I am in the process of building my first 16x24 for starting veggies for the summer. I am putting up a quonset style greenhouse with double 6 mil poly.

I framed the endwalls and put a 20" exhaust fan from J&D Mfg (VES20C) on the west end and two 16" shutter vents on the east end (VRSG16A-C). The fan is rated for 4790 CFM at .05 Static pressure. My calculations 16x24x8 (8 feet high) tell me I should need 3072 CFM to turn the air over once per minute. I figure that static pressure will be closer to .1 so the CFM rating will drop some, but not enough to be below 1 turn over per minute.

What I am worried about is if my vents are too small. I have not put in the door yet so I can still change my vents out. Once I put the door in the horizontal 2x4's that are on the top and bottom of the fan can not be moved.

I have seen so many different calculations on vent sizing. Some sites say you should have venting sized 1.25 to 1.5 times the size of the exhaust fan. Others have a fixed CFM rating for each vent size. I have seen calculations that say to divide the required CFM of your greenhouse by 250 to get the required total vent size needed.

So I get a range of vent sizes from 16" to 30". Can anyone else who has larger commercial size greenhouses advise me on vent sizing? Are the 16" vents large enough? Will two 24" size fans be too big, too small? The cost of the vents is low compared to the cost of the whole structure so I would like to get it right the first time.

I am in zone 5 in southern Wisconsin. My main use for the greenhouse will start (next year) in February (I am puttin in a modine hotdawg HD45-propane) through to late fall. I plan on growing some toms and peppers in containers after veggie transplants are done.

I currently have 60 288 trays of onions sitting outside on the drive way. I has been crazy warm the last two weeks here. In the low to mid 70's and low 80's tomorrow. This is unheard of in March. Normal weather is mid 40's to 50's with some nights normally in 30's. The weather is supposed to be rainy and colder after tomorrow so I am going to put the plastic on, but leave the door out until I get a solid handle on the vent size.

Thank for any help you can provide.