Greenhouse construction is the culmination of diligent planning, whether it’s the first greenhouse you ever built or an expansion to an existing facility. Well before ground-breaking begins, it’s critical to consider the workflow and the crop that will be grown within the facility to choose a structure that will provide a healthy return on the investment.
There’s a lot to consider during the planning phase and it can become overwhelming. From site selection to building material and glazing options to heating and cooling systems, planning a greenhouse is a complex process. Often, cost becomes the motivation for choosing one option over another.
“Cost is one of the main considerations that drives a grower’s decision-making process,” said Jeff Warschauer, vice president of sales at Nexus Corp. USA.
A business can’t survive without staying within comfortable financial limits, but deciding size, layout and building materials on price alone can compromise efficiency, crop quality and end up costing more in the long run than making the initial investment in an appropriately sized facility.
In the article that follows, Warschauer explained how choosing a greenhouse design and construction materials on cost alone can impact the long-term functionality of the structure.
Bigger than the budget
Building a greenhouse is a significant investment for any grower and cost does matter. However, the long-term implications of construction based on price alone can limit the return on investment and create an inefficient or inconvenient workflow.
Warschauer pointed out that growers often determine how wide or how narrow the greenhouse will be based on cost. At first glance, one may think that a narrower structure is less expensive to build. However, that’s not necessarily the case.
“A wider building is most cost effective, with a 42-foot wide building being the most popular,” he said.
Fewer posts are needed to build a wider greenhouse. Less posts equates to savings on material costs. Choosing a wider greenhouse not only saves on construction costs, but the design also encourages more vigorous crop production, which creates a more profitable return on investment.
“A wider structure reduces competition among plants as they grow and spread,” he said. “A wider greenhouse also provides greater flexibility in design for growing systems such as gutters, tables, irrigation, etc., down the road.”
The width chosen also impacts the type of glazing or covering. For example, if you opt for glass, it’s not well-suited for a narrow, round-roofed greenhouse. On the flip side, if you’re using double poly glazing, it’s not suited for a 42-foot-wide structure, said Warschauer.
Studying the difference between glazing options can make one’s eyes glaze over. Covering materials can be largely divided into three categories: poly, acrylic and glass. The initial investment can vary dramatically between the options. It’s important to weigh the initial investment against the longevity, crop performance and maintenance costs associated with each material. What appears to be a lower initial cost may turn out to be the most expensive over the long haul in terms of repair and replacement costs or lower crop performance. Warschauer explains the advantages of each material type.
A double poly plastic cover is by far the least expensive covering. The lightweight nature of the plastic also means that the supporting structure underneath can be made of wood or other materials that are also less expensive. However, this material is at risk for damage from wind, debris and ice; even rodents and raccoons can poke holes in the material. The plastic is rated to last four years at which time, manufacturers recommend full replacement.
“Some people try to eke out extra time before recovering the house, but the plastic becomes more prone to tears as it wears out and becomes brittle,” he said.
The mid-level budget glazing option is corrugated polycarbonate and acrylic. These alternatives are more expensive at the outset, but are rated to last 15 to 20 years depending on their geographic location. Polycarbonate and acrylic structured sheet glazing provides greater insulation but increases initial cost. Given the extra weight of the material a sturdier steel structure is needed to support the greenhouse.
“These greenhouses also use an extrusion system to fasten the sheets to the frame. That in itself adds extra cost. But the longer lifespan and greater insulation qualities outweigh the higher initial investment,” he said.
Over time the polycarbonate sheeting begins to yellow, especially depending on the amount of sunlight it is subject to. Acrylic will also yellow, but less quickly than the poly products. Acrylic has a higher r-value so heating costs will be less when compared with glass.
Glass is the Cadillac option for glazing. It offers the greatest amount of light and the longest lifespan. Warschauer said that upgrading from polycarbonate or acrylic sheeting to glass adds 50 to 60 cents per square foot to the overall cost of the greenhouse.
