The insulated walls of the Zero Room completely cut it off from the outside universe. It is, in fact, a pocket universe within a TARDIS's Micro-Universe. Zero Rooms are used by Gallifeyans to recover from difficult regenerations. Zero cabinets can act as stasis chambers for Time Lords. All TARDISes from the Type 28 onwards have zero rooms. A properly sealed Zero Room has better invulnerability then the exterior of a TARDIS. It should be noted that Zero Rooms are balanced to zero energy and thus don't appear on the Architectural Configuration Indicators.