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Gypsum is approximately 21 percent by weight chemically combined water which greatly contributes to its effectiveness as a fire resistive barrier.  when gypsum is exposed to fire, the water is slowly released as steam, effectively retarding heat transmission.   Fire rated gypsum drywall is more fire resistant because it contains glass fiber reinforcement and other additives within its specifically formulated gypsum core to help it hold longer to fire exposure.

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Because fire rated gypsum board retains its moisture and it is extremely dense which makes it a good sound blocker.