Greenhouse Venting Requirements
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 plan on motorizing the vents so they can be opened before the fan comes on.
I am in zone 5 in southern Wisconsin. My main use for the greenhouse will be to start seeds (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. It 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 the 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.
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Replies to This Discussion
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Permalink Reply by Bill Fleming on -
You should be fine.
Our 30'x50' polycarbonite walled greenhouse has one 20" fan on the west end and two 24"x24" automatic louvered vents on the east side and cools great.
We're in Montana though where sometimes to keep the greenhouse under 85°F it might be in the low 40s outside. The fan may only be on for a couple minutes to drop the temps 20°.
Our summer temps rarely exceed 90° but by that time our plants are all outside or in the hoophouse.
We do use a shade clothes on both our greenhouse and hoop houses though.
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Permalink Reply by Matt on
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Well I stayed with the 16" vents. I installed a venting door as well. This has a slide up glass window like in the house. When I need the extra venting I just open the window. Temps seem to be controlled well now. Although now that I have the greenhouse up I wish I would have went bigger.
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Permalink Reply by Kate Rossetto on
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Hi Bill, I am in Billings, and also have a 14 x 26 foot polycarbonite greenhouse. The panels are installed on a trussed roof. The walls are made out of recycled steel schoolhouse panels that I fitted with the polycarbonate. The greenhouse is attached to the west side of my shop so it gets great southern and western exposure...but it is getting really hot in there. This is my first experience of greenhouse growing and have installed lights under which to start my seedlings. I have had great success with the seedlings. I installed a 20 inch exhaust fan on the west side of the greenhouse and there is also a sliding glass door on the west side. Each schoolhouse panel has a 2 x 4 foot window that opens. So I have ventilation but it still does not keep it cool. I have been told that I need to install shadecloth, but I have also heard that using white latex paint on the roof and south side panels would work too. I have invested allot of money in building this recycled greenhouse and want it to be functional year round, so I installed a gas wall heater. It worked great this past winter, but we didn't really have a very cold winter! What do you recommend I do about the cooling problem. I have no roof vents.
Bill Fleming said:You should be fine.
Our 30'x50' polycarbonite walled greenhouse has one 20" fan on the west end and two 24"x24" automatic louvered vents on the east side and cools great.
We're in Montana though where sometimes to keep the greenhouse under 85°F it might be in the low 40s outside. The fan may only be on for a couple minutes to drop the temps 20°.
Our summer temps rarely exceed 90° but by that time our plants are all outside or in the hoophouse.
We do use a shade clothes on both our greenhouse and hoop houses though.
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Permalink Reply by Bill Fleming on
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Never heard of painting the panels, seems like that would be somewhat permanent and would be counterproductive during the colder times of the year. I've seen temporary paint that can be washed off easily made with lime, water and a little bit of milk to help it stick but think the real solution is going to be a shade cloth.
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