Garden Articles - Greenhouse Basics: Part 2-Major Components of a Greenhouse

Foundation

The foundation is what the frame of your greenhouse is going to rest upon so starting with a square and level foundation is very important. Some plastic framed lightweight greenhouses can be set up over bare ground as long as the site has good drainage. Most glass greenhouses will require a concrete foundation. There are several materials to consider when deciding what kind of foundation to use for your greenhouse. Wood timbers, concrete or concrete block, and brick are all suitable choices. Follow the recommendations of the manufacturer of your greenhouse or your construction plans for the best results.

Flooring

Flooring is a personal preference. Commercial growers prefer a well-drained concrete floor because it eliminates dirt and weeds that can attract insects and harbor diseases. The light color of concrete also reflects light back on the plants. In a home greenhouse dirt is the cheapest option, but can be problematic for the home gardener for the same reasons as the professionals, not to mention be messy. Wood can be used as well, but it will also harbor insects and deteriorate over time. Brick is more expensive but it drains well and looks good. Gravel and rock are inexpensive, drain well and can be watered down in the morning and the heat of the day will cause evaporative cooling. It is also easy to install.

Framing

Framing comes down to what is suitable for your area and what you can afford. Most frames will be made from steel, aluminum, plastic or wood.

1. Steel is very strong and permanent and preferred by most commercial growers, but because of the high cost of shipping it is usually not available in home greenhouse kits.

2. Aluminum is long lasting and especially good for wet climates because it will not rust or rot. It can withstand extreme weather and is very low maintenance. It is frequently used in glass or polycarbonate greenhouses. Aluminum can be expensive and it is a conductor of the outside cold air unless the frame is made to include thermal breaks that keep the outside metal from directly touching the inside metal frame.

3. A greenhouse framed in wood is very attractive but will require more maintenance than other frames. It can deteriorate sooner because of the moist air in the greenhouse and will require a chemical sealant periodically. Wood is a natural insulator so it absorbs heat during the day and can release it at night. Wood frames are especially good for dry climates.

4. Plastic has become very popular in recent years because of its low cost and easy installation. A plastic framed greenhouse is also portable. It is a good choice for mild climates, but can be a problem in high winds and heavy snow. Plastic is a poor heat conductor so heat loss is minimized compared to a similar metal frame. The main disadvantage of plastic is that even with UV protection treatment the ultraviolet rays from the sun will eventually cause it to break down.

Covering

The covering material or glazing is usually the most expensive component of the greenhouse. It is also one of the most important because it is what allows the entry of light and heat into the greenhouse. The ability of a covering to retain or conserve heat is vital to a successful greenhouse. Each type of covering has an R-Value that represents how well a material insulates or "contains" heat. The higher the number, the more efficient the covering will be. There are many options to consider when selecting the glazing or covering for your greenhouse, each has its own advantages and disadvantages.

1. Glass is the most traditional covering for a greenhouse. It makes a beautiful structure that is fairly permanent. It is low maintenance once installed and can be recycled if needed. Glass can be found in single, double and triple layers with R-values ranging from .95 to 2.13, respectively. A single layer of glass is not very energy efficient. There are also many disadvantages to having a glass greenhouse. Glass is more expensive than other coverings and it is susceptible to breakage from hail and baseballs. Clear glass does not diffuse light well causing plants to burn and it can become brittle with age. It is very heavy and will require a substantial frame as well as a concrete foundation that is perfectly level and square. Professional installation may be required.

2. Fiberglass was the first practical substitute for glass. It is translucent instead of clear so it diffuses light and decreases shadowing. It is extremely durable and lightweight but even with a UV protected surface, it will break down overtime resulting in a decrease in the amount of light available to plants. It is made in a single thickness only and is available in flat or corrugated styles. It has an R-Value of .83. The corrugated style makes it more difficult to seal joints completely. Fiberglass is combustible and most types have an average life of about 5 or 6 years.

3. Polycarbonate is one of the newer options in greenhouse coverings. It is available in several widths and in single, double or triple walled sheets. The multi walled polycarbonates give strength and greater insulating values because of the air space built into the product. They also diffuse light and reduce shadows with 80 percent of the light reaching the plants. They are flame resistant and with a UV treatment on the surface can last up to 10 or 15 years depending on brand. The R-Value for polycarbonates range from1.43 for 4mm twin walled polycarbonate to 2.5 for 16mm triple-wall polycarbonate.

4. Polyethylene Film is used by approximately 90% of all commercial growers. It is available in several quality grades. These coverings do not last as long as the others, but structural costs are much lower because the frame can be lighter. They need to changes more frequently but are inexpensive, lightweight and easy to replace. Polyethylene has good light transmission and good heat retention. Two layers can be used with the air space between them inflated by a small squirrel cage fan. When used this way the polyethylene retains heat much better and the R-Value increases to about 1.5 from .83 for a 4 mil single layer. The downside to this type of film covering is its short life span of about 12 to 24 months and there is a possibility of rips or tears. Some high quality UV treated films made especially for greenhouses use can last as long as 5 years but are more expensive. Avoid using film sold at a hardware store for interior use.