There are two common options people usually use for this.
Recycled glass cold frame
- Glass (especially multi-pane) has good thermal insulation
- Look good
- Last a long time
- Tend to be small (only as big as the glass you find)
- Difficult to move
- Shatter if a rock lands on them
- Snow will pile up on the
- Taller inside for taller plants
- Can be as big as you want
- Plastic deteriorates in about 2-3 seasons
- Wind can tear it easily
- Mediocre insulation
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Step 1: Plan
If you want to more easily figure out the ideal angles for your latitude you can use the attached (zipped) spreadsheet or you can simply work it out from the equation in the pic.
Even simpler still, it turns out that for 40º latitude the numbers work out to not far from 60 degrees for the front and the back. This is extremely convenient, since if you remember trigonometry class, 60 degrees is a perfect equilateral triangle! That makes this thing REALLY easy to build! Also, if I assume your latitude is probably like mine -- cold enough that you'll need a greenhouse and yet warm enough that you don't need a full-blown large, heated greenhouse -- I can say 60 degrees is probably a reasonable angle for your location as well!
Step 2: Materials
The only problem is they want like $150 a sheet for the stuff! Again, I probably should have just caved and got the right material but I discovered something else I wanted to try. Basically, that coroplast plastic they use to make signs out of... they actually make "clear" sheets (Cor-X brand) that are, well, not clear like polycarbonate, but more like the polyethylene sheeting you would use for your hoop greenhouse.
You can see what I mean in the picture. Notice how the concrete shows through, yet not as much as it would through glass.
So I grabbed some of that and some regular white coroplast. The greenhouse book I had said that white for the back wall is better than shiny because it prevents hot-spots. You can either buy a new sheet like I did or better yet, gather up and recycle the (usually illegal) marketoid clutter you see alongside the road. For that matter you could use a sheet of anything you can scrounge, really. Maybe even rigid foam insulation with some paint to provide you more R-value on the north side? In other words, pretty much anything and paint it white.
To hold it all together I used a weather-resistant roll of the handyman's secret weapon, duct tape.
(By the way. Cost for 1 1/2 sheets of 4x8 white coroplast plus 1 sheet of clear... these mini-greenhouses work out to about $40 USD apiece.)
Step 3: Assemble
Now it's just a matter of taping the whole thing together. It looks simple, and in a way it is, but you probably need a helper because floppy sheets this big are hard to tame.
Step 4: Experiment Results
Well, obviously I don't know yet. Only time will tell.
I'll visit back here in the future and post some results how well this did/didn't accomplish the goals it set out to.