Greenhouse Structures and Coverings

Your choices will influence what other equipment can be installed as well as the type of plants that can be produced.

By Amy Rigazio

greenhouse - n. a building with roof and walls of glass, often heated, and used for growing flowers and plants that need warmth, or for forcing early produce (The New Lexicon Webster’s Dictionary of the English Language, Encyclopedic Edition, 1989)

The traditional definition of a greenhouse is much too simple to apply to today’s greenhouses. The roll of greenhouses within the agricultural industry has changed drastically during the past several decades. Now, more than half of all greenhouses constructed are not covered with glass, but with some form of plastic. As greenhouse growing accounts for a larger percentage of the agricultural industry, the design of facilities for specific crops becomes an ever-increasing concern for both the growers and the greenhouse manufacturers. A modern greenhouse is a complete plant growing system that not only includes a structure, but the covering or glazing, heating system, cooling systems, environmental controls, watering equipment, as well as material handling and plant support requirements.

The structure
A greenhouse needs to be more than a shelter from inclement weather. It needs to be able to support its own weight and that of the glazing material and it’s required components, plant support, material handling and watering systems, as well as local environmental loads. Since a greenhouse is subjected to many loads (ie. snow wind), it is important to know how it will be used and what it will be covered with before one is chosen. Therefore, the selection of structure and glazing material cannot be completely independent decisions.

Glazing
The glazing will most drastically affect the amount and type of sunlight to reach a crop. The glazing will also determine heat loss of the structure. Many types of glazings are available. The most common are polyethylene films, polycarbonate structured sheets, and glass panels.

Polyethylene film
This is the most common greenhouse glazing in the United States. It typically ranges in thickness from 3 to 8 mil for greenhouse applications (1 mil is 0.001”). Normal installation involves two layers, which provides a total thickness of 8-12 mil. Air is blown between the layers to inflate them to provide air for insulation. Polyethylene film is available with life expectancies of one to four years. One main advantage to polyethylene is its inexpensive cost, although it does need to be replaced.

Light entering a greenhouse through polyethylene film is mostly diffused, -- the direct radiation from the sun is scattered and can be received by the plant from all directions. A poly covered greenhouse lacks distinct shadows on a sunny day. An advantage to diffused radiation is that the scattered light can reach plants, which, under direct sunlight, could be located in shadows. This scattering of light allows plants to be produced on benches underneath a hanging crop without substantial shading problems.

Because of the light weight of polyethylene film, a structural support system is not needed. The material attaches directly to the greenhouse structure around all four edges. Many manufacturers produce various styles of attachment extrusions that can be installed around endwall bows as well as along baseboards.

Polycarbonate structured sheets
These semi-rigid sheets are available in several standard sizes and configurations. The two primary configurations are single layer and multi-layer sheets. The layer configurations refer to the number of walls separated by layers of air. Greenhouse applications generally see single layer (no air layer), double layer (two walls separated by air), or triple layer (three walls and two air separations). Architectural applications have a variety of other configurations available.

The air between the sheet layers helps reduce heat loss, but as the number of sheet layers increases the amount of light transmitted through the panels decreases. The grower must decide which is more important -- light transmission or insulation.

An advantage to polycarbonate structured sheets is their 10-year life span, which most polycarbonate manufacturers guarantee. Polycarbonate's longer life expectance will result in a higher price per square foot than polyethylene film. Light transmitted through polycarbonate, when compared to polyethylene film, has a higher percentage of direct radiation vs. diffused radiation.

Because the sheets are rigid and come in specific widths and lengths, installation on a greenhouse requires more components than polyethylene film. The sheets need to be supported horizontally from underneath at intervals that are specified by the polycarbonate manufacturers. Extrusions and point fasteners are required to attach the sheets to the support structure. Depending upon the layer configuration, splices are used between the sheets to form a continuous covering for the roof and / or walls.

Glass
Glass has the highest price per square foot to install, but also has the highest life expectancy. As glass is inherently resistant to ultraviolet radiation, it does not degrade over time and only needs to be replaced due to loss caused by cracking and chipping damage.

