Can MAPEI Aquadefense be applied over greenboard for waterproofing and vapor barrier in shower stall before tiling?

Thank you for your inquiry regarding MAPEI products! The Tile Council of North America (TCNA) does not recommend the use of sheetrock in a wet area surround for any tile installation. That begin said, the Mapei Aqua Defense will adhere to the sheetrock, but it is not guaranteed to water proof the sheetrock. The only gypsum type backerboard that can be used in a wet area/surround is fiberglass reinforced sheetrock that is generally available in a purple color. Please consider calling us here at Tech Support Mon-Fri from 8 am to 5:30 pm EST at 800-992-6273 for additional support with your project if you would like. Thank you! Product Advisor # 11