Here's some info about a small greenhouse that I built in the spring of 2011. I thought some or all of it might be useful for others who are contemplating similar projects. I wrote this all one evening after putting the kids to bed, so please forgive any typos.
We live in the high desert in central Oregon where the nights are quite cool and the growing season is short. We can have frost in any month of the year (in 2010 we had frost in every month), and we often have damaging hail in late spring and in early summer. We became interested in having a greenhouse, primarily for extending the growing season and providing favorable conditions for tomatoes in particular (which don't like cool nights or hailstorms).
I had researched greenhouse kits, but I decided I could build a more effective and better-looking greenhouse for less money (excluding my labor, which comes cheap ;).
My primary goals for the greenhouse were:
- to extend the summer growing season and provide warmer nights (not to provide for winter growing, except maybe cool-season greens)
- to use pre-used materials where possible
-to keep costs down
-to have its appearance be reasonably okay
Here are a few photos of the finished product.
South side (in the summer the sun was high enough that the volunteer sunflowers didn't shade the greenhouse)
Nearly all of the windows (as well as all of the hardware and fasteners, and some of the lumber) came from our local Habitat for Humanity Restore. All of the windows except the two wood windows on the North side are double pane. I took the glass on the South roof (also double pane) from old aluminum sliding doors. I used the glazing method shown here, which worked great.
The greenhouse is about 11' by 12'.
Having bought the windows and doors with only a rough idea of the design in my head, I largely designed the greenhouse around them. I wanted to incorporate some aspects of passive solar greenhouse design, including:
-an insulated North roof and North Wall (I used 3" of rigid foam, in white so it reflects light back on the plants)
-orientation/roof angle to maximize sunlight and to provide heat gain without overheating in midsummer
-water storage for thermal mass for damping temperature swings
-no-maintenance passive cooling
With these ideas in mind, I did compromise on some of them based on materials and my other goals, including aesthetics (e.g. I wanted to have some windows on the North side, which faces the street, and we decided to use old single-pane windows there because they look cool-- they aren't great for retaining heat). The two doors are also single pane because they were cheap, though I did insulate their lower thin aluminum panels. Passive solar greenhouses typically have a steep south roof to catch the low winter sun and to shed snow, but because we were most interested in spring/summer/fall growing I used a shallower roof angle. This was also partly dictated by the length of the glass I had for the roof.
The foundation consists of 8 pier blocks buried about 1 foot (our frost depth is pretty shallow, and I wasn't too worried about it), with two layers of pressure-treated 4 X 4 on top. The framing is all cheap Doug-fir lumber, stained to look like cedar. Because I'm not a structural engineer and the glass on the roof is very heavy, I purposely overbuilt the structure. You can see some of the framing here:
The roof rafters on the South side are doubled 2 X 6. The central beam is double 2 X 10. The rest of the framing is 2 X 4. The siding is all cedar: shakes on the North side and cedar fencing elsewhere.
The above photo also shows the four 50 gallon barrels along the North wall that hold water for thermal mass. This is a little less water than is recommended for a true passive solar greenhouse (i.e. for winter use), but we didn't want to take up too much space for water, and it still helps mitigate the temperature highs and lows. Steel drums might have been better, since the steel conducts heat to and from the air more efficiently. I designed the roof shape so that the barrels are shielded from the high midday sun during the hottest months, but receive plenty of sun (unless they're shaded by plants) in the spring and fall when the sun is lower.
The ventilation system:
The photos above show the long roof vent and the two North windows, all of which are operated automatically by commercial greenhouse window openers, which are inexpensive and use a wax-filled cylinder with a piston that opens the window based on temperature. The roof vent is about 20 square feet, which is roughly what is recommended for a greenhouse of this size. I used double-wall polycarbonate for the glazing on the vent, because glass would have been too heavy (I also used scraps of this polycarb for the trapezoidal windows on the East and West ends). The basis for the ventilation is that a convection cell occurs, where relatively cool outside air from the shady North side of the greenhouse is drawn in and down (helping to cool the water in the barrels), then rises out of the roof vent once heated.
