PEX

PEX is cross-linked polyethylene, a flexible plastic tubing widely used in residential plumbing and radiant floor heating systems.

It has largely replaced copper pipes in new construction because it's cheaper, easier to install, and resistant to corrosion and freezing.

PEX comes in color-coded varieties—typically red for hot water lines and blue for cold—and connects with special fittings rather than soldering.

The plumber ran PEX through the walls instead of copper.