Your greenhouse will be a very humid environment. Venting it or even air infiltration into your garage may result in condensation and possible structural damage. Otherwise I think you have a great plan.

A few thoughts:

Check into the strength ratings of the various polycarb multi-walls. I have been told (by the supplier) that they typically recommend 8mm twin wall for roofs as it is stronger than the triple wall and all but the very thick multi's. Of course the snow and wind loads for your area will be the determining factor, but I think that at 16"oc pretty much anything can work.

Might need to add forced ventilation, but that can be done later if needed.

You also might want to think about heat storage. Is your garage insulated, if so it might be able to store of your excess heat from the day for use at night (a passive system like just an open door might suffice for that). If not, then the garage probably won't help much. Wayne's point is a valid one, if you are moving GH air into the garage then it will get humid in there. This will be less of an issue during the winter, if the GH is empty. Depending on how you use the garage that may or may not be a problem.

Hmm, something about this project seems vaguely familiar. :)

On the 'other features' I'd add several handy electrical outlets and a light switch right by the door for late-night cilantro urges. I definitely underestimated how many outlets I need, not just for lighting but for heat mats and fans. The best setup would have outlets at both ends so you don't have unsightly extension cords.

I don't think you need evaporative cooling. You'll have great air flow with a door at each end that you can prop open, plus roof vents. We have a misting system which I think is quite a bit cheaper and in this arid climate the plants love the extra moisture.

And on that thought, I don't think the humidity would be a problem in the garage since it is so dang dry here. Would you have a use for heat in the garage on a sunny winter day like today? These are the days when I wish our GH vented into the house. I hate to waste perfectly useful heat in the winter!

One last note - isn't it illegal to steal street signs? Even in MT? ;)

I'm actually building a similiar greenhouse, just started last week. I'm going to be using 4x8" rafters 4' on center. With the low sun angle in the winter I think 16 oc would shade too much. I'm using 16 mm Macrolux multi wall for the roof and 10 mm on the walls. I'll only be using the poly on the lower half of the roof as any sunlight hitting the top half of the roof will give little benefit.

My greenhouse is 12'6" x 16' and I figured the materials at about $6500 so your $5000 budget might be a little low.

Upgrading to the thicker poly will save you huge amounts of heat/money and allow you to use less sun blocking framing.

Thanks all for the very helpful information. I'll certainly post pictures and considerations along the way.

A special note to karin_mt regarding the street signs. Greenhouse, garden, street sign refuge. In MT alone, thousands of street signs are shot in sport every year. With our humble effort, and that of many others' around the state, we hope to save these bold and reflective icons of civic order from wanton destruction. Would you not help us?

cheers!