It is exempt. Even though a door that is used infrequently is still obligated, an emergency door is exempt because it is not designed to be a doorway. Some authorities also exempt it because it is designed for exit and not for entrance.[A]
 Rema Y.D. 286:18; Kuntres HaMezuzah 286:181 writes that even if it is used for emergency entrance, it is still exempt; Agur B’ohalecha 24:9:14; Sha’arei HaMezuzah 5:11:22, citing R. Moshe Feinstein in Ohalei Yeshurun 5.
 Minchas Shlomo (2nd Ed. 2-3, 100:5), cited in Sha’arei HaMezuzah 5:11:22.