That’s a big enough room I would recommend the room to be sealed with a good dehumidifier that’s crucial for a sealed room

fasteddy said:

1. What size room will I need for 16 plants? Would 64 square foot be big enough.

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I'm guessing you plan to hang the plants upside down. If so, 4 square feet per plant should be more than adequate.

fasteddy said:

2. What should the room air replacement cfm% be? 8x8x8 would give me 512cf. Do I need 50%,75%?

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I use a tent that I have set up for drying with a variable-speed exhaust fan. I run it faster for a day or two when the plants are newly harvested. After that, I run it at low speed. I don't know the CFM. I have hygrometers on the inside and the outside so I can compare the two.

fasteddy said:

3. Should the ventilation be updraft or downdraft?

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Probably updraft. That's what I do. Also important, I believe, is air circulation within the dryer. I have fans for that in my drying tent.

I ended up putting my autoflowers I harvested (3 cookie gelato) into the grow tent with a dehumidifier and 100cfm exhaust fan. The hygrometer is reading at 50% so I think i'm ok but the big room is going to be a challenge.......the girls are getting big but fortunately I know an expert hvac guy to give me advice...plus I have afew weeks to go before I worry about it

So the room will be 6ft x 8ft x 7ft. I have a room air purifier for mold etc. and a programable dehumidifier to maintain 62rh. It will have passive intake with 200cfm exhaust. Will that exchange rate negate the purifier etc. and if so what should the exchange rate be. These girls are getting real big so I want to get this right. The purifier can handle a room 225 square feet with a rate of 5 full cleanings per hour and the dehumidifier is adjustable with a room capacity of about the same size as the purifier. The room will be fairly air tight except for the intake and exhaust.