Section 3: Common Greenhouse Designs
There are numerous greenhouse designs. However, most of these are derived from two basic designs: the quonset and the A-frame.
The quonset is based on an arched roof. The arched roof allows stresses (i.e. weight on the roof) on the structure to be efficiently transferred down to the ground. Quonset greenhouses may come in two basic designs. In the first, the arch extends to the ground with no sidewalls. In the second, the arch essentially forms the roof and gable sections of the greenhouse and is set on straight vertical walls.
The A-frame usually, but not always, has a series of supporting trusses that form the roof and gables. The strength of this structure primarily comes from the trusses set on vertical walls. The weight of the structure and other stresses are borne by the trusses and transferred to the vertical walls that in turn transmit the stresses to the ground. A-frame greenhouses may be even-spans or uneven spans. In the former, both roof sections are of equal length whereas in the later they are of unequal length (or missing entirely).
These two basic designs may be single stand-alone structures or combined side-to-side to form ridge-and furrow or gutter-connected structures. In this case, the interior walls where the spans are connected together are usually absent. Most commercial greenhouses now utilize some variation of the gutter-connected design. This is primarily because the gutter-connected design allows for a larger unobstructed interior than would be possible with numerous stand-alone greenhouses. Having a large unobstructed interior improves the ability to automate common tasks such as irrigation and improves space usage efficiency. Additionally, by eliminating interior walls (which would be exterior exposed walls in free-standing structures), the cost of construction materials and heating costs are reduced.
Several potential drawbacks exist for gutter-connected facilities when compared to multiple free-standing structures. Since the entire production area is a single space, the ability to maintain different environmental conditions (such as would exist with numerous individual structures) is limited. Additionally, as the size of the gutter-connected span increases, maintaining uniform conditions throughout the interior space becomes difficult. One way to minimize these issues is to have drop-walls or curtains made of polyethylene film that can be raised or lowered between sections. This allows sections within the structure to be partially isolated so that different temperatures or relative humidity levels can be maintained if only to a limited extent. Finally, it is easier for insect pests to spread through an open gutter-connected design than it would be with multiple individual greenhouses.
Greenhouses, primarily A-frame gutter-connected designs, are often referred to as being of "American" or "high-profile" design or of "Dutch" or "Venlo" design. The American or high-profile design is a traditional A-frame greenhouse with a relatively large roof area compared to the wall area. The “Dutch” or “Venlo” design has higher walls, smaller gables, and narrower individual greenhouse sections and reduced roof area, which reduces the roof surface area (an area of major heat loss) and heating costs. The higher sidewalls of the “Dutch” design provide more room (in height) inside the structure for hot air to rise and thus improves cooling. Although the terms “American” and “Dutch” typically referred to A-frame structures, they are now also used to refer to greenhouse structures with quonset roofs (with “Dutch” structures having smaller quonset roofs per section and taller sidewalls than “American” structures).
Greenhouse sidewalls support the roof and transmit stresses (primarily the weight of the structure) to the ground. In older greenhouses, sidewalls may be only 6' to 8' high (ground to eaves or gutter in gutter-connected design). However, in newer greenhouses, sidewalls are 12' to 14' (or higher) to better accommodate automation (or the production of high wire vine crops). Sidewalls may be covered with a translucent (allows light to pass through) glazing to the ground or the lower portion (typically the lower 3 feet) may be made of some type of insulated none transluscent material (i.e. concrete, brick, concrete blocks, etc.). In the later case, the portion of the sidewall glazed with the nontransluscent material is referred to as the curtain wall. In other cases, external sidewalls may be designed as a polyethylene film curtain that may be raised during low temperatures to retain heat in the greenhouse and lowered during warm weather to provide improved ventilation and to promote passive cooling.
Greenhouses may have stationary roofs with or without roof vents. Roof vents may open at the central ridge line such as in the MX design. Others may open farther down on the roof with the ridge line as the pivot or hinge point. Other greenhouse designs may have a quonset roof that detaches from one side at the top of the wall (at the gutter in the case of gutter-connected structures) and the entire roof opens (referred to as a clam shell greenhouse). Still other greenhouses may have totally retractable roofs. In this case, the roofs may be closed during low temperatures or when light levels are too high and opened to maximize light availability and to allow for passive cooling.
Numerous other greenhouse designs exist but are less common. The gothic arch is a design similar to a quonset, but this type of arch comes to a point at its apex and provides increased support and a larger unobstructed interior. Gothic arch greenhouses may be single-spans or connected as gutter-connected structures. The sawtooth, which is an example of an uneven span, is more common in high temperature locations since the design allows for improved movement of hot air out of the greenhouse roof vents. Sawtooth greenhouses may be single-spans or connected as gutter-connected structures. The lean-to design is most commonly used by homeowners and conservatories and is an uneven span greenhouse that is connected to another structure with the wall of the alternate structure serving as a sidewall to the greenhouse.
Public facilities, conservatories and botanical gardens often utilize unique greenhouse structures to accommodate public displays. These structures may take the form of geodesic domes, cylinders, classic Victorian structures. Still others may be totally unique structures of highly unusual designs.
