Greenhouse Structures and Design
Types of Greenhouses
Greenhouse styles can vary from small stand-alone structures to large gutter-connected greenhouses. Freestanding greenhouses are completely independent structures that come in a variety of different designs. The gutter-connected greenhouses are joined at the eave by a common gutter. Greenhouse designs can be simply classified as attached (i.e., lean-to), freestanding (i.e., even-span, uneven span, Quonset, arch), and ridge-furrow or gutter connected (i.e., sawtooth, Venlo). There are many designs and structures to select from. Thus, it is important to become familiar with the advantages and disadvantages of each.
Free-Standing Greenhouses
A freestanding greenhouse can have a Quonset (hoop), Gothic, gable roof shape, or A-frame. This type of structure is what one usually envisions when the word greenhouse is mentioned. The freestanding design is usually the best choice for the small grower requiring less than 10,000 square feet of growing space.
Advantages and Disadvantages of Free-standing Greenhouses
One advantage of a separate greenhouse is that it is easier to program and maintain the temperature to meet specific crop requirements. In addition, freestanding greenhouses are easier to ventilate without exposing the plants to erratic temperature fluctuations or harsh blasts of cold air. The light in a freestanding greenhouse is uniformly distributed over the entire growing area. In gutter-connected greenhouses, the roof gutters cut down on the light admitted and shade the crops, affecting their growth.
Quonset Greenhouses
Quonset greenhouses, also known as hoop houses or polyethylene tunnels, are constructed from pipe, PVC, or metal, bent into hoops, and draped with polyethylene film (Figure 1.2). Because the walls curve all the way to the ground, there is limited room against the sidewalls for either storage or plants, though it is possible to set a Quonset on columns to give taller plants room to grow.
Gothic Arch Greenhouses
Gothic arch greenhouses have an arched roof line and walls that form a continuous shape (Figure 1.3). The Gothic arch design of greenhouses has many advantages, making it a popular choice among horticulturists. Firstly, it is known for its ability to withstand strong winds and heavy snowfall, thanks to its half-teardrop shape of the roof. The pointed arches of Gothic greenhouses can accommodate a larger air volume than other greenhouse designs, resulting in better heat retention and energy saving.
Gabled Roof Greenhouses
Gable-style greenhouses have sloping, flat roofs connected to vertical sidewalls (Figure 1.4). Gable roof-style greenhouses use traditional welded or bolted trusses to support the roof. These are more expensive and complex to build, but they do provide plenty of internal strength to support curtain systems, boom irrigation, automated basket systems, and other equipment. The use of trusses allows gable-style buildings to span widths up to 60 feet (18m).
A-Frame Greenhouses
An A-frame, also referred to as an even-span structure, represents a traditional greenhouse design with a peaked roof of two equal-length slopes (Figure 1.5). The A-frame is constructed of trusses in the shape of an elongated A (thus the name), either welded or bolted together and affixed to gutter posts spaced on the center. Additional members, referred to as purlins or girts, are placed perpendicular to the trusses along the slope. The pitched roof (often 6/12 pitch) is the perfect angle to facilitate winter light transmission and increase interior condensate control.
Gutter-Connected Greenhouses
Gutter-connected greenhouses consist of several even-span greenhouses attached by a common gutter only at the lower edges or eaves of the roof. These greenhouses are sometimes called ridge and furrow. Gutter-connected greenhouses are composed of several bays or compartments running side by side along the length of the greenhouse. Typically, these compartments are approximately 120 feet (37m) long by 21 to 25 feet (6.5–7.5m) wide. The production area is completely open between the bays inside the greenhouse; there is no division of the structure by inner walls.
Greenhouse Height
The height of a greenhouse is based on the gutter height (distance from the ground to the bottom of the gutter) rather than the peak height. Typical gutter heights for greenhouse structures are 13 to 14 feet (4–4.25m) and are quite suitable for greenhouse crop production.
Light Distribution Patterns
Gutters or curtains placed in a north-to-south orientation will cast a shadow that is constantly changing while the sun moves east to west throughout the day. In contrast, gutters or curtains placed in an east-to-west orientation cast a relatively fixed shadow as the sun moves through the sky.
Advantages and Disadvantages of Gutter-Connected Greenhouses
Gutter-connected greenhouse structures occupy less land and have no sidewalls; therefore, fewer materials are needed for construction. Because there are no walls where the gutters are, more interior space is available than in several freestanding structures. Less energy is required to heat and cool the greenhouse because the exposed wall surface area is reduced. It is cheaper, and thus more feasible, to automate the single consolidated space inside a gutter-connected greenhouse than the multiple equivalent spaces in several freestanding greenhouses.
Sawtooth Greenhouses
Sawtooth greenhouses are constructed with one roof slope having a vertical side, all above the gutter (Figure 1.8). Depending on the manufacturer, the roof can be either arched or straight. In both designs, the distinguishing trait is that one roof slope is shorter than the other, creating a space for a vertical roof vent at the peak where the two slopes of varying heights converge. Louvers or roll-up curtains at the top of each vertical wall can be opened or closed as needed. Sawtooth greenhouses are often naturally ventilated.
Venlo Greenhouses
The Netherlands developed a ridge-and-furrow structure called the Venlo greenhouse (Figure 1.9). The galvanized steel superstructure supports a gable glass roof. The Venlo was designed to use single panes of glass from the peak to the gutter rather than lapped glass and uses no purlins or girts in the roof structure, thereby maximizing energy efficiency and light transmission into the structure. With the roof glazed exclusively in glass, the Venlo offers maximum light to the greenhouse crop, which is especially important in low-light northern regions and on crops that require maximum light.
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