Layed out below are the most common 4 methods I have seen used in installing a vapor barrier. If you talk to different building firms, you usually will end up with many different methods of installing a vapor barrier in a crawlspace. Here are the most common:
one Pea Gravel on top of plastic vapor barrier - This has to be the all time dumbest thing I have truly seen, and yet it is probably one of the most common. I have had builders tell me that the plastic on the ground WILL KEEP ground water and moisture from coming up from the ground. Now if there is no moisture or water with the soil, this might be possible; but if that was the case there would be no need for the barrier in the beginning. So here is the basic idea - ground floor (bottom) - plastic (middle) - 4" pea stones fill (top). Once the crawl foundation is built, builders install a 4 - 6 mil plastic on the ground along with dump about 4" - 6" of pea gravel on top of the plastic. Eventually, what always comes about is that water comes in from the walls and the ground floor and ends up on top of the plastic. So what on earth you end up with is a swimming pool liner that holds water in the gravel for prolonged periods of time. Nearly all the pool and moisture in the gravel back fill has to evaporate into the structure. Another example of building practices and even science turning a blind eye to crawlspaces for decades.
2 . Vapor barrier on top of ground floor - By far the most common practice for installing a vapor barrier. A 6 mil polyethylene vapor barrier is positioned over any ground floor. Here is the idea - ground floor (bottom) - plastic (top). The ground ground could be river rock, pea gravel, dirt floor, sand, etc . The seams are typically overlapped 6" : 12" and almost never taped. While this will temporarily stop some moisture evaporation, it does not seal out water from the internal perimeter wall where most water penetration occurs. Also moisture can come up from the seams, and the plastic is not durable enough to crawl on. It nearly always ends up with many punctures not to mention holes in it.
3. Vapor barrier on bottom of floor joists - This is rarely seen, and also usually only home owners attempt this. This is probably the method that accounts for more wood rot than some other method. If you are even thinking of doing this, quit thinking and call a professional to fix your crawlspace. Most crawlspaces are vented and the cooler surfaces such as duct work, pipes, and the floor will condensate in the summer time. The plastic will trap the condensation up against the floor structure and mold and wood rot might occur. Good intentions do not always produce good results.
4. Vapor barrier fastened to sill plate -- There is a new industry in crawlspace repair that encapsulates crawl spaces. The process of encapsulation is to purchase a heavy think plastic liner on the floor and up the foundation walls. The plastic liner is fastened and covered to the foundation wall and all the overlapped seams are taped. Every potential gap or seam within the liner is meticulously sealed to prevent any moisture from evaporating. Then the vents are sealed in the encapsulation system to prevent hot humid air from entering in the summer. There is another system being sold and put in out there being represented as an encapsulation system, but is far from it.
This "other" system is a 6 mil plastic that is ran up the foundation walls and stapled to the sill plate. non-e of the overlapped seams are taped. It is basically a glorified vapor barrier on top of the ground floor being ran in the walls and stapled to the wood. They seal the vents without properly sealing the moisture in the ground floor or foundation walls. The problem with fastening plastic to the sill plate is that seepage will "wick" up the foundation wall, and moisture will absorb into the sill plate and floor joists. They are giving free access to all the moisture under the liner to rot the sill plates and terrain joists. Not to mention that all the moisture will evaporate up through all of the seams that aren't taped and the nasty liner is only 6 mil and eventually will puncture and tear. Be very careful in the contractor you choose to accordingly encapsulate your crawlspace.