I am renovating my bathroom and am getting close to tiling the shower. I have the hardibacker in place and mortared the seams etc. The guy at Home Depot made it seem like I needed to apply a waterproofing before laying the tile (RedGard 1 Gal. Waterproofing and Crack Prevention Membrane). This stuff isn’t exactly cheap but I don’t want to end up with a leak somewhere. Is this stuff needed or was the rep just trying to make a sale?
It is absolutely needed.
Tile, mortar, and cement board are porous by design so that water and vapor can pass through them.
Using a membrane keeps water from invading your framing and structure. I personally recommend Kerdi, though instead of redgard.
It's needed, at least, strongly suggested if you have dry-walls, if you're on a brick&concrete house you don't need it.