There’s more to consider when choosing a glazing besides cost. Crop performance can be enhanced or hampered by the material chosen. High-light crops grow better under glass whereas low-light crops will do well under a durable poly cover.
“Like any tool, you want to choose the one that does the best job the most efficiently with the best result,” he said. “An A-frame wide-frame glass glazing is the best tool for the job.”
The structure and glazing are only a part of the equation. Every greenhouse needs environmental controls that heat or cool and exchange stale air for fresh air.
Venting systems are critical for crop health and the design of the system largely depends on the crop grown. For example, in ornamental flower growers typically opt for open-roof ventilation systems because flower producers aren’t typically concerned about insects unless it’s a propagation house, said Warschauer.
Atrium type, chimney vents and butterfly are all types of vents and each has its own advantages; some provide better cooling; others enhanced dehumidification. The vents have individual motors with zone-opening ability, allowing you total flexibility in cooling and venting.
“You achieve outdoor environment when the roofs are open and have the advantage over outdoor growing when you can also close the roof and protect your crops,” he explained. “We have roof vents that can open anywhere between 25 and 100 percent.”
Conversely, vegetable and cannabis growers and research labs are much more concerned about pest control and opt for side and end vents with screens rather than an open roof and rely on large mechanical fans to cool the greenhouse.
A lean-to or bump-out on the side of the greenhouse can bring air in and screen it. Screens can be large to keep out big bugs like flies or Japanese beetles. Medium-density screens limit access to white fly and aphids. Screens can also be extremely dense, similar to women’s pantyhose, to prevent tiny thrips from entering the greenhouse.
The construction style contributes to performance. University of Massachusetts Amherst Center for Agriculture, Food and the Environment explained that there are two types of design: free-standing and gutter-connected greenhouses. According to UMass Amherst, “A free-standing greenhouse can have a quonset (hoop), gothic or gable roof shape. The quonset is usually the least expensive and is available in widths to 34 feet. Gothic designs have higher light transmission and shed snow easier. Gable designs may use trusses to span a width up to 60 feet.”
The free-standing design is usually the best choice for the small grower planning on less than 10,000 square feet of growing space. It is easy to build additional greenhouses as more space is needed. A separate growing environment can be provided in each house. Individual greenhouses can also be shut down when not in use. They are also better suited for heavy snow areas and non-level sites. Free-standing greenhouses are less expensive to build, as site preparation and erection costs are less.
On the other hand, gutter-connected greenhouse are a series of gable or quonset arches connected together at the gutter level. Individual bays vary from 12 to 30 feet in width and have a clearance of 10 to 16 feet at the gutter. Bays can be put together to get the width desired. Lengths up to 300 feet are available.
On its website, UMass Amherst stated, “A gutter-connected greenhouse offers the greatest flexibility. The vertical sides give good space utilization and a gutter height of at least 12 feet provides an air buffer and room for an energy truss and energy/shade screen. Gutter-connected greenhouses are usually most efficient and economical in a size of at least 10,000 square feet.”
Within a gutter-connected greenhouse system, the heating system can be centralized to control heating costs. UMass Amherst estimated that this can reduce heating costs by nearly 25 percent in an area of equal size in a free-standing greenhouse.
Wrapping it up
The cost of building a greenhouse varies considerably depending on building materials, glazing, site preparation, environmental controls and other add-ons that are necessary to operate a profitable greenhouse growing operation. Before investing or making decisions based on cost alone, take the time to understand how each option plays a role in crop performance, labor and ultimately return on investment.
To help you get started, the U.S. Department of Agriculture offers free software that can help you calculate break-even costs for crops based on the construction and operating costs of a commercial greenhouse. The software is called Virtual Grower.
This software allows you to input your location, the type of plants you plan on growing, the amount of lighting, the size of the greenhouse and costs for the greenhouse materials. Based on that information, it will provide annual costs for heating, cooling and lighting and give you a better idea of your operating costs before construction as well as a better understanding of the potential for financial sustainability based on those inputs.