Firstly, plan your soundproofing before you put your ceiling in - that's often one of the biggest weak links and a soundproofed room is a system, only as good as the weakest link.
If you need to avoid A/C then you are basically stuffed - airtightness is inextricably linked with soundproofing and where air flows, so does sound.
Best you can do is have a decent amount of proofing but no air when recording and open up for air between takes (and heaven help you if you fall asleep/pass out in there with the room shut up). However I've done this and can tell you that a sealed room with no fresh air gets very fatiguing very quickly (and the more people in the room the faster you run out of air). Also in a sealed room, condensation becomes a real issue and then you really need the dehumidifying part of an A/C.
Tell me more about the room construction - type of roof, number of doors, windows, etc. And how far you are willing to go construction-wise to achieve decent results.
If you need to avoid A/C then you are basically stuffed - airtightness is inextricably linked with soundproofing and where air flows, so does sound.
Best you can do is have a decent amount of proofing but no air when recording and open up for air between takes (and heaven help you if you fall asleep/pass out in there with the room shut up). However I've done this and can tell you that a sealed room with no fresh air gets very fatiguing very quickly (and the more people in the room the faster you run out of air). Also in a sealed room, condensation becomes a real issue and then you really need the dehumidifying part of an A/C.
Tell me more about the room construction - type of roof, number of doors, windows, etc. And how far you are willing to go construction-wise to achieve decent results.