What’s the difference between Rw and STC?

Both Rw and STC are both laboratory ratings of the sound insulation of a material or assembly, where higher numbers represent better sound insulation. They are tests of the same properties according to different test standards. Rw is more commonly used in Europe and Australia, while STC is more commonly used in North America. The same material will normally have Rw/STC ratings within 1-2 dB of each other.