For what it's worth. My first greenhouse was a pit/leanto built onto the side of a garden shed. It was 8x12 and had a door leading into the shed. The moisture from the GH got into the shed and the drywall panels i used to finish the shed ceiling (was a nice shed!) all sagged like 3-4 inches! I might not have had that issue if I used greenboard but i don't know. The shed was 12x12. The point being, there is considerable moisture in the GH. Note: I did use a rubber membrane along the south wall to prevent moisture from getting into the wall framing.

Mike, I tried to make a 5-1-1 mix best I could by sifting bags of pine bark chunks but this summer took someones advice and found a local Agway that has the perfect size bark for the citrus mix. I just need to find a large size perlite supplier. I mixed with retail bags of organic garden soil. I have some citrus in a turface mix too but its heavy and I will eventually repot in the 5-1-1 mix which is a lot lighter. I have also used farfad 52 which edible landscaping recommended. I use Neam oil and q-tips with alcohol and my fingers to remove bugs.

The wall there is partly garage which is unheated and (probably) uninsulated and partly (where the windows are) a chunk of garage that was carved out during construction to serve as a potting room with an open doorway. It's a little weird. Anyway, it's not quite the same as your friend is proposing. It has ceiling vents (which are stuck in the open position) for ventilation and looks like it was a pretty nice kit when installed, although it's just single-paned glass. I'm not sure I would have chosen to install it like that, though. It's slightly higher than the roof and presented a challenge for the roofer. Once we try growing things in there and using water in it we'll have a better idea of the humidity. I'm hoping it'll be more like a sunroom or enclosed porch than a swampy hothouse. I'm looking forward to cleaning it up after the earthquake. No wimpy acrylic ceiling glass here! :P (Don't let your friend put glass over his head--you guys are due too!)

It's been a awhile since I have worked with vinyl siding, but I would be concerned with the max temps your GH will reach in the summer warping the siding. Even if you ventilate it will likely hit upper 90's to 100F air temps, and depending on the color of the siding it could get quite a bit hotter on the siding itself. It might be safest to remove the vinyl siding from inside the GH when you add the attachment struct for the GH. Also textured vinyl lap siding tends to have lots of grooves, hollows, and spots to collect mold, etc, so you might want a better surface in there regardless of the heat issue, and a white one to reflect maximum light back to the plants. I would think that if you are putting a long 2x4 (or whatever) up along the upper attachment point with connections into each stud along its length, that would be adequate. Of course it will depend upon the snow and wind loads you get. You could check with a builder or civil engineer. If you could put a room on the size of your GH and the wall would hold, then it should be fine with the GH. Although you will likely need to use similar attachments for the GH as one would use for a room extension (remove siding under the attachment members, nail/screw into existing garage framing, adding it inside if needed). One concern may be frost heaving. If your gh "foundation" will be nothing more than some timbers laying on the ground, then it is likely that it will move as the ground freezes and thaws over the winter. That will cause added stress to GH where it attaches to the not moving garage. Something to consider. As far as insulation goes, if you don't insulate the garage wall to the GH, then you will loose/gain more heat thru it. If you were planning on using the garage as a heat sink that might be OK (although you'd probably need the rest of the garage insulated to hold that heat). But in general it is probably easier to insulate the GH-garage wall and put in fans/vents if you decide to use the garage as a heat sink/store. If you know how cold the garage gets at night and what the GH temp and outside temp is, you can do a quick calculation of the heat losses thru each surface. That would give you guideline as to whether insulating that wall would be worth it or not. Depending upon the numbers, you might find that insulation there gets you some additional above freezing time at each end of the season.