Carpet is normally installed over a pad which is laid on top of the subflooring. For tile, concrete board is put down over the subflooring and mortar is used to attach the tiles to the cement board.

Some sources say that with a crawl space below, a vapor barrier (4-mil polyethylene) should be put under the pad to keep it from deteriorating. Builders will probably tell you that this is not necessary or they have never heard of such a thing, but we did not get a vapor barrier and our padding has fallen apart after 8 years.