The canopy creates an air gap between the outer wall which is exposed to the outside elements (temp, moisture, etc.) and the inner canopy wall which is exposed to inside elements (temp, moisture, etc.). In my case it also reduces the volume of upper area I needed to heat. If you had a tunnel, it would be a tunnel with closed ends in a tunnel.
My canopy or one piece sub-ceiling with walls is supported by 1” balls mounted on the outer wall. The round balls help to minimize tearing the inner plastic. The first year I made it, I had to run nylon rope across to keep the bows bowed. After a year of being bowed they now retain their shape.
My greenhouse is doubled layered with a 1” air gap between the layers, but the top has gable ends that open for cooling. I felt that the upper ceiling area was a waste to heat and making a sub-ceiling, the canopy, would reduce the area I needed to heat and solve my air leak problems on the gable ends too. It has done both.
The south wall of the canopy only extends down a foot or so because the door is there and the wall is already double layered. There is a plastic roll down door on the outside that I roll down at night as a storm door. The north wall which is the back wall and the side walls of the canopy extend to the floor. The ceiling and walls are all one piece of plastic draped over a pvc frame.
SouthWall2.JPG
Dutch