Dens-Shield, Durarock, Hardiboard, Wonderboard.... We are having a house built with much of the floor being tile, and lots of friends have lots of advice. We have a crawlspace foundation with dimensional lumber on 16" center for the floors (our framer turned out to be from the old school - darn if our house isn't stout!). Our tile suppliers use Dens-Shield. A friend suggest we insist on Hardiboard. Given such a stiff frame, does it really matter which underlayment is used?