At roughly what height above ground would you install the horizontal metal channels for holding the plastic sheet to the greenhouse frame?

I’m planning to make a small (10-foot x 17-foot x 9-foot high) greenhouse by covering the frame from a Coverpro ‘portable garage’ structure with 6-mil UV-treated plastic sheet. To hold the sheet in place and to tension it I’m looking at using metal channels with wiggle wire.

If I’m understanding correctly, some folks will leave part of the bottom of the greenhouse open to allow ventilation air to enter, but at other times will want the plastic sheet to extend all the way to the ground.

What is a good height above ground to position the channels in order to both provide good tension and support for the sheet, but also to permit opening up the bottom area for ventilation?

I would think 18" to provide plenty of air movement, but you might want to set it up to go higher for ease of cleaning and moving materials. Are you planning to install netting, behind the poly barrier?

It depends on how far into the summer you plan on using the greenhouse, we put it all the way to the bottom because we only used it in the spring. Our stuff was gone before the real hot weather. I've seen the strips at the top of the straight upright, then the bottom can be rolled up in hot weather. We had greenhouse sheeting 2 layers, both different and a blower to inflate the gap for insulation. This was in north western Massachusetts. Got most of our stuff from Here

I am going to use the greenhouse throughout the summer months - the growing season in Connecticut is relatively short, and so I will need the max amount of days of growing for my tomatoes.

Although I see that you also described how you did things in Western Massachusetts.

(thanks for the link to the greenhouse supplier - I already ordered the sheeting and the metal channels and wiggle wire from Amazon.)

I am planning to add ventilation fans at one end of the structure, with screening for air intake at the opposite end, but if the sheeting is raised for ventilation I'm not sure whether the fans will be useful.

I had not given that any thought yet - netting for keeping out critters (including birds) and/or insects, or what else? Chicken-wire size or smaller holes?

I definitely will need to keep critters and birds at bay - planning to have an outer fence with electric fence wire, but that won't keep out the birds (and it will be my first time using an electric fence, so I don't know how successful it will be at keeping out smaller critters).

It's good to keep to air moving even if the sides are open. I would run fans that oscillate blowing on or around the plants as well as the venting. We had a 4 foot vent fan, there was also a 12 inch vent fan that ran when the furnace was running.