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Whether you decide on a lean-to or freestanding structure, careful thought should be given to the different parts of a greenhouse. The foundation, the flooring, the frame and the covering material all play an important role in how much the greenhouse will cost and how well it absorbs and retains heat.
The frame of your greenhouse rests on the foundation and it is probably the most important part. Clearly, you’ll want to make sure you start off with a square and level foundation. Some plastic framed lightweight greenhouses can be set up over bare ground provided the site has adequate drainage. Glass greenhouses require a concrete foundation.
There are several materials to consider when deciding the kind of foundation to use for your greenhouse. Wood timber, 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 is a personal preference. Commercial growers prefer a well-drained concrete floor because it eliminates dirt and weeds that 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 with drainage issues and insect; it can also be messy. Wood is sometimes used, but it will also harbor insects and deteriorate over time. Brick, although more expensive, drains well and looks good. Gravel and rock floors are easy to install, inexpensive and drain well.
Framing comes down to what is suitable for your area and what you can afford. Most frames are made from steel, aluminum, plastic or wood.
1. Steel is very strong and permanent; it is also preferred by most commercial growers. However, because of the high cost of shipping, steel is usually not available in home greenhouse kits.
2. Aluminum is long lasting and especially good for wet climates since it does not rust or rot. It can withstand extreme weather and is very low maintenance. Aluminum is frequently used in glass or polycarbonate greenhouses. Aluminum 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 requires more maintenance than other frames. Wood frames are especially good for dry climates. Because of the moist air in the greenhouse, wood has a tendency to deteriorate if a chemical sealant isn’t applied periodically. Wood is a natural insulator so it absorbs heat during the day and can release it at night.
4. Plastic has become very popular in recent years because of its low cost and easy installation; plastic framed greenhouses are also portable. It is a good choice for mild climates, but the stress of high winds and heavy snow can cause it to break. 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.
The covering material of the greenhouse, also known as glazing, is usually the most expensive component of the structure. It is also one of the most important because the glazing 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. Higher numbers mean better efficiency.
There are many options to consider when selecting the glazing or covering for your greenhouse, each has its own advantages and disadvantages:
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.
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 stray baseballs. Clear glass does not diffuse light well causing plants to burn; clear glass can also become brittle with age. It is very heavy and requires a substantial frame as well as a concrete foundation that is perfectly level and square. Professional installation is often required.
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, fiberglass breaks down overtime resulting in a decrease in the amount of light available to plants. It is made in a single thickness and is available in flat or corrugated styles. 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. It has an R-Value of .83.
Polycarbonate is a newer option in greenhouse coverings. It is available in several widths and in single, double or triple walled sheets. Multi-walled polycarbonates give strength and better 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. Polycarbonate is flame resistant, and with a UV treatment on the surface, can last up to 10 or 15 years. The R-Value for polycarbonates range from 1.43 for 4mm twin walled polycarbonate to 2.5 for 16mm triple-wall polycarbonate.
Polyethylene film is used in approximately 90% of all commercial greenhouses and is available in several quality grades. These coverings do not last as long as the others, but its structural costs are much lower due to the lighter frame required. It needs to be change more frequently but is 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 fan. When used this way, the polyethylene retains heat much better and the R-Value increases from .83 to about 1.5 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; there is also the possibility of rips or tears. Some high-quality UV treated films made especially for greenhouses can last as long as 5 years but are more expensive. Avoid using film sold at a hardware store for interior use.
You are no longer limited to a glass greenhouse. A great variety of styles and materials exist. The greenhouse you decide to build will depend on your goals and financial constraints.
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