Building a geothermal greenhouse on your property means you have a reliable way to keep crops alive all year long. And guess what–you don’t have to live on top of a geyser to construct one!
I have almost shed tears when crops died that I put a lot of work into. Having a way to keep my babies—er, plants—safe from storms, pests, and deer is a dream come true.
There are several benefits to having a geothermal greenhouse. For starters, they are excellent for keeping your plants safe. Additionally, since it is heated through the ground, it’s free to maintain and you won’t lose your crops in the event of a power outage.
Check out the video to see how I built my geothermal greenhouse-LDS Prepper Style.
How Does a Geothermal Greenhouse Work?
First of all, yes, you can use geothermal energy to heat your greenhouse even if you don’t live right above a hot spring. Soil, especially as you dig deeper and deeper, naturally holds a certain amount of heat. Most methods of capturing geothermal heat involve some kind of pipe or tubing system that is placed underground.
Heat naturally moves from warm areas to cooler ones, so the pipes underground form a conduit that warm air naturally fuses into.
For my system, the pipes will have an inlet and outlet that ends above ground. I’ll use a fan to push air through in the inlet and as the air moves through the pipes it will capture that warmth and push it out the outlet.
It is important to use corrugated pipes for a project like this because the corrugations (little ridges) provide additional surface area underneath for the heat to transfer into your pipes. Additionally, be sure to choose perforated drain pipe for the portions of pipe that will be underground. The perforations will allow moisture to escape.
To calculate how much pipe you need underground you take the square footage of your greenhouse and multiply by 10-15%. So my greenhouse is 1,800 cubic feet. 1,800 x .10 = 180 feet. To be on the safe side you should try to use closer to 15% of your cubic footage, so 1,800 x .15 = 270 feet. I ended up getting 270 feet of pipe underground.
Now, most people who use geothermal energy to heat their greenhouses can’t realistically keep their greenhouses at say, 80-90 degrees Fahrenheit all winter long. A more realistic goal is to keep it above 50 degrees Fahrenheit, or at least above freezing.
Since I live in Idaho, anything above 50 degrees is downright balmy in the wintertime and will serve my purposes just fine of keeping my crops above freezing.
Pre-Made Greenhouse Kit vs. Custom Design
My greenhouse is a 200 sq. foot, 10’x20’ greenhouse, so on the smaller side. Almost all the design was taken from the YouTube channel “LDS Prepper.” David who runs that site has two big geothermal greenhouses that he built from kits, but I framed mine out with lumber.
Many people use a pre-made greenhouse kit that usually comes with a metal frame. Those kits are great if you don’t have the tools to make a custom design.
I specifically chose to build my greenhouse from lumber because I wanted some specific things out of my greenhouse I couldn’t get with a kit. Mostly, I wanted to be sure my greenhouse fit my backyard space well, but additionally, I wanted to make sure the walls were comfortably high for me.
Lastly, I wanted this design to have a double layer of plastic. Many people design a double-layer plastic greenhouse by draping the plastic sheeting over the walls of the greenhouse and then attaching a blower to the inner space so that the air kind of “inflates” the walls of the greenhouse.
This air barrier is what insulates the greenhouse. The main issue I see with this style of insulation is that at the bottom of the wall where the plastic is held together there isn’t room for air to flow, which could mean heat loss.
It also relies on an electric fan to keep the plastic apart, which, from an emergency preparedness standpoint isn’t ideal.
To avoid this potential heat loss and make it less reliant on electricity I’m attaching plastic to each side of the 2×4’s and I’m inserting a 2’’ Styrofoam board into the wall and roof panels on the north side of the greenhouse. This means that there will be a uniform distance between the plastic sheeting throughout the greenhouse and no need for a fan.
Now, it did cost me a little extra to design it this way because I’ll have to put wiggle wire on both the inside and outside of the greenhouse, but I think it will be less of a hassle and less of an energy drain at the end of the day this way.
Cost of this Geothermal Greenhouse
Now, for some people cost might be a determining factor when it comes to choosing a pre-made kit versus building your own. Here is the approximate cost breakdown for my geothermal greenhouse.
- Lumber: $450
- Wiggle Wire: $425
- Greenhouse Plastic: $215
- Drain Pipe (100 ft. roll x 3): $375
- Gorilla Tape: $50
- Mini Excavator (2 days): $415
- Trencher (1 day): $100
Note: If I had to do this whole project over again I would double the number of roof trusses from one every four feet to one every two feet. We get a lot of snow where I live and I think the added stability would give me more peace of mind.
