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Greenhouse Kit Buying Guide

Eventually every gardener considers the purchase of a greenhouse. A greenhouse offers the opportunity to enjoy gardening every month of the year. This guide was developed to help you select from the variety of greenhouse kits available today.

What size greenhouse kit?
When choosing a size for your greenhouse, consider if it will be used year-round, seasonally, or mainly as a sun space. If you want a greenhouse to house an extensive collection of houseplants, then it is best to plan on building or purchasing a structure that is larger than your current needs. Most hobby greenhouse owners find they need a larger greenhouse than they originally thought. Upgrading later on may be more costly than ordering a larger greenhouse kit from the beginning. Another important size consideration often neglected is height. You want to make sure your greenhouse has adequate head room. You need to consider both peak and eave (sidewall) height. Taller houses are also easier to heat and ventilate because the air has a greater buffer area.

Is a building permit required?
Check local ordinances for required setbacks from property lines, design requirements, and other requirements. Call your local building department. Some hobby greenhouses may not need a permit, but it is a good idea to check anyway.

Selecting a Site for Your Greenhouse

Choosing the right site for your greenhouse will not only determine how well it works as a greenhouse, but how much you will enjoy it. There are several factors to consider in choosing the site to locate your greenhouse.

1. Sunlight
If the greenhouse is going to be used primarily for starting seeds and transplants or plant propagation in the summer, place it in partial shade to minimize heat buildup. You can use a shade cloth to control the amount of sunlight reaching the interior if a partially shaded site is not available. If the greenhouse will be used for growing in late fall and winter, or growing plants to maturity, it will need maximum exposure to the sun. It should receive a minimum of 6 hours of direct sunlight everyday. It is best to position the greenhouse with the ends facing east and west. This will provide more heat gain from the sun during the winter and create less shadowing in the greenhouse. If the southern exposure is restricted, but open to the east, southeast, southwest, or west, turn the greenhouse to the winter sun. Remember the difference in sun angles from summer to winter (the sun is much lower in the winter). 

Greenhouse Site Selection

Sometimes a shade tree can be an advantage, providing some shade for the greenhouse during the hot summer and letting the sun in after losing its leaves in the fall. The problem with overhanging trees is one of falling branches that can damage your greenhouse.

2. Accessibility
You want your greenhouse kit to be close by and easy to get to. A good site should be sheltered from high winds and easily accessible from your home and garden. Remember moving soil, plants, fertilizer, and yourself to and from the greenhouse will be much easier if it is in a convenient location. Access to utilities such as electricity and water are important requirements to remember also when selecting a site for your greenhouse.

3. Weather
Many regions have chronic weather problems such as heavy rain, snow, and/or strong winds. Heavy rains may cause drainage problems in and around the greenhouse. To avoid standing water, choose a spot on high well-drained ground or install a drainage system before the greenhouse is erected. Snow is usually not a problem as long as you provide adequate insulation and heating. Strong winds can be a real problem. In cold weather, winds blowing over a greenhouse can drain it of its interior heat escalating energy costs. Windbreaks are your most effective weapon. A windbreak is an obstacle that "breaks" up the force of the wind. Trees, shrubs, fences, and other structures can all be effective windbreaks. Remember that a windbreak can also obstruct light. Try to locate one where it will block the least amount of light.

4. Ease of construction and maintenance
A level, well drained site will obviously be easier to work with and maintain than a low, swampy, or sloped area. It is also a good idea to locate your greenhouse away from children's play areas.

What's the Best Material for Greenhouses?

There really is no best material for a greenhouse. It is hard to know what's best when there are so many different materials used to make greenhouses. Aluminum, galvanized steel, wood, PVC, glass, fiberglass, polycarbonate, polyethylene, etc. They all have their place in greenhouse construction. Your needs and budget will determine which is best for you.

Greenhouse Coverings

Most important is the covering. It will determine the amount and type of light reaching your plants, the overall appearance of your greenhouse, its safety, ease of maintenance, and longevity.

Greenhouse Covering Insulation (R) Values
4 mil polyethylene
4 mm twinwall polycarbonate
6 mil polyethylene
6 mm twinwall polycarbonate
6 mil poly double layer (inflated)
10 mm twinwall polycarbonate
11 mil woven polyethylene
10 mm triple wall polycarbonate
3 mm glass (single layer)
16 mm 5 wall polycarbonate
Two layers of glass (insulated)
Fiberglass or polycarbonate (single layer)
R value is used to measure the effectiveness of thermal insulation. A larger number represents a more insulation and therefore greater heating and cooling efficiency. Specific, brand-name, product R values may vary slightly from these figures.
- Glass -

The traditional greenhouse covering, preferred for its permanence and beauty. Glass is one of the least efficient materials for retaining heat, because it transmits heat and cold quickly and has very little insulating value (that's why it is used in cooking utensils and thermometers). Greenhouse glass should be double or triple strength to increase heating efficiency and decrease breakage which can be dangerous when installing as well as a problem in the completed greenhouse.

Glass is much heavier than other greenhouse coverings, requiring more substantial framing. Other disadvantages include: it doesn't diffuse light, so there's a risk of burning plants; glass breaks more easily than the plastic coverings (important if you have hailstorms, trees nearby, kids that play baseball, etc.); and finally, slight deviations from horizontal and vertical frame alignment or settling of the foundation can crack it. Most glass greenhouses use either engineered aluminum, steel, or laminated wood frames with full foundations. Never install glass on breezy days. Because of the need for many smaller, overlapping, glass segments in these greenhouses, site selection should take wind into consideration. Air (heat) leakage is greater in glass greenhouses because of the many panes needed.