Panel sizes have increased over the past 50 years, but the support structure required for glass still surpasses that required for other glazing materials. Safety glass is available, and municipalities often have regulations for the type of glass permitted in roof in roof applications.

The percentage of direct radiation transmitted though glass is the highest of all greenhouse glazing options. Its high clarity, though, does lead to shadowing within a structure, as well as an amplified heat loss.

Structures
Once the light requirement of a crop has been determined and a glazing material has been selected, the structure can be chosen. Every greenhouse manufacturer produces structures with specific characteristics, but the basic types of structures are the same. The two most popular styles are quonset and gutter-connect.

Quonsets
Quonset structures are freestanding greenhouses that have either an arched profile or a gothic profile. Arched quonsets are often referred to as hoop houses because of the curved shape of the bows. These structures are designed to support polyethylene glazing material for either seasonal or year-round use. Hoop houses are generally no wider than 30’ with bows spaced between 4-6 feet apart.

As every manufacturer has individual designs, the heights of the structures, as well as the physical components, vary. These structures usually do not require concrete anchors, which may qualify the building to be designated as a temporary facility by some municipalities. As with all greenhouse construction, the local zoning board should be contacted to determine what permits, if any, are required.

Gothic-style quonsets are freestanding buildings that form a peak at the ridge. Gothic buildings are usually glazed with polyethylene film, but often can support polycarbonate sheets with the addition of framing members. These buildings are designed to handle a greater snow load capacity because of their roof pitch and increased member sizes. Gothic quonsets are available in wider widths than hoop houses, have higher ridge heights, and the bows are spaced either 4 or 5 feet apart. Most polyethylene covered gothic quonsets do not require concrete anchors, although they are usually recommended when the structure is glazed with polycarbonate.

Quonset Advantages
Quonset structures have many advantages for growers. The most obvious advantage is in the low cost per square foot of a polyethylene covered hoop house when compared to a more traditional style greenhouse. With no external framing required to support the polyethylene film, a quonset can be fabricated from lighter weight material and still safely protect plants from the outside environment.

Another advantage to freestanding greenhouses is the minimal number of structural components. This not only reduces the shading caused by overhead obstructions, but but also most growers can build the facilities without professional help.

Production in individual quonsets allows a grower to isolate each crop in a completely contained environment. Crops requiring different environmental conditions can be grown in adjacent structures without compromising the needs of any one plant.

Gutter-connected
Gutter-connect structures are more traditional in appearance, with straight walls and either a pitched or curved roof. The name gutter-connect refers to the ability for an infinite number of houses to be connected together, at the gutter, to form one large greenhouse range. Because these houses are designed to form ranges, the available sizes are limitless.

The widths of individual houses, also called bays, typically range from 17-42 feet. Available heights vary, but sidewall heights are usually between 8 and 15 feet tall, while the height to the roof peak depends upon the bay width and style.

Construction of a gutter-connect greenhouse, regardless of the number of bays, is often a job for professional contractors. These facilities require either concrete anchors or slabs to carry the tremendous loads applied to the structures. The height of the facilities also can make construction difficult and dangerous for individuals without the proper equipment.

Gutter-connect greenhouses are designed to be permanent facilities able to support greater environmental loads, as well as internal equipment and system loads.

Increased Versatility
In many ways, gutter-connect greenhouses are more versatile than quonset houses. Most gutter-connect structures can be glazed with polyethylene film or any configuration of polycarbonate sheeting. Glass covered greenhouses are only offered by certain manufacturers.

Because of the heights available and the load capacity of the structures, internal curtain systems can easily be installed to help increase heat retention at night or increase shading during the day. A curtain can be used in conjunction with a high light transmitting glazing so that when the curtain is open, the maximum amount of light reaches a crop, but heat can be retained when closed.

More extensive plant support systems can be installed in gutter-connect facilities than in quonset structures. Material handling systems, such as monorails, can more easily be suspended within the facility. As a range is typically enclosed by four walls and a roof, a single environment exists. If crops with different biological needs are being grown, internal walls or curtains can be constructed to form several smaller internal environments.