Each lesson culminates in an assignment which is submitted to the school, marked by the school's tutors and returned to you with any relevant suggestions, comments, and if necessary, extra reading.
What are the Options for Covering a Greenhouse
Through constant research new covering materials (or new ways of using old materials) are improved and released onto the market. Materials are developed to suit regional climatic conditions or even microclimates. Manufacturers for example have addressed issues such as excessive dripping, UV breakdown of the cladding and heat retention in poly-houses. This now makes the Poly-house an attractive proposition for a lot of growers as it is also a very cost effective and easily dismantled alternative to houses that have rigid cladding.
When choosing covering materials for a growing structure it is important to consider:
- Insulation (the materials ability to hold heat in).
- Light transmission (how much of the light reaching the greenhouse will travel through the covering). Some materials will become increasingly opaque over time reducing the amount of light being transmitted.
- Cost (some materials are far more expensive initially to buy).
- Durability (how much wear and tear they can withstand).
Glass is very rigid so it has low flexibility and can't be used on tunnel type structures. It is one of the best materials for insulation, light transmission and durability (will last 50 to 100 years), but the most expensive. It is generally more resistant to storms than most other materials, however they are often damaged during hail storm and can be dangerous, and difficult, to clean up if it is broken. It is more readily cleaned than most other covering materials (this is normally only required for some of the longer lasting materials). Glass for greenhouses is usually doubler strength, triple strength and tempered triple strength is also available. Low iron glass is sometimes also used and has the best ability for light transmission.
This is semi-rigid, with two layers of plastic joined together by a corrugated ribbing in between. Insulation qualities are good, light transmission is good but less than glass or PVC film, cost is reasonably inexpensive given that it will normally last for at least ten years. It has good resistance to inclement weather, and won't shatter like glass. Small puncture holes can be readily patched. It comes in large sheets or rolls and is easy to work with or handle. The large size of the sheeting also reduces heating losses because of gaps between sheets, which can occur with other materials used in smaller sizes (e.g. glass sheets). It is flexible enough to be used on tunnel and arch type greenhouses. Algae can sometimes be a problem on and between the sheets. It also tends to collect dust.
Corrugated PVC Sheet
A semi rigid material, of moderate to high cost, average durability, with good insulation qualities, but light transmission is lower than most of the alternatives.
Long lasting, very strong, expensive. Available in clear or smoky grey sheets, corrugated or 'Grecca'. Excellent light transmission, flexible and easy to work with. Collects dust. Looks good. They tend to yellow over a period of time.
Corrugated Fibreglass Sheet
Semi rigid, similar to acrylic coated PVC sheet but more expensive. Light transmission not as good as alternatives (i.e.: glass, coreflute, PVC Film). Becomes yellow and brittle with age. Not as widely used today.
A very flexible material that is cheap to buy. It has a short lifespan (a few years normally), and is susceptible to damage from hail, winds, storms, etc. There are many different types of films some have reasonable insulation and good light transmission properties, others are poor. Some have inhibitors that reduce the effect of Ultraviolet light, which is the major contributor to the breakdown of most of these films. Condensation forming on the inside can result in dripping (on plants and you), and can lead to heat loss. In areas where damage from weather is not a major problem then PVC film can be the most cost efficient covering material. It is sometimes used as a double cover to improve insulation, but some light transmission is lost. These films can flap noisily in the wind, particularly if poorly erected. Polyhouses are better sealed than glasshouses and tend to get hotter than glasshouses, and more humid.
Reinforced PVC Film
Same as PVC film, but with woven thread embedded in the plastic to provide reinforcement this improves its strength and durability.
There is slightly less light transmission, which at times can be an advantage, depending on the type of plants being grown. It is more expensive than standard PVC film.
Tin-skin plastics covers are also available and are manufactured using the latest technology. They are usually 200 micron on the outside and 150 micron inside and feature UV stabilised polymers, EVA infra-red heat retention and anti condensation drip protection