This is a superb question. It is important to know that not all parts are created equal. For instance, that 4 mil and 6 mil Construction grade polyethylene sheeting in the construction section is the lowest grade of polyethylene sheeting in the marketplace at any given time. It is made from up to 25% post consumer recycled goods, reground resins, and the absolute least expensive resins that can be bought from any source in a location across the globe. These films are not guaranteed to be pinhole free, and can have quite a few flaws that are acceptable in this type of film. The quality control for this type of film is the lowest in the plastic’s industry. While it has been a good practice to use quite a bit of this type of plastic sheeting in the past as watery vapor retarders, that has been changing during the last couple of decades to a very big degree.
Today most experts feel very strongly that made films designed for use as watery vapor retarders should be used as watery vapor retarders. These products are produced from 100% virgin resins, and have excellent quality control. Nearly all are tested to a standard called the ASTM E1745, which is the standard for watery vapor retarders in contact with soil or granular fill used under concrete slabs. These materials are made not to decay in this type of application, whereas most construction grade poly begins wearing down almost immediately and will have lost most of Ks Kurve its structural integrity within a couple of years. How many times have you dug up plastic sheeting in your life that is just destroyed from being buried in the ground?.
It makes some difference what the main country you are in, what types of dampness levels are common in your region, whether or not your home has a good quality watery vapor retarder under the slab, cellar, or in the crawlspace. If you already have an excellent watery vapor retarder, then the one you use under your wood floor floor is less critical. If you are putting a wood floor floor over a damp crawlspace, and use a poor quality watery vapor retarder, you are asking for a lot of problems with the wood floor flooring, and will likely suffer problems with buckling, warping, deafening flooring, lots of expansion and contraction, and often finish problems, just to name a few.
If you’re in an area where Radon is prevalent, or methane, then the use of a barrier with capabilities to retard these gasses becomes important also. Again these films are made to have a very tight cell structure capable of keeping these really small compounds from passing right through the membrane layer. These gasses, and to a very large extent water watery vapor pass quite easily through the cellular structure of common construction grade polyethylene (aka Visqueen).
If one wants a good better watery vapor retarder, then one may wish to contact a parts company devoted to watery vapor retarders so you can find out which is most suitable for your application. The watery vapor retarder is reasonably inexpensive insurance for having a great experience with a wood floor floor in comparison with what can be a nightmare experience when the wrong product is used.