This page is a compilation of my experience with the greenhouses here at Arcosanti, greenhouses I have seen, and greenhouses people I know have used with a great degree of success. I also highly recommend reading Alternative Technologies Transfer to Rural Areas (ATTRA) website page regarding greenhouse design. It's a comprehensive paper on all aspect of solar greenhouses.

For me I distinguish between two basic greenhouse style: south facing and cold frame (or hoop) greenhouses. The prototype greenhouse at Arcosanti incorporated many key aspects of south facing greenhouse design with a few nice additional features, but also a few important shortcomings.

SOUTH FACING GREENHOUSES
The most important aspect of south facing design is to maximize solar gain in the wintertime. This angle equal the latitude of the greenhouse local plus 10° to 15°. Ideally the rays from the sun on the winter solstice impact the greenhouse glazing perpendicularly, this maximizes the solar gain of the interior space.

Second to the slope of the greenhouse's glazing, insulation and appropriate thermal mass are the most important aspect of greenhouse design. These two things harness or mitigate the solar gain through the glazing. Depending on the extremes of climate greenhouse require two to five gallons of water (or equivalent thermal mass) for every square foot of glazing. This mean a greenhouse with 1500 square feet of glazing requires 3000 to 7500 gallons of water to stabilize its temperature making it ideal for plant growth. If possible, the thermal mass should receive direct sunlight in the winter to store more of the energy embodied in the rays and receive shade in the summer so it remain cooler. This can be accomplished in a multitude of ways, but the simplest way I have seen places the thermal mass against the back wall of the greenhouse where it naturally receive winter sun and summer shade.

Insulating the building goes without saying as this allows the thermal mass to hold temperature good for plant growth not that of the ambient outside air. In colder climates insulative curtains that can cover the glazing at night will help to reduce heat loss.

The third most important criteria for a greenhouse design is the number of hours per day that the growing area receives direct sunlight. Since the plants' growth rates vary directly with the number of hours of sun they absorb, more direct sunlight on the growing area means the faster the plants will grow. I must qualify this statement slightly as plants can only grow with adaquant moisture and temperatures. If the greenhouse dries out or becomes to hot or cold then plant growth stops. Therefore just like growing plants outdoors western shade can actually be beneficial even though the plants receive less light. Therefore in warmer area greenhouses are oriented up to 15° east of true solar south, or they may be built asymmetrically to capture more morning sunshine.

The last basic design requirement all greenhouses must possess is the ability to vent excess heat and moisture when necessary. In south facing greenhouses vent should be installed at the top of the greenhouse and below the glazing for best possible air movement. These vents need only be one-quarter to one-half the width of the greenhouse even in hot humid climates, additional venting effects the climate very little.

This list of basic characteristic a greenhouse should possess leave lots or room for design creativity and the following are some of the features I like the best in greenhouses. I prefer the terraced designs because they provide many different microclimates, and also feel that greenhouses need to utilize as much vertical space as possible thus seemingly expanding the square footage. In greenhouses I feel pathways should be kept to a minimum so gardening space can increase. Depending on the scale of the design I think insulative curtain make a big difference in nighttime temperatures.

COLD FRAME (HOOP) GREENHOUSE
Cold frame greenhouse provide much less protection from extremes of temperature than do well insulated south facing greenhouses, however the are much simpler and cheaper to build. The way I see it, hoop-houses can provide three key services based on their design. With benches and added heat they make ideal environments for starting seedlings in the winter and spring, they can provide frost and minor temperature protection for perennials normal too fragile to grow in a cold place, or extend the season of annual vegetable growing dramatically both in the spring and fall.

A typical hoop house will provide an environment approximately 15°F warmer at night than the ambient temperature without additionally heat. It also provide a good amount of frost protection when temperatures hover near freezing.

When starting seedlings in a hoophouse located in a cold climate the greenhouse will need additional heat. The most ingeniuos heating system I have seen to date uses a old metal woodstove and concrete blocks to channel and store the heat. With this system the stove sits at one end of the greenhouse with concrete blocks stacked forming the benches on which the seedlings grow. The blocks are stacked so there holes lineup horizontally. The heat leaving the stove through the chimney is channeled into this concrete duct where it heats the block on its way out vent at the other end of the greenhouse. The only alteration to this design I might suggest would be to replace the typical wool burning stove with a Rocket stove, whichs burns more efficiently and generates higher temperature with smaller amounts and quality of wood, and incorporate rainwater as a thermal mass similar to following perennial hoophouse design.

When using hoophouses to grow otherwise non-hardy perennials, the best design I have heard of uses a twelve foot tall hoophouse that channels all the rainwater and snowmelt from the outside of the greenhouse into canals inside the greenhouse helping to further buffer the temperature from the outside, increase the humidity inside, and can also be used for irrigation when necessary. Preferably wood planks would cover the canals to act as walkways and increase growing space. Other advantages to growing inside a hoophouse you never have to worry about late spring frost destroying your crops. I feel this design needs a way to drain the canals if the high humidity becomes a problem and increases disease, but other than that I think this design has great potential. I can't wait to build one next time I settle down.

Finally, when using any variation on the hoophouse design. Make sure to rotate your crops to avoid the buildup of pests and disease, or allow your greenhouse to move. Some people literally slide their hoophouse onto another piece of land each year as a means of soil and pest management. For those interested in knowing more please refer to Eliot Coleman's book, "Four-Season Harvest."