What is the downside with using green dry wall vs cement backer board in a tub/shower surround? If green board is completely sealed with RedGard or similar product, will it not be impervious to moisture?
According to the manufacturer, drywall is not a suitable substrate for RedGard®.
When you're building a tub/shower surround, there are two questions that need to be answered:
- Which substrate material should I choose?
- How do I waterproof it?
The answer to question #1 is easy. Cementboard, always (unless you're using a fully-integrated system like Kerdi, but if that's the case, you're probably not asking the question here). Not drywall, not greenboard. Always use cementboard.
Question #2 has a variety of answers, but it needs some answer. You can't have no waterproofing layer, because neither cementboard nor tile are waterproof. They are not affected by water, but they will transmit it through themselves to whatever is below, which is probably wood if you're in the USA. Not good. The wood will rot and grow mold and termites will eat it. So you need some kind of barrier to keep the water off the wood. Once you've committed to using cementboard, painting it with RedGard or a similar product is a popular and effective approach. A more old-fashioned one is to put a sheet of 6-mil poly under the cementboard. Fussier, and with more detailing required, but also effective.
But you can't have no waterproofing layer!
drywall should never be used where its in contact with water regularly. if its some splash or moisture occasionally, you can use water resistant drywall.
we are already starting to see problems with various waterproof membrane systems they have introduced in the last few years. redgard is just the newest crappy product they are trying to get people to buy. its just urethane foundation waaterproofing with red dye instead of blue. the won't even warranty it if you don't use their mortars and reinforcements. its just an extra cost security blanket for people as a substitute for doing things properly. you dont need it at all. just design your space intelligently and do the work with care and the right materials and attention to detail.
cement board is the only appropriate material (short of lath and mortar on a plywood backer) as it won't break down when it gets wet. all tiles leak eventually - cementboard will just withstand it longer without being compromised.