www.ifansolution.com/pex-pipe/