Does PEX tubing constitute an offgassing problem when applied under existing hardwood flooring?
Additionally, are there any water contamination issues with using PEX tubing for human-consumption water supply?
PEX is cross-linked polyethylene, so there is no measurable offgassing from the material.
Through one of several processes, links between polyethylene molecules are formed to create bridges (thus the term "cross-linked"). The resulting material is more durable under temperature extremes, chemical attack, and better resists creep deformation, making PEX an excellent material for hot water and other applications.
PEX was developed in the 1960s. PEX tubing has been in use in many European countries for plumbing, radiant heating, and snow-melt applications since that time. PEX was introduced in the United States in the 1980s, and has seen significant growth in market demand and production.
We use PEX for house plumbing in almost every job that the municipality we are building in allows. It is cheaper, easier to work with, and results in a much better-performing finished plumbing system. Due to its small gauge compared to traditional metal plumbing materials, PEX minimizes the annoying and wasteful wait for hot water that most traditional plumbing systems have.
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You should also read Michael Heacock's Ask A Pro Q&A, "I'm seeking bids to repipe my house. PVC is recommended by all three plumbing bidders. Is it safe?"