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Greenhouse Construction Load Limitations Light Penetration Cost

Greenhouse Construction

When considering greenhouse designs, three major factors should be considered: load limitations, light penetration, and cost. The primary load considerations include snow and wind. Roof slopes of at least 28° and heated air in the greenhouse should prevent snow accumulation on the roof. Bracing along sides of the greenhouse and roof should be sufficient to withstand wind, particularly in the spring. Bracing along the roof also should be sufficient to withstand crop loads if tomato or cucumber vines are to be supported by twine attached to the bracing. A concrete footing is preferred for a permanent greenhouse. A wide door at one end of the greenhouse will ensure easy access for equipment.

Without sacrificing strength, support structures should be kept to a minimum to maximize light penetration. Glazing materials should be highly transparent. Overhead electrical lines, irrigation systems, and heating ducts should be kept to a minimum. Support structures should be painted with a reflective, light-colored material for maximum light reflection.

Most greenhouse crops grow best in light whose wavelengths range from 400 to 700 nanometers. This range of wavelengths is called photosynthetically active radiation (PAR). Most greenhouse coverings will accommodate these short waves of visible light. Polyethylene and fiberglass tend to scatter light, while acrylic and polycarbonate tend to allow radiation to pass through directly. Scattered or diffused light tends to benefit plants by reducing excess light on upper leaves and increasing reflected light to lower leaves.

Plastic glazed greenhouses have several advantages over glass greenhouses, the main advantage being cost. Plastic also is adapted to various greenhouse designs, generally resistant to breakage, lightweight, and relatively easy to apply.
Types of Plastic Coverings

* Acrylic is resistant to weathering and breakage and is very transparent. Its ultra-violet radiation absorption rate is higher than glass. Double-layer acrylic transmits about 83 percent of light and reduces heat loss 20-40 percent over single-layer. This material does not yellow. Its disadvantages are that it is flammable, very expensive, and easily scratched.
* Polycarbonate resists impact better and is more flexible, thinner, and less expensive than acrylic. Double-layer polycarbonate transmits about 75-80 percent of light and reduces heat loss 40 percent over single-layer. This material scratches easily, has a high expansion/contraction rate, and starts turning yellow and losing transparency within a year (although new varieties with UV inhibitors don't yellow as quickly).
* Fiberglass reinforced polyester (FRP) panels are durable, attractive, and moderately priced. Compared to glass, FRP panels are more resistant to impact, transmit slightly less light, and weathering over time reduces light transmission. This plastic is easy to cut and comes in corrugated or flat panels. It provides superior weatherability only when coated with Tedlar. Fiberglass has a high expansion/contraction rate.
* Polyethylene film is inexpensive but temporary, less attractive, and requires more maintenance than other plastics. It is easily destroyed by ultraviolet radiation (UV) from the sun, although film treated with UV inhibitors will last 12-24 months longer than untreated. Because it comes in wider sheets it requires fewer structural framing members for support, resulting in greater light transmission. Using a double layer of 6 mil polyethylene on the outside and 2 mil as an inner barrier will help conserve heat; this inner layer also will help reduce water condensation. The inner layer should be 1-4 inches from the outside layer with layers kept separated by a small fan (creating an insulating dead air space) or wood spacers. Two layers reduce heat loss 30-40 percent and transmit 75-87 percent of available light when new.
* Polyvinyl chloride film has very high emissivity for long-wave radiation, which creates slightly higher air temperatures in the greenhouse at night. UV inhibitors can increase the life of the film. It is more expensive than polyethylene film and tends to accumulate dirt, which must be washed off in winter for better light transmission.

Colorado Capitol Going Solar KRDO, CO - Nov 16, 2008 The administration likes to talk about a "new energy economy" for Colorado, and making government buildings more efficient is a step in that direction. ...

Town hall could be going solar in near future Brick Township Bulletin,  United States - Nov 13, 2008 BY PATRICIA A. MILLER Staff Writer The Mayor's Advisory Committee on Renewable Energy (MACRE) has zeroed in on town hall for its first solar panel project. ...

Merrillville's town hall going solar Chicago Tribune, United States - Nov 2, 2008 AP MERRILLVILLE, Ind. - The Merrillville Town Hall is going solar under a $27000 project to equip its roof with 30 solar panels that will turn sunlight into ...

Learn more about going solar Marin Independent-Journal, CA - Oct 27, 2008 GoSolarMarin and SolarCity have scheduled two educational forums to provide program details, an overview of the benefits of solar power and answer questions ...

North Carolina landfill going solar Cleantech Group, CA - Oct 27, 2008 Progress Energy Carolinas will buy power from a one megawatt solar plant to be built on a former landfill. Raleigh, NC-based Progress Energy (NYSE: PGN) ...

Pueblo Chieftain

Alamosa hospital going solar Pueblo Chieftain, CO - Oct 24, 2008 CHIEFTAIN PHOTO/MATT HILDNER -- The San Luis Valley Regional Medical Center will draw 89 percent of its power from a solar plant scheduled to be built just ...

Sacramento State is going solar Sacramento Bee,  USA - Oct 24, 2008 By Walter Yost California State University, Sacramento, along with other schools in the state university system, is taking part in a new solar energy ...

Businesses going solar phillyBurbs.com, PA - Oct 31, 2008 By JOHN ANASTASI After more than 50 years in the ceiling and insulation business, Bensalem's A. Tripod Associates is expanding into the growing solar energy ...

Going solar? Insulate first! Brattleboro Reformer,  United States - Oct 29, 2008 I moved to Brattleboro 28 years ago to work for an organization that was all about promoting solar energy -- an industry that blossomed out of the 1970s ...

Tax Incentives for using Solar Energy KATC, LA - 1 hour ago And the government has stepped in to offer some incentives of going solar. "There is a federal tax credit for solar energy as well. ...

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