| The idea is that you only want one moisture barrier, and with so many | houses now using housewrap, if you put plastic on the studs you are | creating an area in which any trapped moisture has no where to go. As | the dewpoint changes, this trapped moisture will, at times, condense, | leading to mold problems. |
That's not actually how it works. The housewrap should be a wind barrier that allows moisture to migrate through.
"The TYPAR Weather Protection System. It provides exceptional air and water holdout, optimal moisture vapor transmission"
(I wouldn't trust housewrap to stop liquid water, but that's another debate. :)
The inside walls can then be covered in plastic as a moisture barrier. So moisture is never getting into the walls from the inside in the first place. That's also a nice bonus where winters result in very dry indoor air.
Though in most of the places I work houses are not wrapped. They may have tar paper, but the walls are usually leaky in terms of air flow. Old houses are not wrapped unless they're wood and have been completely redone on the outside, with all the old siding stripped off. Where I live that's not at all common. In fact, I'm often dealing with houses that have incomplete or no insulation.
| In my shower I just put the hardibacker right on the studs, then | slathered it with redgard before I began tiling (using thinset mortar, | of course). |
I'd consider that an unnecessary experiment. Hopefully it works out OK. But your thinset is now not bonded to the hardibacker. It's bonded to the Redgard plastic coating. You've lost the advantage of producing a composite mortar wall. With concrete board, thinset and tile you'd end up with essentially a single mortar panel.
We've discussed this issue before. People have different opinions. To my mind there are an awfully lot of new inventions that are not time-tested and for which there's really no standards system to decide whether they actually make sense. I'd consider hardibacker, Redgard and waffle sheet underlayment all to be in that category -- claiming to solve a non-existent problem. Though I would be interested if someone came up with an easier- to-use version of concrete board that's also stronger. It's too easy to cause cracks in the concrete filler by bending and hitting.