Pex is a flexible plastic tubing developed in the late 1960s for water piping. Available in Europe for years, it was first brought to the United States in the 1980s. Pex is easier to install than copper pipe, withstands extremes in temperature and is resistant to chemicals used in today's plumbing environment. Designed for long pipe runs with few fittings, it's durable. Pex has its problems, however, and those should be understood when considering the piping.
One major problem with Pex pipe is that it's not suitable for use outside. It is strictly designed for indoor and underground use as water piping. It's resistant to freezing temperatures but will not resist a direct and hard freeze. It's also not good for Pex to be exposed to direct sunlight outside for long periods as it will break down the material's strength and lead to full failure.
One annoying and damaging problem with Pex piping is its apparent attractiveness to mice and rats. These rodents are known to chew on Pex pipe, sometimes breaking through completely and causing water leaks. Possible reasons for this include that they are attracted to the bright colors, that they like the taste of the plastic and that they hear the sound of the water and chew to try to get a drink.
Pex pipe is known to occasionally aid in the development of some bacteria-forming biofilms when used with non-chlorinated water systems. This is most possible in areas of infrequent use, where the water is able to sit undisturbed in the pipe for long periods before use. Chlorinated water supplies don't have this problem, but for non-chlorinated supplies, the pipe system should be sterilized once per year.
One of Pex's greatest strengths is also sometimes a problem. Because Pex water lines are often one long, continuous span of pipe, if repair is necessary it can make it more difficult than with copper pipe. Pex fittings are more expensive than standard pipe fittings and so replacing a bad section of pipe may be more costly than expected. Pex pipe fittings also require special tools that are also quite expensive to purchase.
Pex's flexibility, savings on installation and reliability is its main hallmark. Overall, Pex is a solid alternative to traditional copper pipe. The problems associated with Pex are mostly minor or are easy to correct and avoid. As long as these problems are understood and watched for, Pex will do the job of providing a quality potable home water supply well.