QUESTION

Why use Sealant in Transition joints? - someone else mentioned that about sealing the transition joints, I'm just curious as to why is that important, if there is a vapour barrier behind the wall?

ANSWER

ANSWER - two reasons:
1. Transition joints are movement joints to allow expansion and contraction of the various materials.

2. Transition joints are normally areas where multiple products merge and they have a propensity to be defective in one way or the other and becomes the path to subsurface areas where collateral damages can occur from moisture migration.  So the sealant seals that joint to help prevent moisture migration to behind the tile.