The plants will be fine with it closed as far as having enough "fresh air plants need to live". The problem with an enclosed space (in addition to excessive temps) is that if there isn't some kind of airflow tyou can get problems with disease, damping off, etc....It might be good to unzip it for a while in the afternoon if you can.

For example, something that indoor growers do, especially when growing in a confined space is attach a small clip-on or desktop fan to the shelf to gently blow over the plants. Airflow not only reduces the amount of surface moisture which can lead to fungal diseases, etc...., but it also helps to grow sturdier, stockier seedlings.

I'm not sure how practical that would be for your situation but it was just an FYI. I'd say unzip it for a while each day if you're home and it isn't cold out.

That's very helpful information, thank you!

I am in the same boat as you kinda. My DW made me go to Big Lots with her this weekend and they had a 6 X 6 "greenhouse" with shelves for 60 bucks. I like toys and bought it! It has the same type zipper door, just a little bigger.

Not real sturdy but I can make it sturdy with a few sticks of EMT that I have laying around.

I haven't completely put it together yet. I plan to put it on a wood foundation, but it's been raining. So I've been pondering on ventilation for it.

Since I'm an electrician and have all kinda good junk stored in my out buildings, I've came up with a plan.

I wish I had a program to give you a drawing, but I don't. So I will try to explain the best that I can.

Since I plan to put it on a wood foundation I can run 4" PVC pipe trough the wood. Then extend the pipe to the top of the greenhouse, heat rises, I'm sure you know.

I have an old fan that will attach directly to the PVC that I will put on a thermostat, used of course! Then on the opposite side I will put another 4" peace of PVC through the wood with a clothes dryer type flapper valve for fresh air.

The flapper valve will open when the fan comes on and close when its off.

CO2 is what plants need from fresh air. Seedlings are so small that they are not consuming much of it. Your cover won't be 100% airtight anyway, which is fine.

You'll be surprised how hot it gets inside in full sun. Ventilating it by hand will require a lot of attention.

I hope you did something to anchor it to the deck. I would use metal conduit straps and screw it down to the deck rail. If you can push it over with one hand, it's only a matter of time until the wind does that for you. You don't want to waste all of your hard work!

Wertach, that sounds like a fun project you have going. I wish I had enough level space in my yard for a 6' x 6' enclosure! I think with that size you are going to need ventilation for sure... Cole Robbie is right, it gets surprisingly hot. The sun came out today and it was over a 100 degrees in there. I hope I got to it before the seedlings fried. I am checking the temps constantly until I get the hang of using this thing.

CR, I appreciate that advice! It's heavy enough when it's loaded with trays, but I know eventually I should tie it down, just to be safe.