DIY greenhouse: How do I connect and seal the windows?
 

I've had the idea, for a long time, to build a greenhouse from discarded windows/glass doors. I've collected a lot of material and there's a couple ads in the Craigslist Free Stuff section right now for more. Footings and framing would be similar to a house or shed, but arranged to fit whatever windows I'm using. I could screw through their frames into the wood to secure the windows/doors to the frame.

How would I join the windows at their edges? For horizontal edges on the roof I can lap them, and for vertical edges on the walls caulk should be fine. What about vertical edges on the roof and horizontal edges on the walls? I guess I could lap the horizontal edges on the walls too but it would look awful. Maybe just caulk them and accept a little bit of leakage in driving rain? I still don't know what to do about vertical edges on the roof.

Edit: Actually, I guess the vertical joints everywhere will have 2x4s under them, so caulk should seal them well. Nevermind, I seem to have worked out the questions. Topic is open for comments and ideas though. :)

Do you care if things frost heave? With glass you very well might. If you go with piers, make sure your area doesn't have a lot of clay in the soil, since that can freeze to the pier and then the whole thing can heave. You can install foam (to about 48" out) to get around this or use pier forms that flare out at the bottom. That requires a lot of digging, so at that point you might want to consider a rubble trench foundation as well. Both are good options for DIY.

I'm not positive I am quite visualizing your description properly. By "vertical joints" I assume you mean a butt joint between two window frames on the roof? If caulk alone doesn't cut it, you can always put a strip of wood, metal or whatever over the joint and then seal that. Gives you a little more room for flex. Hopefully I have an idea what you meant.

I wonder, will old glazing compound stand up to long standing pooled water? If the roof windows have mullions and the angle isn't steep it will collect.

I built my own fixed windows for the mountain view here, but was starting with raw glass so most of my experience won't translate to your frames. And they're vertical, although we do get wicked winds.

Well, frost heaves would explain why my dog-area fence gate doesn't line up anymore after four years. I put in some decent footings and figured that would be good enough.

You understood my description correctly. Good ideas for sealing.

I was thinking of taking the glass out of the frames and then dealing with the glazing. I think all my material is double-pane modern stuff which won't make a very effective greenhouse if I don't at least break the seal and let the argon out.

Not sure I understand the letting the argon out comment. Is this a single pane greenhouse or a double? Lot more R value with double.

Are your winds strong? And if so, are they consistent in direction? Just some things to consider.

If it's just glass then I can give some input. The guy who sold me my glass (I bought a whole case since it was cheaper and then had him cut it to the sizes I needed) has very little faith in caulking done on the outside. He really prefers bedding the glazing in caulk when it's installed. Alternately you can get this double sided sticky foam strip stuff. A lot of vinyl windows use that and it's what I went with. Costs more than caulk, though, but I was paranoid about the wind.

I have a shed that's attached to my house and the piers heaved, so it angled up on the one side in winter. After a while the door wouldn't close. That's how I learned about the clay problem, since the piers were well below frost depth. Foam did the trick, but required a lot of digging!

Oh, yeah, are you going with all glass or just on the southern exposure side? Cut down your work a lot and you can use high R-value walls on the north then. I've always thought earth sheltered greenhouses were cool, but your topography has to cooperate.

Do you have an evaporative cooler for the greenhouse? You'll want it. I grew up along the gulf coast where it's normally 80% most of the time and even there they used evap coolers. It doesn't do much there, but it makes sure the air is as saturated as possible so sensitive plants don't dry out.

Keep in mind that even with single pane non-insulated windows you will want to have a forced air cooling system in a glass greenhouse. Your car is a perfect example of why lol.

Big ol' vent won't hurt either. My mother's greenhouse had a gas cylinder that would open a roof panel at a set temperature. I don't know much about the shades you often see on greenhouses.

Hmm...sounds like there's a lot more to greenhouses than I thought. I don't know anything about them, and I have what I call a "black thumb" -- opposite of a green thumb, anything I touch dies. The idea was to build it for the wife.

I thought greenhouses were supposed to get hot inside, and therefore single pane would be better than the high R-value double pane argon-filled glass.

I was planning to do it all glass with manually-operated vent windows. If I use something other than glass, my cost goes up; salvaged glass and salvaged 2x lumber is the material I have and can get for free. I don't have a source for plywood or siding.

I do have a little bit of OSB but I was planning to use that for a roof on my shed. My shed had a former life as a Schlitz truck body and has a roof but it's ugly. I put vinyl siding on it and built some nice gambrel roof trusses to put on top of it (which would then give me some more storage space on a second level under the new roof), but never got around to finishing the roof. I fear that the trusses are probably rotted out, they've been sitting outside for two or three years. I guess maybe this year I'll either put them up or rebuild them (or just build a gable roof).

In the winter running them warm works great, but it all depends on how your summers get. If you seal it up and let all that infrared in through the glass and then hold it in you've got a recipe for roasted tomatoes... fresh from the vine!

I think(read: THINK) it'll be okay if you put in a ridge vent of some sort, lower wall vents, and use that heat blocking screen outside the windows that get the most sun. Just for safe measure I'd install a simple thermostat with an exhaust fan though. Now, in winter you'll want it warmer, which might be when you just remove the screening.

Passive convection venting...sounds like a good idea and easy enough. Ridge vent is easy. Then a fan for the hottest part of summer.

Then agian, pre-roasted tomatoes might mean some good marinara sauce, right? :D