Build the Foundation of the Greenhouse
- 4’’ corrugated drain pipe (minimum 180 feet underground)
- Excavator (rented)
- 2’’ Fiberglass boards
- Spray paint
- Wooden stakes
- 10′ long 4”x4”s (6)
- 7” metal spikes
Step 1: Mark the Foundation
First, I staked out the outline of the greenhouse with wooden stakes. I intend for this to be a 10’x20’ greenhouse. I strung a string between the wooden stakes a guide for painted lines, and then I used spray paint to paint the outline of the greenhouse.
I have two rectangles painted on the ground. The outer rectangle will be where the Styrofoam insulation goes and the inner rectangle will be where the interior wall is.
Tip: To make sure the foundation is square measure the two diagonal distances from front to back. If the cross distances are equal, you know its square.
Step 2: Dig the Foundation
I rented a mini excavator to dig out three and a half feet down. I had to have my son-in-law measure the depth as I dug so I didn’t go too crazy and dig way too deep.
Step 3: Lay the Pipes
The pipes need to be a foot away from the Styrofoam walls and a foot away from each other on center. I laid down seven rows of 4’’ corrugated perforated drain pipe all along the length of the trench and once I came to the end of the trench I looped it around and spread it back the other direction, making sure to keep the pipes a food away from one another.
I used temporary wooden stakes to keep the pipes in the right place while my son-in-law tamped the dirt around the pipes.
You need to tamp the dirt down pretty hard so that when you add more dirt on top it won’t crush the pipes.
Eighteen inches of dirt will go on top of the first layer of pipes, and then another layer of pipework will get laid down on top of that in the same configuration as the first layer. The two layers of pipes will be connected and this is essential for capturing the most heat.
There will be two ends of the pipe that are left sticking out of the ground, which I will cut down to a manageable length once I have more of the interior infrastructure of the greenhouse in place.
Step 4: Dig a Trench Around the Perimeter
I used a gas-powered “trencher” to dig a four-inch-wide trench exactly along the orange spray-painted border. In the trench, I’ll place an insulating Styrofoam board.
The trench was three feet deep, but ideally, it should have been three and a half feet deep (same as the foundation). I couldn’t find a trencher to rent in my small town that would go more than three feet deep. I think this should still work though because the floor of my greenhouse is six inches above grade, and those extra six inches are also insulated with the fiberglass boards.
The corners were a little tricky to dig out and I ended up having to just adjust the height of the fiberglass foundation board to accommodate for the slightly shallower trench in some places.
As I said I am building this greenhouse to be six inches above grade, which is why in the video you’ll see the fiberglass boards sticking up above the ground rather than being flush with it.
Step 5: Level the Foundation
The last step before building the structure is to level out the dirt floor you now have and tamp everything down.
Step 6: Build the Foundation Frame
I used six 10 foot long 4’’x4’’s. The width of the frame is exactly 10’, which meant I had to trim off just a little bit off the 4’’x4’’s because each one was just a little longer than 10’. I ended up trimming off about 7 inches off each end.
I attached the long sides with a metal bracket plate and three-inch decking screws. To secure the corners together I used 7’’ spikes.
The frame will sit just on the inside of the insulation board.
Build the Frame of the Greenhouse
- 12′ long 2×4’s (5)
- 10′ long 2×4’s (14)
- 8′ long 2×4’s (8)
- Chop saw
- Metal mending plates
*Note: Most of the 2×4’s will get cut down and I tried to give you the exact lengths below.
Step 1: Construct the Walls of the Greenhouse
The frame of the greenhouse will have six-foot-tall sidewalls with a roof pitched at 22.5 degrees.
The length of the greenhouse will be exactly 20 feet long and the short ends will be just under 10 feet long since they’ll be fitting inside the long side.
See 14:46 on my video to see how I joined the 2’x4’s together to make 20-foot sections.
I’ll frame it with 2’x4’s and that includes a 2’x4’ that will lay on top of the foundation frame. To have a six-foot-tall wall with two 2’x4’s on the top and bottom of the frame, the height piece will be 69’’ tall.
Each length-side wall will have five panels, and each one will be feet wide, *except* for the panel closest to the front of the greenhouse, which will be just under four feet.
Update: I also realized once I had the frame up that I needed to add a second 2’x4’ on each corner post so that the wall material I’m using had an anchor point. See 33:40 in my video to visualize the extra 2’x4’. I also added a second 2’x4’ around the door frame so that I have a spot to put the wiggle wire on and a spot to screw the door hinge onto.
Step 2: Construct the End Walls
The front and back of the greenhouse will be made of three panels each and will be identical although I’m only putting a door on the front side. The door frame will be the right size for a pre-hung door if that’s what you want to use.
- Four 35 ¾’’ 2x4s
- Four 84 11/16’’ 2x4s cut at a 22.5 degree angle on the top of each piece
- Two 38 ½’’ 2x4s
The door ends up being 38 ½’’ x 82 ¼’’.
