I’m replacing my fiberglass surround above the bath tub with hardie board and ceramic tile. My question is, due I really need to apply a sealer such as Redgard to the hardie backer cement board? I thought that the grout would keep the water from getting behind the tiles and it’s not like a shower only stall where the walls and floor get soaked. Maybe I’m being cheap, but that stuff is $50 per gallon and it would take two at least.