I have had two hobby greenhouses. The first problem I had was overheating in the summer and the second problem I had was heating cost in the winter. You are correct in considering cooling vents and heating before you build.

Think about a misting system too.

Thanks for the info. I think Im going to use one of my rain barrels for water. I have a 12 volt pump that I used for watering. Its quite handy. What would you reccomend for the covering. That polycarbonate stuff is kinda expensive. Plus I need something that comes in roll form. What reccomendations do you have for that? I thought I would use just a electric space heater for heat. Where would you get a mister, and how much do those cost. Im wanting to use this for seed starting, and for propagating berry bushes. thanks

You might also want electricity for lights for working at night, and an emergency heater should the gas run out (if you're using gas heat that is). Just be careful with the electricity of course ~ water being so prevalent in a g-house and all. My electrical plug is up at the top of one end of mine, up out of the water.

I have a greenhouse made of cattle panels and it's wonderful. Materials were about $600 for a 20' long one, but would be a good chunk less if you used the regular 16' cattle panels instead of the 20' ones (16' panels 9' apart gave me about 5'10" of height ~ 8' apart I think would get you 6' tall ~~ 20' ones like I have makes it about 7' tall). It's plenty sturdy enough to hang two rows of hanging baskets down each side. I built it like the one here, but used 4x4s on the bottom and closed in the ends with plywood on a wooden wall frame. Here's a not-so-good picture of it before the second layer of plastic was taken off.

I don't use automatic vents, but would like one in each end eventually. It's not been too hard to remember to open it up during the day and close it at night, but automatic is always nicer. I do have a large window in one end, aluminum framed one installed upside down so when it's open the open area is at the top.

I use two layers of 6 mil plastic held apart by lengths of pvc pipe insulation to form an insulating space. Enough plastic and pvc insulation to cover my 10'x20' greenhouse costs about sixty bucks. The plastic ($40 of that sixty) only lasts one year, but the insulation lasts years. It would be about the same price or a bit more expensive (even pro-rated per year) to buy four-year greenhouse plastic, but worth the extra since you only put it on once every four years and it won't crumble to pieces come spring (those pieces are a pita to pick up if you don't get the plastic off in time).

In my climate, I don't need supplemental heat in it if I'm only trying to keep things from freezing. Tomato and pepper seedlings, rooted cuttings, etc., all do fine when placed on the ground in my unheated g-house. Actually, even my tropicals do okay, though the tall ones smetimes get nipped on extra-cold nights (only a couple times a year). I'd bet I could put some floating row cover over them then and they'd still be okay. But you'd probably need extra heat. If you did two layers of plastic, you wouldn't need as much as with only one layer.

One other thing I did that's handy ~ I put it under some deciduous trees so it would be shaded in summer and get sun in winter. This works out really nicely. The plastic lasts a bit longer this way and I can still use it in summer as a potting shed.

These can also be used for any kind of shed. At the link, you'll see pictures of the first one I built years ago that's now being used as a chicken coop. Handy!

I forgot to mention that you probably don't need a real floor. When I wrap the plastic around mine, I make it long enough to tuck under each side and cover the floor (the entire house is wrapped up like a burrito), and then spread pine bark mulch on it. The mulch keeps the moisture in very well and keeps it nice and humid in there. And setting the plants in big, heavy pots directly on the plastic holds it on the house.

If you want to use real greenhouse plastic that's too expensive to buy enough to tuck underneath, I don't see why you couldn't just use weed barrier fabric, or some sort of crushed gravel for the floor.

Now I don't have any experience with ground freezing, so if it does where you are, you may need to build a floor to keep that cold out. OR, I've read that you can stack hay bales or some sort of insulating material on the ground around the house directly up against it to avoid that ~ what I've read is that keeps it from freezing under the house. Or the first suggestion of plastic and mulch (maybe an extra thick layer) over it may be enough on it's own.

Go on ebay, check out greenhouses, and in book section. If you can get one that is titled The Greenhouse Business. It is a big green book. It has a real good example of the greehnouse and what it takes to make one. It has many examples of material and equipment that goes in one, so you can make up your mind on what you want to use.