Ventilation and door construction in vocalbooth

Hey guys, I'm planning to construct att vocalbooth in a apartment (top floor). I will use it only for vocals :roll: I have read a lot in this forum and got a whole lot of tips especially - http://johnlsayers.com/phpBB2/viewtopic ... ght=#51627 So it will be something like that. But I haven't got my head around the ventilationproblem. My room will probably be around 4,5m3. How much ventilation is necesary? If I use some kind of insulated ductwork - how big(and long) does it have to be and do I want a fan(or something) outside of the booth blowing air into the booth or will it be enough without? Should I use a silencer...? The other question is about the door and the benefit :?: of using 2 instead of one. Since I'm only going to use the booth for vocals do I really need an acoustic(best kind) door or could I use one outer door(which is heavier than innerdoor) or 2 innerdoors (less weight but with a air space between). I read some about making sure that the door close airtight so that I will do. Hope you guys can advice me //Cheers

Anyone? 8)