Build the Roof of the Greenhouse
- Metal mending plates to join 2x4s
- Miter saw
The roof of this greenhouse will have a 20 foot long 2’x4’ along the peak of the pitch of the roof. The roof will have a 5-12 pitch and the length of the trusses for the roof will be 64 7/8’’ long.
Step 1: How to Calculate Roof Pitch
Remember that a 5-12 pitch means that for every 12 inches of horizontal length the pitch rises 5 inches. Since I have a 60’’ long horizontal distance from the side wall to the center of the greenhouse, and 60 divided by 12 equals 5, that means I have (5’’x5’’=25’’) 25’’ of height to the top of the pitch from the center of the greenhouse frame.
Using the basic geometry formula Pythagorean’s theorem, you can also figure out how long the truss should be if you’re not using my measurements. You can just google Pythagorean theorem to find an online calculator.
Step 2: Building the Roof
Technically, using the Pythagorean theorem, my truss should be 65’’ long however mine ends up being a little short because my miter saw can’t accommodate the precise angle of the pitch that I need.
Let me explain:
For a true 5-12 pitched roof you need the boards to sit at 22.62 degrees. On my miter saw I have a setting for 22.5 degrees, so that is what I’m going to use. All this means is that the length of my trusses will have to be just a hair shorter than 65’’ to accommodate for the slightly different angle.
To determine the true length of the trusses I cut two trusses with the 22.5-degree setting and then cut them down from 65’’ to make sure I get a perfect fit.
To cut the angles on the trusses I start with an uncut 2’x4’ and set the miter saw to the 22.5-degree angle notch. Then when you have the newly cut angle you take a speed square and line it up so that it’s flush against both ends.
Be sure to check out my video around 25:45 to see how I did this if you’re still scratching your head a little.
Step 3: Cut a Notch In the Trusses
Now the trusses will have to sit directly on the walls of the frames so there needs to be a notch cut out of the end of the truss. I cut a 66-degree angle that is 3 1/2’’ deep and then directly perpendicular to that cut I cut a 22.5-degree angle straight down.
Check out 29:00 on my video to see how I did this.
Add the Styrofoam and Plastic Walls
As I mentioned above the entire greenhouse will be covered in a double layer of heavy-duty plastic sheeting. In addition to the plastic sheeting, however, I’ll also have 2’’ Styrofoam boards on the north wall, north side roof, and the two end walls. On the south side of the greenhouse it will just be the plastic sheeting but between the plastic sheets will be a layer of plastic chicken netting to keep my chickens out.
In the heat of the summer, I’ll be able to roll up the plastic sheeting for improved airflow.
- Plastic animal netting
- Metal “wiggle wire” channels (enough to line the inside and outside, top and bottom of the greenhouse frame.
- Crystal Clear Gorilla Tape with UV protection (6-8 rolls)
- Styrofoam panels (2’’ thick)
- Styrofoam cutting tools
- 6 mil UV protected greenhouse plastic
Attach animal netting to the walls of the greenhouse. I ordered Sta-Green Wildlife Netting from Lowe’s and it wasn’t as thick as I wanted it to be but I think it will work ok.
Along the base of the roof on the inside and outside of the greenhouse I attached metal channels called “wiggle wire” which will hold the plastic. These channels will get attached to the bottom inside and outside of the frame too.
The wood has a rough texture and I don’t want it to rip the plastic, so I’m covering all the surfaces where the plastic might touch the wood with Crystal Clear Gorilla Tape with UV protection. I especially did this on the corners of the trusses.
The north-facing walls and the north-facing roof of the greenhouse will get insulated with 2’’ Styrofoam panels. I cut the Styrofoam with a special heated Styrofoam cutting tool I bought on Amazon.
Tip: When you’re using a heated cutting tool for Styrofoam it lets off some noxious chemicals so make sure you have a respirator or at a minimum a very well-ventilated area.
Attaching the Plastic:
Finally! Attaching the plastic to the wiggle wire is pretty easy. The wiggle wire has a space to push the plastic into and then it pinches and holds the plastic in place. Check out this video to see how it’s done. I left the plastic sheeting long at the bottom of the frame so that I can bury it in the ground and add another level of protection against pests.
I made my door frame the size to fit a pre-hung door, but I’m going to build my own door because it’s cheaper. I made the door with 1’’x3’’ pieces of wood so that a Styrofoam panel would fit inside of it. I made sure to line the wood with the Gorilla Tape and then I cut the plastic sheeting and lined both sides of the door.
I put some foam weatherstripping on some 1’’x2’’s and attached that to the frame of the door to improve the weatherproofing.