My new greenhouse

If you live in an area with any kind of winter a greenhouse really expands the range of possibilities by increasing productivity and making gardening more comfortable and interesting. I first worked in a greenhouse over 20 years ago and quickly learned to appreciate their convenience and comfort. This inspired me to build my own greenhouse at the first opportunity and I have never looked back. Now I would find it very hard to live without a greenhouse and so every time I move I have to get creative and build another one. Over the years I’ve created quite a variety of styles: a couple of salvaged wood and polyethylene lean-tos (against the house), a couple of PVC hoop houses, a lean-to against a garden shed (made from salvaged cast iron window frames) and most recently a gable roof greenhouse I built using some single sheet polycarbonate glazing (I salvaged these from a commercially made plastic greenhouse that blew away when I neglected to weight it down sufficiently). All of these have worked well enough, but they weren’t very energy efficient. They didn’t retain much warmth on cold winter nights (they would all freeze on the coldest winter nights) and didn’t get warm enough for starting warm weather crops until March. My garden wish list has always included a solar greenhouse, which would retain enough solar heat to get early warm weather crops, and just recently I finally worked my way down to it. Building the greenhouse is a logical early winter project because you can use it immediately and for the rest of the winter, so I decided to build a solar greenhouse.

When I use the term solar greenhouse I feel I should qualify it by adding that all greenhouses are solar to some degree, but I use the term to mean one that receives and stores enough heat from the sun that it doesn’t require any supplemental heating. To achieve this the walls and roof that don’t face the sun are solid and insulated. This means some loss of reflected light, but this is more than made up for by the increased ability to retain heat overnight. There is also some thermal mass (usually 55 gallon drums filled with water) to absorb heat during the day and release it at night, which evens out the temperature.

I already had all of the lumber I needed to frame the greenhouse (in fact building it helped to clean up my garden), so my only significant expense was for the glazing (I only have roofing paper on the roof as yet). After some searching I found a local source for double wall polycarbonate glazing panels at a fairly reasonable price (at least I think it was reasonable - $65.00 for a 4x8 sheet, or $360 for five sheets). Polycarbonate is a fairly unpleasant kind of plastic and its manufacture involves some toxicity, but it is such a perfectly suited and durable material that I think its use can be justified for this purpose.

The climate here is fairly mild and for economy I didn’t make my greenhouse as energy efficient as I would if I lived in a colder area. I made the walls out of 2x4’s (rather than thicker 2x6’s which would have allowed for more insulation), I didn’t pour an insulated concrete foundation and I haven’t added insulation to the walls, or water barrels for thermal mass as yet. I’m trying to create an efficient greenhouse as cheaply as possible and plan to see how well it works as it is (I will add more energy efficiency if needed). My aim is to have a greenhouse that is warm enough to start warm weather crops in February (and maybe get a banana plant to fruit here).

Design

Before I started building I searched online and checked out various books for inspiration, but didn’t find quite what I was looking for, so I ended up designing it myself. I had some definite design criteria in mind when I started. I wanted a single flat front for simplicity of construction and wanted to use whole 8ft panels without any cutting or waste. I wanted to be able to cut the side panels from a single sheet, cut diagonally to produce 2 identical pieces (an unforeseen problem is that polycarbonate sheets only have UV resistance on one side, and by cutting it in half I got two identical left side pieces. This meant I had to cut the right hand side from another sheet to get the UV resistant coating on the outside. I also needed a fairly steep front to get enough headroom close to the front. Another important criteria was that I wanted the whole greenhouse to be movable (if I move again I want the option of taking it with me). I built it in sections that could be easily moved by two people and simply screwed together. I also wanted it to be inexpensive, so I used Douglas fir framing lumber for the greenhouse with pressure treated bottom plates where it is most likely to rot.

Building the greenhouse

I’ve been building stuff for years so this was pretty straightforward. It would have been a lot quicker to build each section completely on the ground and then simply assemble it all at the end. This wasn’t really possible because I did a lot of the design while actually building (I would come upon an unforeseen problem and have to find a solution). Instead I half built it on the ground and then moved it and did the rest of the work on site.

The finished greenhouse

The greenhouse is now close enough to finished that I’ve given it a coat of primer and have built benches around the inside. So far I’ve been very happy with the design, it actually meets all of the criteria I had and I’m now getting a feel for how it works (though I’ve already thought of improvements for next time). At the moment the sun is pretty low in the sky and it’s only getting about 5 hours of direct sun a day, which is enough to get it up into the 90’s F. My next step will be to add thermal mass in the form of barrels of water along the back wall and then I’ll think about insulation.