I also installed a used 50-watt solar panel that powers a radiator fan to exhaust air out of the West wall, but the passive ventilation proved this to be unnecessary (see below), and I didn't end up using it.
In the beginning of May we planted tomatoes, eggplants, three chile varieties, and basil in the greenhouse. We probably could have planted things a bit earlier (maybe next year). The temperature in the greenhouse after planting never dropped below about 45 degrees. Here's a chart of the greenhouse temp., outdoor temp., and greenhouse humidity for four days in mid-June:
A first thing to notice is that on the first three days, which were progressively warmer, the difference between the high temp in the greenhouse and the outdoor high became smaller. This is mostly because of the way the ventilation system works: The openers on the North windows essentially sense the temperature of the incoming air, so on cooler days they open less than on warmer days. This helps the greenhouse heat up more on cooler days (when extra heat is good, because cool days are often followed by cold nights, so it's good to heat up the air and the water in the barrels in preparation) than on hotter days.
You can also see that the greenhouse lows tend to be around 10 to 15 degrees higher than the outdoor lows, and that all temperature swings in the greenhouse are more gradual than outdoor swings, so you can get a sense of the inertia of the greenhouse as a complete system. It's definitely not as efficient as a well-insulated true passive solar greenhouse would be, but it's definitely better than an uninsulated kit greenhouse.
One more interesting thing to note is that the greenhouse humidity plummets when the vents open in the morning, and then rises again rapidly when they close in the evening.
The highs in the greenhouse were a little higher than tomatoes prefer, but only by a few degrees, and I think that that extra heat is probably good for reducing the nighttime lows.
Here's a chart of a few days in September, when the outdoor highs were in the mid- to upper 90's:
This is very similar to typical weather in July and August. Most summer we have a few days the reach 100 degrees, but we didn't have any in 2011.
Notice that the highs in the greenhouse are now lower than the outdoor highs, and that difference is greatest on the hottest days. This is partly a product of the ventilation system and the thermal mass, but also indicates a cooling effect. This is largely caused by the plants in the greenhouse (mainly tomatoes, which filled all available space after I got tired of pruning them). The plants block the sun so it doesn't heat surfaces in the greenhouse, and also cause a great deal of evaporative cooling through transpiration. Our climate is very dry, and I don't think this effect would be as great in more humid climates.
So, although I was afraid the greenhouse might overheat on hot days it didn't (and in fact stayed cooler than outdoors). However, if wasn't full of plants I would expect it to be at least as hot as the outdoors, and maybe a bit hotter. It might get hotter in more humid climates as well.
Here's one more chart, this one from October:
Now the outdoor temperatures are similar to what they were in early June, but the greenhouse doesn't warm up as much as it did then. Again this seems to be mostly because of the cooling effect of the plants in the greenhouse. As of this writing (in mid-October) I'm not sure how late in the fall the plants will survive, but I'm guessing fall frosts will pose more of a hazard than spring ones because of this effect. I expect that repeated nighttime lows below around 20 degrees will kill the tomatoes.
Overall, I'm happy with the performance of the greenhouse. It will be interesting to see what the temps are like this winter, and whether we can grow some greens. We also hope to start seeds in it in the spring. Things I'd do differently:
- Obviously, more insulation could never hurt. Insulated doors would probably make a big difference. The roof vent could also probably be opaque and insulated, though this would cut down on sunlight a bit.
- More water storage also couldn't hurt, which necessitates more space, which leads to:
- I'd make it bigger! And taller! (the indeterminate tomatoes that I had pruned to a single stem reached the roof by early July).
That's all I can think of. Thanks for reading. Time for bed. Feel free to contact me with questions-- see web site below.
Andrew Mowry, 2011
Links (remember when all web sites had a list of links at the bottom?):
Here's what I do for my day job: Mowry Stringed Instruments
Here's one of my favorite sites, with lots of info about greenhouses and other solar projects (I'm hoping for a reciprocal link ;): Build It Solar
One of my other favorite sites for people who like to make things: Instructables