Greenhouse Ventilation

Hey guys I am in the middle of rebuilding my greenhouse for the outdoor season and had a ventilation question, my GH is 20' x 18' stick framed to 9 1\2' tall with a pvc roof. I used 6mil agricultural plastic that's 4yr rated and lets 90% light in with UV filtering. I don't have any pics at the moment to post but here is a link to a video that I made that you guys can watch that shows the entire build process of my GH.


Last yr when running a hoop style pvc GH I was constantly fighting with heat in the dead of summer and had tons of cheap box fans I installed to try and keep good ventilation. This yr I went all out... killed the hoop house made a solid structure and ventilation has been the biggest priority for me. I got online and found a GH ventilation calculator and it recommended a 4230cfm fan to exhaust the GH, I personally thought that sounded a little on the low side so I used that as my bare minimum for cfm. I went to Harbor freight and found a sweet industrial shop fan that said it was 7700cfm so I bought that since it was more powerful than the bare minimum standard that I set..and to my surprise when I got home the box said on high speed its almost 10,000cfm! My guess is that it's not running 10k but somewhere around the 8k to 9k realistically. So now that I have a good fan for exhaust i installed it and all the walls of the GH suck in which is excellent, meaning the fan is really moving that air...now I'm looking for some sort of louvered intake port to install. My question for this is the guy at Lowe's told me that a 12" x 12" louvered vent would be plenty efficient for the intake air....seems like not enough to me...maybe 2 of them if anything.

So should I go with what the Lowe's dude said and only go with the 12" x 12"? It just seems like its to much restriction, but maybe that extreme negitive pressure is what I want. Anyone with good GH experience please if need some advice!

THX
BG420

I'd remove the plastic walls & use mesh, problem solved......

Agree with DJJ, use a shade screen cloth instead of plastic. I would also use a oscillating fan, whenever a breeze wasn't naturally flowing
here is a good spot for greenhouse help. I bought a fan from them, its a beast.
http://www.littlegreenhouse.com/fan-calc.shtml

ya I did that last year using shade cloth in some area's and screen in others and the air movement was still slow and stagnant. With this new exhaust fan I'm running I have the best air movement I've seen since starting GH growing. Any professional GH you see has good ventilation and I am just trying to figure out what is a proper intake size for some louvered vents. Should I be aiming for a higher neg pressure so the intake port is pulling air in at a rapid rate or aim for a slightly less neg pressure by using a bigger intake port so the air is being pulled in at a more gradual rate.

Oh sweet that's exactly what I was looking for GB...Thx Ya I plan to have about 1'-2' opening at the bottom lined with bug screen on the sides of my GH that have the roll up sides so that the fan is bringing in air from the lower parts of the GH making for air movement under my girls as the ex fan blows out all that hot air up top. I'm in Oregon so the flowering season will have rain and cold at the end before harvest and being in a GH makes it so nice to be able to let your plants finish all the way rather that cutting early to avoid boyt. and rot issues. Just looking for what is recommended for intake on a GH my size and thank you because you answered this for me!

this is just my 2 cents but I can see you having problems in the future with this design. It looks good, dont get me wrong but I think you may have put yourself in some future situations with its current layout. I see you pretty much used schedule 40 pvc for the roof. it good for hoop houses but a greenhouse this scale you should have went with schedule 80 electrical pvc. the white pvc isn't meant to sit in the sun, the uv rays will beat up the plastic causing it to become weak over time. all you need is one big gust of wind causing the pipe to flex and it could snap and rip your plastic to shreds. I would also suggest adding some more support to those pipes, maybe some perlins or another support beam. the first heavy rain your gonna get that roof will collapse. the arch isnt high enough for the run off and it will just puddle on top and eventually cause sags and rips. If your not financially tied down to this design I would invest in a higher arch and schedule 80 electrical pvc. I spent a few months designing a greenhouse very similar to this and had my engineer buddy look at it and he pointed out the flaws on my designs just I did for yours.

Do you get snow? Hopefully not! Glad you got good ventilation figured out.

Should be nice to tend in there after hooping it. Have a great season!

Ya I agree my roof should have gone up at least 6 more inches but so far all rain has ran right off of the roof so far....knock on wood!...lol. I really didn't think about the pvc and using the sch80 electrical stuff. My first hoop house the pvc lasted for 4 yrs and I am hoping for that here because my plastic is a 4yr plastic and if it lasts that long I will replace the pvc to sch80 when I replace the plastic. I have had 25mph gusts so far and it actually held up like a champ and barely budged. Making you own DIY greenhouse is always a learning experience and I know I still have a lot to learn thx Zest!

Thx Bugeye, ya I get snow here but its more or less in the way of a light dusting. The only issues I've had so far in the cold is after the summer my plastic has pretty much had it and it extremely brittle, so when it gets cold the plastic just wants to tear and crumble if it gets hit to hard. I hope to have remedied this issue by buying the agricultural greenhouse plastic, it's supposed to handle rain, heat and snow and has a anti dust film on it to help dust from clinging to the plastic.

Damn you Zest!!!! You are absolutely right on that the pipes need more support, I can definitely see where in time either side of the beam will sag. I will put up another beam on either side. Thanks a ton bro probably saved my roof...lol...and thx for everyone else's advice too, this is why I only get on rollitup!

BrownGuy420 said:

Damn you Zest!!!! You are absolutely right on that the pipes need more support, I can definitely see where in time either side of the beam will sag. I will put up another beam on either side. Thanks a ton bro probably saved my roof...lol...and thx for everyone else's advice too, this is why I only get on rollitup!

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no problem. also think about investing in some turn buckles for extra support.