Passive air flow in 8x12 building. Sealed. Need a 6'' or 8'' fan with filter. Size? Where to buy? Some have said the biggest filter is the call but I really don't understand why 6'' fans are rated higher than the cfm/5 fomulas for this size and people say 8''? Want to silence whichever I buy and l will need a passive hole for intake but don't know what size hole to cut and how to stop back draft from leaving through it when fan is off? Woul a back draft damper open and close when fan is running/off? and biggest question is where can you get a back draft damper. I really could use your advice so please help.... Thanks.