Structural Components
Rafters are the primary vertical supports of a greenhouse. They may be placed closer or further apart depending on strength requirements. Heavy glazing materials or wintertime snow loads may require more closely spaced rafters. Curved arch rafters are best suited to flexible glazing sheets that can easily conform to the shape. Truss type rafters are required for greenhouses more than 50’ wide.
Side posts and columns also provide vertical support and determine the height of the working area in a greenhouse. They may range in height from one to ten feet and this determination will greatly impact the overall efficiency of the greenhouse.
Purlins are horizontal supports that run from rafter to rafter and may be connected by a cross tie. They provide additional structural rigidity, which is especially important in areas with high winds.
Framing Materials
Framing is the basic supportive structure for your greenhouse; the most popular materials include aluminum, steel, and wood. Rigid PVC pipes or rebar can also be used in simple tunnel construction, although those materials are typically chosen for economy rather than durability.
Aluminum is the longest lasting and most economical option for commercial greenhouses since it’s lightweight and can be extruded and formed into virtually any type of structural component.
Wood deteriorates quickly in a humid greenhouse environment, and while some types of pressure treated lumber can perform well, certain types have been found to give off fumes that can harm plants. Be sure to keep up on the latest research both before and after purchase if you elect to use wood in your construction.
Steel is much heavier than aluminum, making it more expensive to ship and less suited for certain structural components that need support over long spans.
Covering Materials
The primary purpose of a greenhouse is to allow light in while preventing heat from escaping. This means the cover of a greenhouse must be essentially clear, insulating, affordable, and durable. Choices are wide-ranging and ever-increasing:
- Glass is the traditional covering for greenhouses, since the days of Victorian England. Glass gives the best light transmission but is heavy enough to require a special structural system, which makes it fairly expensive. Glass isn’t a great insulator, either, unless you use double or triple panes, which makes panes expensive and difficult to replace.
- Fiberglass panels are often used for commercial greenhouses - they are rigid, durable, and lighter than glass, so they don’t require as much structural support. However, fiberglass is very vulnerable to the effects of ultraviolet (UV) light from the sun. Over time, fiberglass panels transmit less and less light as they lose their transparency. In fact, fiberglass panels may need to be replaced as frequently as every five years.
- Polycarbonate panels are very strong and lightweight, compared to glass, and some are available with UV treatments to prevent yellowing. Panels may also be available in double or triple panels for added insulation. However, polycarbonate is very easy to scratch and is prone to problems with dust, algae, and condensation.
- Acrylic (also Plexi-glass, Lucite, etc.) is very strong but tends to contract and expand during temperature changes, so it needs special attachment hardware. Acrylic is one of the more expensive types of rigid glazing. Multiwall acrylic options offer protection against UV rays and provide 60% more thermal insulation than glass.
- Low grade Polyethylene (PE) film is inexpensive, flexible, and can be easily applied and trimmed to most shapes and curves. It can be installed in double layers, enclosing a pocket of air in between, to create a dead space which provides additional insulation. However, the least expensive grades of polyethylene sheeting aren’t very durable. Exposed to the elements, a PE cover can quickly become brittle and prone to shredding if it’s not replaced every year.
- Higher grade polyethylene greenhouse covers come in continuous woven sheets of a thick, flexible material, available in either single or double layers for better insulating properties. These reinforced polyethylene covers are translucent, allowing them to diffuse and bounce available sunlight to reduce shadows to aid plant growth. Some covers also offer:
- UV protection - prevents yellowing, brittleness and breakdown from sunlight
- Thermal protection - reflects infrared radiation (IR) or heat back into the greenhouse, which can maintain higher ambient temperatures at night.
- Anti-condensation - prevents condensation from forming on the plastic, which maintains light transmission and prevents plant damage from constant dripping.
Whatever your choice of material, commercial greenhouse growers need high quality, durable covers that are easy to order, ship and install with minimal delays. BTL Liners offers clear woven polyethylene greenhouse covers in 9-mil and 12-mil versions, both available with UV protection and an anti-condensation coating. Each of BTL’s ArmorClear products can be custom cut and fabricated to your exact needs, eliminating waste with panels that are too small or too large. At BTL Liners, we offer the fastest turnaround time in the industry so your greenhouses can be up and operating in a flash.
