In looking at information about greenhouses, I was impressed to see that owners often had more problems cooling their structures than heating them. Since our summers here in central Virginia do occasionally top 100°, this was an important concern. That was the reason I decided to go for a model with full-ridge ventilation, and added a ventilation system consisting of a 20" exhaust fan and two motorized intake shutters.

There's disagreement in the literature about the placement of an exhaust fan and intakes: some writers advocate placing them at foliage level, so that the flow of fresh air passes through the plants, while others place the fan up high and the intakes low. To me, the latter arrangement makes more sense, since it exhausts hotter air and brings in cooler air; however, because I have a kneewall, placing the inlets low down involved leaving two openings in the wall, as seen in the photo on the preceding page.

Unfortunately, I didn't check the workmanship on these openings carefully enough, and when it came time to install the intake shutters, we discovered that they needed a lot of work. The major problem was that the floor of each hole sloped inward, so that rain would drain toward the inside instead of the outside. Our first attempt to solve this was to build up the inner side of this floor with a quick-setting instant cement. This stuff, of course, also wanted drain to the inside while it was still wet, and my quick trowel-work only resulted in flicking bits of cement over everything in the vicinity, including ourselves. Also, being inexperienced, I found it difficult to create the smooth, level finished surface I wanted.

And then --- after the cement had set, of course --- I got the bright idea of setting the intake louvers on door thresholds, which would give me the needed slope outward. So, guess what we did the next day ? . . . chiseled out all that cement I had slopped on the previous day, and yes, as claimed on the box, it does set up harder than concrete. Here's what the opening looked like with the threshold (or, to be more accurate, the half-threshold) in place. You can see the jagged results of our chiseling.
And here's the louver in place; I still need to do some caulking, and I plan to seal the exposed cement to protect it from water damage. I'll also fit a screen into the recess in summer, and a square of styrofoam in winter. Well, that little project took us only about six hours. . . .
On the inside, a small motor is mounted on the frame, with a linkage arm to the shutter control rod. I'll have these motors operating through a thermostat so that first one louver will open, and then the second one will open at a warmer temperature. The pipe at the lower right of the picture is the water supply, to be plumbed later.
At the other end of the greenhouse, we faced the problem of how to mount the heavy exhaust fan high up in the gable, and fortunately thought of separating the fan from its frame, which was the important part to install. Cutting out a 20"-square hole in the twinwall polycarbonate proved challenging without a fine-toothed circular saw. After trying a hacksaw, we finally used a heavy-duty utility knife. This was a tense moment, and I just kept telling myself, "Okay. . . okay, if we goof I can just buy another panel, so cool it." But it turned out reasonably well: a few ragged edges (thank God for caulk!), and the frame wasn't square so we had to trim the opening and trim again, but it worked.

To support the fan, we bolted one side to the center support of the gable framework, the other side to a vertical aluminum bar (green arrow) attached to the roof framework, and supported the bottom with an angle bar (black arrow) which we attached to the framework with T-bolts. This created a pretty solid mount.

The photo on the left shows the outside view. Because of our mounting method, the fan doesn't fit right up against the inside of the glazing, so I stuffed foam strips (the kind used to insulate around window air conditioners) into the resulting gap; these strips create the gray outline around the fan seen in this photo.

And here's the final product, from the inside. Much to my dismay, I discovered that the louvers of the fan flap freely in the wind, which not only is unpleasantly noisy but also will make heating this baby prohibitively expensive. I didn't realize, when buying the fan, that you could get either the model (like mine) with free-floating louvers that are opened by the breeze created by the fan, or one with motorized louvers, as on the intake shutters. Now I need to see if I can retrofit to the motorized version.