Any patch of glowstone as a floor larger than 2x5 seems to attract zombies, we put glass over the glowstone and they are still spawning there. We kill them and with a 5x3 area 3 appear the second it becomes night, more if its a larger area (20x20 gets about 10). Slime on top seems to stop the appearance, however who wants to bounce their way through the halls all the time?