Minecraft wiki entry for Airlock, associated text: For the first step, the hallway must have a ceiling height equal to the highest water level possible plus 1, to compensate for the torch. Also, each new column must be 1 block higher, as the torch is 1 block higher each time. A torch is placed on either wall at a level where water can contact it and there is enough space above to provide the required amount of sand to seal the Chamber. Sand is now placed above the torch until the sand reaches the ceiling, or until it reaches the desired level.