Greenhouses are made of a variety of coverings and frames. The materials you choose can make a big difference to the heating efficiency of your greenhouse.
Traditional greenhouses originally were covered with glass. However, glass is one of the least efficient materials to retain heat, so greenhouse glass should be double or even triple strength to increase heating efficiency and decrease breakage.
Fiberglass usually comes in corrugated form and is translucent rather than transparent. Fiberglass scatters light to create a virtually shadowless greenhouse. Fiberglass retains heat more efficiently while transmitting less heat into the greenhouse than glass.
Fiberglass must have its gel coat maintained or it will eventually be baked off by the sun, will yellow, collect dirt, decrease the amount of light that can enter the greenhouse and become unsightly. UV treated polycarbonate provides much of the clarity of glass and is stronger and more resistant to impact than other glazings. It is also more fire resistant than other plastics. It comes in double walled sheets with many structural walls separating its two flat sides. It diffuses light to minimize shadows and has a 12+ year lifespan.
Polyethylene film covers about 90% of all greenhouse square footage in the US. It's popular with commercial growers because of its low cost and simplicity of maintenance. It lasts two to five years, depending on the thickness and UV treatment used, then can be easily replaced with new poly film. Used in single thickness, it is good for simple cold frames and greenhouses used for starting seeds and other seasonal needs. When used in two layers with the space between being inflated by a fan to create an insulated air space, polyethylene film retains heat more efficently than glass. It is important to buy it from a greenhouse supply company to be assured of the UV coating.
Traditional greenhouses were constructed of wood, usually redwood or cedar because of their natural resistance to the elements and glass. However, wood frames are larger and heavier than metal, thus increasing the shadow in the greenhouse. Eventually wood will deteriorate in the humid climate of a greenhouse.
Galvanized steel is used in most commercial greenhouses because it is long-lasting, inexpensive and strong enough to require less framework, creating less shadow in the greenhouse. It is usually covered in poly film.
Aluminum frames are usually used with glass or rigid coverings such as polycarbonate. It can be anodized in many colors and has low maintenance requirements. Not as strong as steel, an aluminum frame will have more members and create more shadow.
PVC pipe has become increasingly popular as a frame for greenhouses over the last five to ten years because of its extreme low cost, portability and ease of installation. Although not as strong as metal or wood frames, it can be strengthened with metal wire supports. Ideal for extending the growing season, starting seeds, growing transplants and winterizing plants and when covered with polyethylene film, is the most economical type of greenhouse.
To grow year-round or in excessively cold or windy climates, however, a metal frame that could be fitted with an insulated covering would be a better choice.
Other features to help growing
Lighting: Lighting your greenhouse benefits your plants and also allows you to garden after sundown.
Heating: Heating your greenhouse will allow you to grow all year round.
Ventilation: No matter what type of heating method you choose, you must keep the air circulating to prevent the heat from collecting at the top of the house. Heat retention is increased when the greenhouse flooring or foundation is rock, gravel, or concrete.
Cooling: A must if growing in hot summer. Most plants are happiest in temperatures ranging from 60 to 75 degrees. A vent in the rear of the greenhouse with minimal dimensions of 36" x 36" is advised and fans are easy and convenient methods of cooling. Shade cloth with 65% to 85% shade capacity is a very inexpensive way of cooling a greenhouse.
Misting: An irrigation mist system will help maintain moisture in the greenhouse, which is very important in prolonging plant life.
Knowing how you intend to use the greenhouse and establishing a budget will help you decide what kind of greenhouse you need and how many options and accessories to add. Because most gardeners tend to expand their gardening once they have a greenhouse, it is a good rule of thumb to get a greenhouse bigger than you think you need.
-- Tips courtesy of Service Magic