PEX pipe
Also called: PEX tubing, PEX-A, PEX-B, cross-linked polyethylene, PEX plumbing
Cross-linked polyethylene tubing for hot and cold water supply: flexible, $0.40–$2 a foot, joined with crimp, clamp or expansion fittings instead of solder. The material in most repipes today. PEX-A is more flexible and uses expansion fittings; PEX-B is cheaper and uses crimp rings.
PEX arrived in US homes in the 1990s and is now accepted by every model plumbing code. It bends around framing, so a run from the trunk to a fixture often needs no fittings at all; it doesn’t corrode or pit the way copper does in aggressive water; it’s quieter; it tolerates freezing better than rigid pipe; and it costs a quarter to a half of copper. Those are the reasons a whole-house repipe in PEX is $5,000–$10,000 where copper is $10,000–$20,000.
PEX-A is the most flexible grade and uses expansion fittings: the tube end is stretched with a tool, the fitting inserted, and the tube shrinks back over it. Full-bore fittings, kinks repairable with a heat gun, a $300 tool (or $40-a-day rental). PEX-B is stiffer and cheaper and uses crimp rings (copper) or cinch clamps (stainless) closed with a $60 tool and checked with a go/no-go gauge. Both are fine for a repipe; don’t mix systems, and use the manufacturer’s fittings so the 25-year warranty holds.
Rules that matter: red for hot and blue for cold (or all white with markings), support every 32 in horizontally, nail plates where it passes within 1.25 in of a stud face, bend supports at 90° turns out of walls, insulation in unconditioned spaces, and keep it out of sunlight — UV degrades it. Rodents chew it, which is the one argument for copper in a crawl space with a mouse problem.