For the record...the national codes no longer allow greenboard in wet areas. Not all local codes have changed. There never was a really good reason for greenboard. If it is wet, it should be cement board (cbu). If it is dry, then regular drywall is stronger, cheaper, and accepts tile and thinset quite well. Greenboard also tends to droop on a ceiling, and requires 12" on center supports (less on a wall), which most people don't honor. It is false confidence when using it.