Tech solutions: What style of greenhouse is best?

Columns - Technology

July 21, 2011

Q. What style of greenhouse is best?
Planning for a new greenhouse is one of the most important steps a grower has to take. Proper site selection, orientation, size, glazing material and many other factors enter into the decision.

Generally for someone starting out in the business or expanding a small operation, the free-standing greenhouse is the best choice. For a larger established business that has good markets, the gutter-connect is preferred. Here are some considerations to help you make a decision.

Free-standing greenhouses are less expensive to build as site preparation and erection costs are less.

Free-standing greenhouses
A free-standing greenhouse can have a hoop (gothic or Quonset) or gable roof shape. The gothic shape is the most popular today as it sheds snow easier than the hoop shape. The square foot cost of a free-standing greenhouse is usually less than a gutter-connected house.

Hoop house designs are available in widths from 12 to 34 feet with the wider widths usually having a collar tie or truss to support more of the roof load. Gable designs are built to about 60 feet clear span. The wider the greenhouse, the greater is the efficiency in the use of space with less influence from sidewalls.

Hoop houses are available with 4- or 5-foot hoop spacing. Gable houses are usually 8 or 10 feet between frames. If the greenhouse is to be covered with polyethylene film, 48, 96 and 144 feet are standard lengths to better utilize the available plastic sheet lengths.
Gable houses can be built to any multiple of the frame spacing. With rigid glazing (polycarbonate, acrylic or glass) length is less critical.

Non-level sites
Although it is best to select a level site or have it leveled, free-standing greenhouses are commonly built on sites with one house lower than the adjacent one. Avoid this if possible as it makes materials handling and walking more difficult.

With individual greenhouses it is easier to add to the growing space. Some growers add one greenhouse every year as the business expands. Adding space can also be done without disturbing plant production.

Before beginning, plan on where future greenhouses will be built. Also, if possible, arrange the greenhouses so that they can all be attached at one end to a head house. This improves labor efficiency.

Choice of glazing depends on the greenhouse cost and style. Hoop and gothic shapes are usually designed to be covered with polyethylene film or flexible polycarbonate. Gable designs can be covered with any type of glazing.

Heavy snow areas
Free-standing greenhouses allow snow to slide off allowing light to enter. It is best to provide a minimum of 10 feet between houses for snow removal with power equipment.
Separate environments

With free-standing greenhouses it is easier to provide different temperatures as needed by the plants. Each house has its own heating and cooling system and separate temperature/humidity control.

Construction costs
Free-standing greenhouses are less expensive to build as site preparation and erection costs are less. A hoop house can cost less than $10 per square feet.

Winter shut down
Individual greenhouses are easier to close during the winter when fuel costs are high. The use of hot air furnaces or unit heaters eliminates the need to drain or blow out hot water pipes.

As the glazing area to floor area ratio is less in a gutter-connected greenhouse, heating costs are as much as 25 percent less than an equivalent floor area of free-standing houses.

Gutter-Connected Greenhouse
A gutter-connected greenhouse is a series of gable or gothic arches connected together at the gutter-level. Post lines support the gutters and drains are provided inside to carry the water away. Gutter-connected greenhouses are usually most economical when built greater than 20,000 square feet.

Advantages of gutter-connected greenhouses include:

Less land is needed
About 30 percent more growing area can be placed on the same amount of land area as free-standing greenhouses.

Individual bays are available in standard widths of 12, 24, 30 and 36 feet with the wider widths using trusses to carry the intermediate gutters. The trusses also support the energy/shade screens, hanging baskets, water, electrical and heating system pipes and supplemental lighting.

The length of the greenhouse can be any multiple of the support post spacing. Greenhouses greater than 200 feet may be more difficult to cool with fan ventilation.

Utilities are easier to install. Electrical service, water supply and computer systems are centralized making installation and maintenance less expensive.

Heating costs
As the glazing area to floor area ratio is less, heating costs are as much as 25 percent less than an equivalent floor area of free-standing houses. Heating can be simplified with one or two boilers instead of multiple heaters.

Cooling costs
With natural ventilation, open roof designs, fans are not needed and the temperature is more uniform throughout the growing area.

Greater labor efficiency
Carts and conveyors can be easily adapted as everything is under one roof. Movable bench/tray systems also adapt more readily. Labor is easier to control.

Greater plant density
Most gutter-connected greenhouses are built with a 14- or 16-foot gutter height to allow for the energy truss and to provide space for one or more levels of hanging plants. This takes advantage of the controlled environment that is provided and reduces the energy costs per plant.