If you are unsure about your building talents, you might do well to avoid glass as the frame must be absolutely square and rigid. If you must have glass, consider hiring a contractor for your greenhouse installation.

- Plastics -

These coverings include fiberglass, polycarbonate, acrylic sheets, and polyethylene film. All plastics resist hailstone damage and are shatterproof, a distinct advantage over glass. Rigid plastics are stiff, but not brittle. They can be flexed to fit over a curved surface and are available in large sheets. This reduces the number of potential air leaks by reducing the number of joints in the covering.

One of the most popular covering options, UV treated polycarbonate provides much of the clarity of glass and is stronger and more resistant to impact than other greenhouse coverings. It is also more resistant to fire than other plastics.

Polycarbonate is available in several different thicknesses and normally comes in single, double, and triple walled sheets with many structural walls separating its two flat sides. Single wall polycarbonate is generally used for its low cost and attractive appearance, but it lacks the strength, heat retention, and light diffusing properties of double and triple wall polycarbonate. The multiwall structure gives polycarbonate greater strength and superior insulating values with the air space built into the product. Multiwall polycarbonate also provides your greenhouse with an even diffused light that minimizes shadow and is optimal for growing plants. Another advantage of polycarbonate is its +15 year lifespan in most areas. Double walled polycarbonate is used to cover the Sunshine and Cross Country greenhouses. Grow More greenhouses come with thicker triple wall polycarbonate panels and Cross Country greenhouses are available with 5 wall panels which are the most insulated panels available. These are more expensive than single and twin wall polycarbonate options, but they will pay for themselves in reduced heating costs in cold climates that require frequent heating.

Polyethylene Plastic Film
A favorite of commercial growers because of its low cost and simplicity of maintenance. Use it for 3 to 5 years (life depends on poly thickness and UV treatment used) then recover with new poly. Used in single thickness, polyethylene film is good for simple cold frames and greenhouses used for starting seeds and other seasonal needs. When two layers are used, and the space between them is inflated with air, the polyethylene film retains heat more efficiently than glass houses, saving roughly 40% in heating costs.

Drawbacks to polyethylene plastic include a relatively short lifespan vs. other coverings, possibilities of rips and tears, and a translucent appearance much like fiberglass.

There are differences in polyethylene film. Cheap, thin, plastic films sold at many hardware stores and home centers are unsuitable for greenhouse use because they do not have UV inhibitors. Greenhouse polyethylene films are specially coated for protection from UV (ultraviolet) rays which shorten the lifespan of unprotected film. There's a minimal cost difference and a considerable difference in performance on your greenhouse.

Greenhouse Frames

Most greenhouse kit frames are made from wood, aluminum, or galvanized steel. Which material is right for you depends a great deal upon where and how you will be using your greenhouse.

Galvanized Steel
Most commercial greenhouses have galvanized steel frames because they are long-lasting, low cost, and require less framework (thus less shadowing) than any other framing material thanks to steel's natural strength. Steel's greatest value in greenhouse construction is its strength. You want as much light to enter your greenhouse as possible and steel frames can be thinner than others, creating less shadow. Its other big advantage is its low cost. Steel greenhouses are normally covered with polyethylene film because most frames are not designed to accommodate rigid panels without additional hardware. Be sure that any steel tube greenhouse you purchase is made with heavy-duty galvanized or stainless tubing which is made for outdoor construction purposes to protect it from a greenhouse's normal humid and corrosive (fertilizer salts) atmosphere.

Galvanized metals will eventually wear off their protective finish and rust from high humidity levels present in a greenhouse. Steel is much heavier than aluminum and generally requires additional hardware to mount a rigid covering to it.

Aluminum is used primarily in conjunction with glass or polycarbonate coverings and hobby greenhouses. It can be anodized in a variety of colors and has low maintenance requirements. Because of its higher initial cost, aluminum is most often used with glass and rigid plastic coverings in structures like the Grow More, and Cross Country greenhouses. Aluminum is the longest lasting of all of the framing materials mentioned because it will never rust, rot, or break down from UV rays.

Aluminum does not have the strength of steel so frame members either must be larger or more numerous. Look for engineered shapes in aluminum that are designed to increase frame strength, because you want as little frame shadowing as possible while not sacrificing the integrity of your greenhouse's frame.

Wood is most commonly used either for sunrooms or in homemade greenhouses. They are popular because of their attractive look, the ease in which accessories can be added to them, and the low amount of heat loss they produce compared to similar size metal frames. Wood frame structures are most often covered with a rigid plastic or glass. Though very attractive in sunspaces, wood has a limited lifetime in a greenhouse's damp atmosphere before it starts to deteriorate. Redwood (used in the Sunshine greenhouse frames) or cedar is recommended because of their natural resistance to the elements and insects. Applying a chemical sealant or stain to the wood periodically can also greatly increase the life of the material.

Wood frames are generally larger and heavier than equivalent metal frames which increases the amount of shadow in the greenhouse. Wood hobby houses are generally small scale with a limited ability to expand once construction is complete.

Rule of Thumb

If you buy a greenhouse kit based solely on your current gardening expectations, it will probably be too small within a year.