Carpet Pad with Moisture Barrier
We sometimes encounter wet carpeting with a moisture barrier in water damaged areas.
Guess there can be a pretty strong attraction to them as an extra charge upgrade. If something gets spilled, by the kids, grand kids or...pets, it doesn't soak on through the carpeting down into the pad. It is also theoretically easier to clean up since it is right on the surface until you can wipe it up.
There are two cons to these moisture barriers. Should anything happen where some water of quantity gets on the floor or a spill is near the edge/seam, the liquid will go under the pad. Under the pad means under the moisture barrier. It is essentially on the wrong side of the barrier.
Pulling the carpet back, and probably removing the padding is the only choice to get the water/liquid out. Very disruptive.
Also, can't say I am in favor of anything that traps moisture inside a building. It doesn't evaporate well at all and can lead to other, undesirable consequences.