Constructing and Operating a Greenhouse in USDA Zone Four by Stuart Culp - Click images to enlarge

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.

Greenhouse space is much like computer memory -- you never have enough. After the addition of a few more non-hardy water plants, visits to the Caribbean and an ever growing obsession with tropical plants, the basement grow room was soon full to overflowing. Several issues were just not solvable:

1. The room was so tightly packed that watering was achieved only with great difficulty. I finally used a 6 foot length of PVC pipe with my watering can to reach many of the plants. The addition of an automatic watering and misting system only partially solved the problem and created a rat's nest of tubing. 2. Because of the need to keep the lights fairly close to the plants, only the lower 4 or 5 feet of the room was really usable. 3. The electrical input to the room necessary to operate the lights kept the room too hot-around 85°F or more most of the time. In the evening, when the lights went off and the temperature dropped a bit, the humidity skyrocketed and mold became a constant problem. The addition of automatic venting controls just didn't seem warranted for such a small room. 4. Although money wasn't really a driving issue, I was using about $800 of electricity per year to light the room. 5. What started out as a method of keeping the plants alive over the winter so they could be put outside in the summer backfired. Except for the water plants, everything started dying back in the summer, due to the cool evenings and unpredictable daytime temperatures, and needed the winter in the basement to recover. What started out as the simple matter of carrying out a few pots in the spring became a monumental, back breaking chore as plants (including trees) grew and multiplied. With my tenting and outdoor activity going full swing in the summer, very little time was available to enjoy the plants anyway.

It became an exercise in frustration. I had three options:

1. Get rid of 90% of it. By then my obsession with tropical specimen plants had outgrown my original water gardening hobby, and this simply was not an option. 2. Move to a warmer climate. Unless willing to move to southern Florida or to some similar growing zone, some method of protection such as a greenhouse would still be necessary. Family connections and a love of skiing also weighed strongly in favor of not moving. 3. Build a greenhouse. With hindsight, this decision was long overdue!!

Choosing a Site

The most difficult and pondered decision was where to put it. Ours is a typical suburban home on a 100 x 150 foot lot with trees in all the wrong places. The texts and references, of which there are many good ones, instruct on how to analyze shade over the four seasons, how to orient the structure to receive the best light and even how to pick the best roof angle to maximize light entry. But those instructions were just not consistent with my requirement that it be attached to the house in some way so that I could have easy access in all weather and in all seasons.  The final solution was to construct a hallway from the family room to a point where the winter shadows didn't reach and start the greenhouse at that point. In order not to block the view of the woods to the rear of the house, the greenhouse had to be angled 90 degrees from the ideal and a large tree had to be removed. The rationale that tropical specimen plants tended to require relatively low light levels proved correct. In fact, considerable shading is required in the summer, both to keep the plants from scalding and to keep the temperature under control.

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.

The Glazing

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".

Simply put, the insulation factor, or "R" value of a substance, is a representation of how rapidly heat will pass through it. Four factors determine how much heat will pass through: The R value, the surface area, time and the temperature difference between the two sides. "Amount of heat" is represented by British Thermal Units, or BTU's. A BTU is the amount of heat necessary to raise one pound of water one degree Fahrenheit. Heaters are rated by BTU per hour (BTU/hr). The equation that ties all this together is:

Where A=the area in square feet and T=the temperature difference between the inside and the outside.

To work with the equation, we need to know a couple of things about the greenhouse. First is the temperature value for the equation. Lets assume the worst case nighttime temperature will be -20°F. and under those conditions you don't want the inside temperature to go below +50°F. That makes T=50- (-20)=70. Next, we need to know the total area of the glazing. Let's assume 727 square feet (guess where that number came from). Finally, pick a number from Table 1. Let's use 1.0, which is what double pane glass would give.

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 16000 BTU.

Not wishing to compromise on the size (area) of the greenhouse and not knowing about the existence of multi-wall polycarbonate at the time, I was stymied for nearly a year. Only after a visit to A World of Orchids in Kissimmee, FL was I aware of the possibilities of that material. The equation now became:

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.

How Large and How Constructed

Having made the decision that aluminum framework was too expensive, the choice of construction material became wood frame. There are some really nice looking wood framed units available from a number of manufacturers, but two major drawbacks forced me to reject all of them: 1. The framework did not appear to me to be strong enough to withstand the snow load that might be expected and 2. The triple wall polycarbonate glazing was not available -- at least not in anything close to what I thought the price should be. I needed design drawings and a specification. The size of your greenhouse will, to a large degree, be dictated by your budget (or the size of your obsession). Just remember--whatever you build will be small on the day you move in. In my own case, a 14' x 20' floor area was the largest I could accommodate with one heater. It has given me plenty of room and although it went over budget it worked out about right. As far as cost goes, the price doesn't relate directly to size, because of fixed, or relatively fixed, costs which are independent of size. Examples of these are the heater, ventilation system, foundation, doors, lighting and any access passageway. The Foundation I can't emphasize enough the importance of the foundation in a cold climate. In my area the building codes require that the foundation footing be below the frost line, which can be 4 feet. You wouldn't want to violate that rule anyway, because frost heave would cause too many problems over the years. It should be insulated. In my own case, I use a dirt floor and have many specimens planted in the soil near the foundation. Heat loss could not be tolerated near or below the ground. All the plans of a well insulated GH can be ruined by a lousy foundation. The footing should be tiled and drained. If you are not sure of your soil percolation or if you have no drain field then install a sump pump. Believe me -- I know!! I could write a humorous story on that subject (it wasn't funny at the time)!

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: