Introduction: Solar Hothouse


As a young boy I always loved walking through my grandfather's huge hothouse, admiring all the cool and exotic plants, and have wanted one ever since. Having a limited budget, I was fortunate to be able to find materials, such as the shower doors, storm windows, and door, either free or re-purposed from St. Vincent de Paul or Habitat stores, and craigslist. The cedar planks and fir posts were milled from windfall trees out of the forest in our back yard. My neighbor down the road offered to slice them up for free with his saw mill. The curved aluminum struts for the ceiling were scrounged from an ancient dish antenna. The exhaust fan from a wrecked Honda was found at a local wrecking yard, as was the battery which runs the exhaust fan. Among items I had to purchase outright (and not recycled) are the solar charger, double wall polycarbonate panels, passive vent actuators, some concrete, and anchor bolts and nuts, and screws.

I didn't draw detailed plans per-se, but did sketch out the layout of the shower doors to get some dimensions. Since I was able to find eight doors, I made the overall length to accommodate their collective width. The height evolved in much the same way. I knew I wanted about three feet of solid wall at the bottom, then add the height of the shower doors, then another section of cedar plank plus the height of the roof structure, and that allowed about the right height overall. And so went my design process. Consequently, the project took on a somewhat organic flow. Of course, these dimensions can be adjusted to suit your materials and design.

Step 1: Foundation

Like most such structures, foundation is important. And a light structure like this can be easily damaged by strong winds.

First, level the area. Then set a 1"layer of sand and level again (a large landscape rake works well for this). Then you can lay a layer of gravel or bark, or whatever you prefer to walk on. This can all be compacted if you like.

I decided to use galvanized post anchors set in concrete. Layout the location for your posts. I can't over-emphasize the need for the accuracy of laying out the post locations. If you don't get things lined up, square, and level, you will fight that problem for the entire remainder of the project. Since my structure is twenty feet long, I wanted a post in the middle, so I located three on each side. Use a string line, tape measure, and level to make sure of your post placement. Dig the holes about 1 1/2 ft. deep and about 9-10" diameter. You will need a concrete form to contain the concrete that flows above ground level, say 2-3". If you use cedar planks like I did, you don't want them resting right on the ground. You can buy cardboard tubes made for concrete forms, but I just nailed together some scrap wood. Mix the concrete and pour into the holes one at a time. Shuffle the post anchor into the concrete while checking the height and location with you level and measuring tape, and string line. Once you get it about right you need to brace it or block it in place with scrap wood or bricks, etc. Then check it again, adjust as required, and repeat for each post.

Let the concrete dry for about three days undisturbed. Then you can start bolting the posts in place.

Step 2: Walls

Once the concrete is cured and the posts are bolted in place, you can start nailing or screwing the wall planks on. I like screws because the are more secure, but also more expensive, your choice. This is a pretty simple process. Set the planks in place, check they are level and the ends are where you want them, and fasten to the posts. If you want get fancy and have a more weather tight joint, you can mill an overlapping rabbet joint or tongue and groove on the edges of the boards. Or you could just overlap the edges.This is probably a good idea in colder climates. Build your wall up to where you want the windows to set, constantly checking for level. and that the posts remain plumb. You may have to brace the posts to keep them vertical until you get a few boards on.

Based on the location of your top wall board and the dimension of the shower doors, add a stout length of wood across the top of the vertical posts. I'll call this the window header,

Now set one of the widows (shower doors) on top of that last wall board, and fasten your retainer board over the top edge of it, screwing it to the window header board. I used a plank in which I cut a relief the thickness of the window and about an inch high, so it overlaps the top of the window to keep it in place. From there you can push the remaining windows up into that relief and set the bottom on the top wall board. once the windows are in place, you can start adding vertical strips like 1" x 2"s to secure the sides of the windows. These vertical strips should be the full length of the wall and screwed to all wall planks.

Cedar weathers pretty well, but for added protection, this would be a great time to paint all exposed wood with some kind of preservative. This is an easy and economical step considering the added longevity provided for all your effort.

Step 3: Roof Structure

The roof structure was probably the most challenging for several reasons. First, I had these cool curved aluminum struts I salvaged from an old 10' dish antenna that I really wanted to use. But I knew I couldn't build it on top of the hothouse. At 6'-6" and 245lbs. I could visualize a horrible YouTube scenario. And, how would I possibly reach the edges of the poly panels to screw them down? So it had to be built on the ground, then lifted in place. I choose a clean level spot on the driveway for this.

I laid out the strut locations on a 20ft long 1" x1" piece of square aluminum tubing (a gift from an artist friend) and using simple metal brackets I cobbled up, screwed them in place. To hold the polycarbonate panels in place, I used 1/2" x 2" cedar strips which I heated in the shower for a while and bent in a simple jig to form the curve, then clamped to the struts to dry. I then covered the top of the struts with a foam tape to close any gaps, and placed the panels on them, screwing the cedar strips over them and into the struts. Once this was complete, I found that the structure was still too flexible to move, so I added another aluminum square tube on the outer edge. to make it more ridged.

At this point, four people with push sticks could have lifted it in place, but I couldn't find three more who were available, so I had to improvise. I fixed an 18' long wood beam to the bucket of my tractor and drove it into the end of the roof structure. At 20' out it was pretty sketchy, but after a bit of careful finesse (what a time for the steering on the tractor to go out!) I was able to set it up on the 8' walls. I had previously notched the window headers for the curved struts to set in so that the poly panels would set on the corner of the window headers and close off that junction., and with a bit of encouragement, all went into place. Then I screwed the struts, through the cedar strips, into the headers. This is where the previous careful attention to measurement, squareness, and levelness of the foundation, posts and all the rest really paid off.

Step 4: The Fun Stuff

This part is optional, but I already had this solar panel in place to charge a battery for a rain water pump for the garden. The panel is about 25' away so I dug a trench to run buryable cable from the charger into the hot house. There it attaches to a 12v car battery, which connects to the re-purposed car radiator (exhaust) fan. There is a limit switch in this circuit which is activated when the automatic thermal vent opener opens the vent window to a certain point, turning on the fan. This activation point is adjustable so that the fan isn't on until the temperature is high enough to really need it, which is determined by how far open the vent is. The battery will also power a small 12v car stereo (plants love music) and maybe some LED party lights.

Step 5: Where to Get

Double wall polycarbonate hothouse panels :

Windows, door: Any St. Vincent DePaul or Habitat store, craigslist

Roof struts (1" sq. Alum) : re-purposed from an old 10ft. dish antenna (there are still some around!)

Concrete, post anchors, bolts and screws, switch: Any home improvement or hardware store

Cedar planks: If you don't have access to windfall trees and a local sawmill, these are also available at home improvement stores.

Solar charger: Oak Harbor freight, ebay

Battery and fan: Local auto dismantler

Temperature activated vent openers: Many sources, but check your local Home and Garden Show for best prices.

More information: Bill

Cost: Really depends on what you can scrounge and re-purpose, but I've seen similar sized hothouse kits for 10 times my $600 investment.

Reclaimed Contest 2017

Third Prize in the
Reclaimed Contest 2017