The third day of erecting the greenhouse was spent mostly on installing the vent. This is a flap attached to the ridge of the roof, which opens as needed to release hot air from the inside. A feature of the National Greenhouse design that influenced my decision to buy this model is the ball-and-socket hinge attaching the vent to the ridge, which is shown in the larger diagram below. Other greenhouses I considered connect the vent to the ridge by two J-shaped flanges hooked together by their short arms, as shown in the smaller diagram; this arrangement didn't seem very substantial.

Installing the vent was a step I had been dreading, since it involved threading a 20-foot long bead (the "ball") into the socket on the ridge. But with three people on strategically placed ladders, it turned out to be a piece of cake (lubricating the piece with soap helped). After this, we put in the mechanism for opening and closing the vent.

Having been buoyed up by our progress in erecting the framework, I was discouraged to find, on closer inspection, that things weren't quite right with the vent --- for one thing, there were huge cracks where the vent didn't contact the header where it was supposed to form a seal. Well, that one turned out to be simple: I had put the wrong gasket in the header. Correcting that mistake helped, but after my helpers left I went back up on the ladder and discovered that the vent just didn't fit well at all --- it seemed warped.

So today's project was taking the vent apart and readjusting it. That sounds simple, but it involved first undoing the whole opening/closing mechanism. . . . progress backward. But it was worthwhile (even worth falling off the ladder), because now the vent almost fits!

Here is the vent from the outside (left), and the inside (right). It is fitted with screens, which makes it look darker.

Five lifter arms attach to the vent, and to the vent shaft inside. Each of these arms passes through a brush inset in the screen, which helps keep out undesirables when the vent is open:

Here's the motor that operates the vent:

As the motor pulls the silver tube into the black one, it rotates the shaft in a counterclockwise direction, which pushes the elbow arms up, which raises the vent (at least, that's the theory. . . . the electricity isn't hooked up yet, so we shall see).

So after two days' work on the vent, it seems to be okay except for the rather prominent gaps (green arrow) along its sides:

So, another phone call to National on Monday, to find out what the problem might be. I've been pleased with the support National has provided: all my phone calls and e-mails have been answered promptly and helpfully.

I sent close-up photos of the vent problem to National, and Vince DeForest pointed out that the vent overlapped the vent header unevenly. So, our fourth day of erecting the structure was spent in disassembling the vent header and figuring out why it didn't match up with the vent. . . . Much dejection, but we fixed the misalignment, and now the vent lies snugly along the length of the greenhouse. The cracks at the ends of the vent, however, are still there, but I think they can be sealed by attaching soft foam rubber strips along the under edges of the vent. And National will send flashing for the vent ends that will give them a finished appearance.