How it Got Started
Everyone gets started in the greenhouse hobby in a different way, I suppose. If you don't have the benefit of knowing someone who has a greenhouse and who can help you along, the choices can be overwhelming. Maybe my experience will inspire someone who really wants one but can't get "off the fence" and underway to GET GOING.
In the late 1980s I had been water gardening, in a small way at least, for about ten years when I added a couple of tropical water lilies to my collection. Immediately I was faced with the issue of what to do with them over the winter. Some merely treat them as annuals and throw them out. Others elect the wet sand or other storage technique similar to that used to over winter garden bulbs. But in zone 4 it takes too long to get a water lily going in the short summer, and besides that I had the idea of enjoying blossoms all winter.
That was the beginning of my basement "greenhouse". In March 1991 an article which I wrote describing that effort was published in the Water Garden Journal, the official publication of the International Waterlily and Water Garden Society, Inc.. An up to date version of that article is here, together with some comments based on ten years or so of experience with it.
Choosing a Site
A somewhat site related problem is that in the winter the short days combined with the perpetually cloudy skies in this area induce rather deep dormancy in many plants, most particularly the water varieties. Better orientation would possibly alleviate this situation, but spot artificial illumination may be the best solution if I decide to keep those species. If you are going to raise high energy plants such as tomatoes, etc., you should pay more attention to orientation and get the most sun possible. Bear in mind that the plants on the near-to-sun side of the greenhouse will shade other plants if they are tall, so you will need to place them accordingly. If everything is on waist-high benches it won't be as much of a problem. The winter sun is surprisingly low in the sky, and shadows can get quite long.
Single pane glass is the traditional greenhouse glazing. It is easy to clean and is transparent, but it is a horrible insulator and breaks easily. Even if heating cost was of no importance the constant condensation, water running down the walls and even ice formation would be unsatisfactory. Vulnerability to hail (a constant problem in my area) falling limbs and small boys also make glass unsatisfactory. One greenhouse manufacturer offers an option whereby a second layer of removable panes could be added, thereby increasing the insulation factor, but the added insulation in my view was not significant and the task of removing the panes for cleaning seemed far out of proportion to any benefit.
Several manufacturers offer double insulated glass sealed
with inert gas and containing ultra violet (UV) and infra red
(IR) barriers, but this arrangement is quite expensive and should
be combined with professionally installed framework. This, in
addition, requires an appropriate foundation. The result is a
beautiful greenhouse if you can or want to afford it. Although
I was not in a position to have to look for the least expensive
approach, the professionally installed aluminum and glass structure
was simply beyond my obsession. Having raised the subject of
insulation factor, here is the appropriate place to talk about
it. If you are considering building your own greenhouse or in
having someone build it to your specifications you will need
to have an understanding of it and may even want to "crank
through the numbers".
Where A=the area in square feet and T=the temperature difference
between the inside and the outside.
Now the equation looks like this:
25,000 BTU is a common heater size, and I didn't want to use
two heaters Furthermore, there's something you need to know about
gas heaters. Heating units such as household furnaces, greenhouse
heaters, etc. are rated with BTU INPUT. It is the OUTPUT you
are interested in. Manufacturers can't guarantee the efficiency,
which is dependent upon things that the user does. Older household
furnaces may be about 60% efficient, whereas the latest models
may be 90% or more. I selected the 25000 BTU vented Southern
Burner heater. I still feel it was an excellent choice after
2 winters of operation. After consultation with the manufacturer,
I finally came to the conclusion that the usable output was about
Still not enough. I compromised by saying that at temperatures below zero I would rely on supplementary electrical heat, but that still would not keep the internal heat to 70 degrees without supplemental heat, which is what I needed for the tropical plants. The first winter of operation proved that assumption to be correct, although I never did have to use an electrical heater to prove the point. Then I had an idea: By stapling (stainless steel staples!!) clear, 6 mil plastic to the internal framework on the sides and ends, I could add more than 2 inches of dead air space -- another R of 1 to those surfaces -- thereby, raising their R value to 3.5. The equation now became:
Close enough! Some other factors give a bit of safety to the number: 1. The outside door is sealed with 2" of pink poly in the winter. 2. The vents are sealed with pink poly and plywood in the winter. 3. The thickness and cumulative effect of all the framing effectively reduces the overall area, and 4. The portion of the structure which connects to the hallway to the house is not exposed to the outside temperature.
The UV coating on the polycarbonate should provide protection
from the sun and extend the life of the plastic. The need to
clean the area between the plastic and polycarbonate has not
arisen in over a year so far. A possible downside of this arrangement
for some would be that the light transmissibility of the polycarbonate
is something like 80% which is further reduced by the addition
of the plastic sheet. I have not found this to be a problem.
You basically have two choices -- either concrete block or pressure treated wood. Concrete block is the traditional choice, but it throws another contractor into the loop, and after it's all done you still have to go through the added insulation process. Pressure treated wood sounds a little spooky on the surface. After all, wood rots -- right? Not this stuff. It works great and can be insulated as it is being constructed. It has the additional advantage that you can nail, screw or otherwise fasten things to it easily. I added brown aluminum flashing to mine on the outside as a finishing touch.
The Drawings and Specifications
Making drawings and a specification is not as difficult a job as it might seem. Even if you plan to do all the work yourself you still need to prepare them for the zoning review. The zoning permit and process was simple for me -- practically nonexistent -- but for some it could be quite formal and demanding. The technicality of whether or not it classes an addition to the home and how it affects your home value and insurance are details you need to work out with your builder, the zoning officer and your insurance agent. Here are the drawings and specifications that I used:
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