[ncoak]

in a square room, you'll be dealing with lots of waves cancelling each other out from bouncing off all the parallel surfaces. reflective surfaces like glass are especially bad for this, in addition to windows being a good source of noise from outside.

this same effect is magnified in a small room like a closet, especially with a sensitive mic like a condenser - the vocal recording won't have any air/space to it and will sound boxy, even with the walls completely deadened. if possible (accounting for noise from your computer/equipment) i'd treat the room as a whole (incl. ceiling) and record there. closet should be a last resort. of course if you're recording with a dynamic you'll be able to minimize the effect, in which case the closet may not be your worst option.

if the closet is big enough though (esp. height-wise), the pros of isolation might outweigh the cons of the treated main room.