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  • From: "Jim Fruth" <jfruth@tds.net>
  • To: "NAFEX" <nafex@lists.ibiblio.org>
  • Subject: [NAFEX] Make your own greenhouse for cheap
  • Date: Mon, 17 Dec 2007 11:35:45 -0600

Steve Herje said, " If you don't have a green house (I will one day),"

I'm sorry. I've been sitting on this idea for how long now and didn't think of telling anyone until reading Steve's post. Here is how I built my greenhouse for a fraction of the cost of a commercial model.
Go to Jenkins Crafted Canopies, http://jccshade.com/ . They sell ready made fittings that you can use to make your own greenhouse for almost any size you want it to be, providing you can live with a gabled structure. These fittings mate with any of the several sizes of electrical conduit. I say "almost any size" because the width of your greenhouse will have to fit whatever width of polyethylene film you use.
For mine, I used treated 2x4 for the foundation and drilled holes part way through for the uprights and corner braces at the corners. On the outside edge of this "foundation," I inserted eyescrews the length of every 3" securing them into the ground with bridge spikes (Bridge spikes are 12"+/- long nails that are available from a hardware store). You'll want to nail your greenhouse to the ground so a strong wind won't blow it away. After a season, the spikes begin to rust thereby holding more strongly to the soil.
Erecting the structure is pretty straight forward, especially if you use Jenkin's examples as a pattern. Constructing the door frame can be tricky, though. I made mine (on the gable end) with mid-height horizontal supports that connected to the corners. The lintel is long enough to touch the underside of both rafters and is secured to the rafters with hose clamps.
At the 3' level along the perimeter there is a 2x4 that is grooved to mate with the uprights secured to each upright with "U" bolts. I also used "U" bolts to secure a wooden door frame to the structure. At the bottom, along the foundation, I nailed 2x2's between each upright. Around the outside, at the bottom, I have 2" styrofoam insulation 3' high which is attached using sheet-rock screws. When you attach the styrofoam to the upper 2x4, be sure that there is at least 1-1/2" at the top edge to attach the plastic. around the perimeter.
Attaching the plastic: My local lumber store stocks 20' wide rolls of polyethylene so I made my greenhouse to fit. The greenhouse needs two sheets so there is a dead-air space between them to act as an insulation barrier. Attach the first sheet, tight enough, but not stretching it, to the upper portion of the 3' high perimeter 2x4 on both sides. Stapling it to the perimeter 2x4 with 1/4" staples is sufficient. The uprights should be no closer together than 2' apart or the bubble will occupy too much of your workbench space. Attaching the second sheet of poly might take some practice. It should have 6" to 10" extra on each side so it can balloon out to make the insulating space. Folds and extra plastic on the gable ends will take some practice to get it right. Since there is extra plastic on the gable ends, I recommend that the gables be on the East and West sides so a maximum amount of light can enter the sunlit side. Secure the two edges of plastic around the perimeter with 1-1/2" cloth tape - I used lawnchair webbing with 3/8" staples; lawnchair webbing should last several years.
A word about the plastic.....................If you want to spend the extra money, you can buy UV treated polyethylene that will last several years. I use one season plastic so I can cut out the sides (leaving the roof) during summer because a totally enclosed greenhouse will become like an oven by Mid-Summer.
I bought a commercial squirrel-cage blower ($75) for creating my dead-air space because it comes with convenient fittings. A friend of mine fabricated his own fittings and his works as good as mine except that he has to re-fabricate every time he puts new plastic on. You'll want to use outside air to inflate your dead-air space. Using inside air can create a partial vacuum inside especially if there air leaks in the dead-air space. Something else I buy from a greenhouse supply is clear UV resistant 3" wide tape to fix air leaks.
Keeping the greenhouse cool in the summer can be a challenge without spending alot of money for vents and blowers. I finally resorted to cutting holes in the sides of the plastic to augment my system of blowers blowing hot air out.
I never use my greenhouse until it becomes time to transfer seedlings out of my (crowded) indoor greenhouse, usually in March. I bought a fishhouse heater (under 10,000 BTU, I forget exactly) and never turned it higher than its minimum setting. It is amazing how little it takes to heat this greenhouse. One 100 lb. bottle of propane carries me through the Spring with gas left over.
If you have any questions, just ask

Jim Fruth
Brambleberry Farm
Pequot Lakes, MN 56472
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