HMS Tiger's armour in the treaty era

Was HMS Tiger's armour inadequate by the end of the 1920s?

It remained adequate for a battlecruiser role, but not for a treaty-era capital ship competing with battleships for limited tonnage.

Tiger's nine-inch main belt and six-inch upper belt remained defensible for traditional battlecruiser work. The problem was institutional as well as technical: treaty fleets could not freely build separate battleship and battlecruiser categories. A navy retaining Tiger would have to explain why scarce capital-ship tonnage should not instead buy a better-protected battleship. Treaty accounting could therefore make a once-appropriate design obsolete before its machinery or guns became useless.