Currently, if liquid spills on carpet, it can soak down to the foam pad,
then down to the floor.
This is bad if it is a pet stain, because once soaked into the floor, the
odor is hard to remove. So, why not put some waterproof sheet between the
carpet and the foam pad during installation? This way new stain won't soak
the floor, and old odor from the floor won't come up to the surface.