Greenhouse Building Materials: Should I Choose Glass, Polycarbonate, or Poly Film?
If you're planning to build your own greenhouse or to purchase a premanufactured greenhouse kit, you will need to decide what transparent material will be best suited for the walls and roof or your greenhouse. Depending on your application, you may need to consider various types of glass, polycarbonate, greenhouse plastic, or other materials. Every material will have its own set of price, strength, durability, maintenance, and aesthetic considerations. Not all materials are practical in all climates or for all styles of greenhouse construction. The materials guide below will introduce you to the most common materials used for the light-transmitting roof and walls of greenhouses and explain which materials are best suited for which types of greenhouse.
Overview of Materials
Polycarbonate is a less expensive option than glass but has a number of features that may make it the best option for some greenhouse installations. Polycarbonate is lightweight and easy to work with, which do-it-yourselfers may find helpful, and when properly treated with UV stabilizers, you may get anywhere from 10 to 20 years of useful life out of the panels. Polycarbonate panels are also available in a wide range of twinwall and triple-wall panels, so you can use polycarbonate in many applications where insulation is important. On the downside, like any plastic, polycarbonate will eventually degrade from UV exposure.
The lowest-price option, poly film can be a good option for greenhouses where budgets are small and long-term useful life is not as important. Poly films are easy to work with, but they are the least permanent option for greenhouses.
Greenhouse Glass Panels
Glass is the most expensive, but generally the most durable, option for greenhouse construction. If your budget and application makes glass an appropriate material choice, make sure you ask the following questions as you evaluate different glass panels.
What are the safety features of this glass?
Annealed glass can be dangerous for greenhouse applications. When it breaks, annealed glass shatters into long, sharp shards which may cause injury.
Tempered glass is four to six times more shatter-resistant than annealed glass, and when it breaks it breaks into small square pieces, making it unlikely to cause injury. Tempered glass is a better choice than annealed glass for greenhouses. There are different varieties of tempered glass (single tempered, double tempered, and more) with various tensile strengths.
Do I need single-pane or double-pane glass?
You will need to consider your overall project budget, including your long-term heating and cooling costs for the greenhouse, to decide whether single- or double-pain glass is more appropriate. Single-pane tempered glass is a fine choice for low-budget greenhouse projects where heating and cooling costs are not major concerns. For projects where greenhouse heating and cooling costs are a concern, you'll be better off with double-pane tempered glass (or possibly triple-pane glass if you live in a particularly cold climate).
To maximize the heat-reflecting properties of glass in hot climates, you may also consider low-e tempered glass panels, which will block heat more effectively than regular glass.
How difficult will it be to maintain these glass panels?
If you're choosing double- or triple-paned glass, make sure that the panels are sealed properly to prevent condensation inside the panel.
You should also be aware that a variety of glass coatings can be used to make your glass panels easier to maintain over time. For example, self-cleaning glass (SunClean brand) is coated with a material that uses sunlight to break down dirt accumulation on the panels and causes water to sheet off the panels, rather than bead up on them. Though these coatings increase the price of the glass, they may spare you a lot of glass washing over the years.
Greenhouse Polycarbonate Panels
Polycarbonate panels are made from clear, rigid plastic that transmits light almost as well as glass. Panels are typically available as flat twinwall panels, which contain two flat polycarbonate panes separated by an air space. The air space between panes improves the insulative properties of the panels.
Polycarbonate panels are more expensive than poly films but less expensive than glass for greenhouse applications. The benefit of polycarbonate is that it approaches the durability of glass though it is about one twelfth the weight, which makes it much easier to handle and install than heavy glass panels. On the downside, polycarbonate panels will begin to yellow over time, which may be an aesthetic concern for home greenhouse applications.
Twinwall polycarbonate panels include a rating, in mm, that indicates the size of the separation between the polycarbonate panels (e.g. 4mm twinwall panels have a 4mm air space between the panels). The larger the gap between panels, the better heat insulation the panels will provide.
Greenhouse Poly Film
Also referred to as greenhouse plastic or agricultural plastic, greenhouse poly film is a strong, flexible, translucent sheet of polyethylene. Poly film is available in different thicknesses, often ranging from 2 to 11 mil for greenhouse applications, with thicker (high mil) film typically lasting longer than thinner film. Poly film transmits visible light while blocking ultraviolet (UV) light, and the translucency of the film causes good light diffusion, which aids plant growth. Poly films are appropriate for use in a wide range of climates, from freezing temperatures to very hot temperatures.
The benefits of poly film are that it the least expensive material option for greenhouses, easy to work with for a do-it-yourself project, and available in a wide range of opacities and thicknesses. However, poly film has a shorter useful life than glass or polycarbonate panels, so the ease of initial installation should be weighed against the projected ongoing cost of replacing poly film over time. If you decide poly film is the right choice for your greenhouse, here's a list of questions to make sure you answer before you buy.
What's the film's useful life?
Poly films are often rated in terms of the number of useful growing seasons (e.g. 1 year useful life, 4 year useful life). Consider whether it makes sense to replace your film every year or ever four years, or if it would be more economical to spring for a more durable material, like polycarbonate panels or glass, up front. The useful life of a poly film is determined by a number of factors, including the climate where the film will be used, film thickness, whether the film has been treated with a UV stabilizer, and how well the films were installed.
If a UV stabilizer has been applied to the film, check whether the stabilizer was applied to both sides of the film or just one side. If the film was treated on just one side, you'll want to make sure you face the treated side toward the sun when installing it.
When you install poly film, try to minimize the chance of flapping, vibration, or tearing by placing greenhouse rafters close together, sanding rafters smooth where they will touch the film. Remember that poly film will expand when it's warm, so installing it on a warm day will help to ensure the film remains tight in both cold and warm weather.
Do I need woven poly film?
Woven poly film has a higher tensile strength than non-woven poly film of the same thickness, and woven films resist sagging, tearing, shredding, and punctures better than non-woven poly film. Woven films may be appropriate if you live in an area where hail, snow, or wind are a concern, or if you live with cats who may like to claw the poly film.
What film opacity do I need?
Your climate and the plants you're planning to grow will determine the amount of light you need transmitted into your greenhouse. Check the opacity rating of the film you're considering. Films are available in a range of opacities, often from 30% to 70% opaque.
Do I need black or white silage film?
Black and white silage films help create the right conditions for flowering for certain types of orchids and tropical plants.
Do I need condensation control?
Some poly films are treated to minimize condensation which can form on the underside of the film in humid greenhouses. If you need to minimize the chance of dripping onto plant leaves, or if you want to maintain even light diffusion through the film, you may need condensation control film.