A comment
I have been cultivating vegetables in closed ground for a long time. Almost every year in the country begins with the construction of a greenhouse from improvised materials. However, high humidity, frost, sun. During the season, both the construction and the majority of the covering material were rendered unusable. Having calculated how much I spent money over the past 5 years, I realized that I could already buy not just one, but at least one and a half factory greenhouses.
I chose the Kremlin Farm 3x4m on the site.the frame is reliable, durable of galvanized pipes 20x20 and 20x40 mm. Double arcs, withstand heavy snow load. The walls are vertical, it is convenient to walk at full height, seedlings grow under the wall itself, and feel comfortable.
For airing 2 vents on the ceiling and 2 rails. Doorways wide enough for a trolley to drive through. The only negative is the high cost, but you have to pay for convenience and quality.
pros
convenient in operation, reliable, tight