We live in a plastic-laden age, so when it comes to choosing a material for a year-round greenhouse, the choices can get overwhelming. Polycarbonate, polyethylene, Polygal, Lexan…? Knowing how to navigate the array of ‘polys’ – and other materials like glass – is a major hurdle for first-time greenhouse builders.
Below, we run through the major categories of plastics and glass for a year-round greenhouse. As background, our specialty is energy-efficient, year-round greenhouses that use passive solar greenhouse design. However, this overview can be used for any structure, from low-cost hoop houses to high-end conservatories.
How to Choose a Greenhouse Glazing Material
Before diving into the specifications of the materials, it is helpful to know how to evaluate them. The best choice for your greenhouse depends on your climate and growing goals. Do you want to grow year-round despite freezing winter temperatures? A multi-layer insulating material that can be well-sealed will greatly benefit the greenhouse. Or, do you live in a mild climate and only want to grow cold-hardy crops through the winter, or not grow in the winter at all? A single layer rigid plastic or polyethylene film may be the best choice. To evaluate a material, consider the following factors:
- Cost – Consider both upfront cost and lifetime cost, based on how often it is replaced
- Durability / Longevity – A number of factors play into how long the material will last. Consider the wind and snow loads at your site when weighing this factor. Also consider whether your greenhouse requires a building permit, in which case the structural loads of the material will have to meet building standards.
- Warranty – This varies by manufacturer and product; some materials come with a warranty against accidental breakage and hail damage.
- Light Transmission – How much light the material transmits is a very important factor for the performance in a year-round greenhouse. But, it can be difficult to ascertain the ‘right’ amount of light transmission. It greatly depends on your climate. For most year-round greenhouse growers with low light in the winter, the higher the light transmittance, the better. Greenhouses in sunny climates will likely benefit from lower light transmission or some shading. Importantly, higher light transmission is inversely correlated with insulation. Materials that transmit more light are thinner, and usually, have lower insulation ratings. This makes it a tricky balance for most growers. (Get in touch with us for a climate analysis of your greenhouse if you want a specific recommendation.)
- Insulation – For most climates with cold winters, higher insulation ratings are better. Additionally, greenhouses in very hot climates will benefit from more insulating glazing materials as they will block heat from entering, and keep the greenhouse cooler. Insulation is rated in R-value (higher numbers are more insulating). Sometimes, the inverse rating – the U-value — is also given.
- Transparent v translucent – Transparent materials are those you can clearly see through, like a window. Most double layer plastic glazings are translucent: they diffuse light and can’t be seen through. Translucent materials are usually better for growing; however clear view windows provide a nicer view.
- Availability – Many materials must be special ordered from distributors. Single-layer polycarbonate or fiberglass may be available from local hardware stores in your area. Shipping costs are another factor when choosing a material.
Polycarbonate is probably the most common material today for the backyard and commercial year-round greenhouses, due to its many advantages:
- Lightweight and easy to install
- Can hold up to high wind and snow loads
- Long-lasting – many products come with a 10-year warranty
- Can create an insulated greenhouse
- Can be high light transmittance
- Can be sealed well to the greenhouse, often with attractive aluminum trim
The disadvantages are that it will eventually yellow over time with sun exposure, but in recent years the quality has increased greatly. Typical life spans are 10-20 years. It is a greater cost compared to polyethylene film, but generally provides more durability, insulation and a nicer aesthetic.
The fact that polycarbonate can be insulating makes it a good choice for an energy-efficient year-round greenhouse. There is a huge range of polycarbonate products, from standard double layer (8 mm) to 5-layer product (32 mm and an R-value of 5.6, similar to many home windows). They also provide different rates of light transmission – from very high (over 90%) to under 50% for thicker and tinted varieties. Thus, the grower can select a polycarbonate material to suit their climate. Growers in hot climates can reduce heat gain with a lower light transmission product; while those that struggle with freezing winters can retain heat with a thicker, insulating material.
The most common products are one or two-layer products. These are cheaper upfront, but less insulating and thus decrease the energy-efficiency of your greenhouse. For those in climates with freezing winters, that means you will probably have to heat a year-round greenhouse. The investment in a more insulating, double or triple layer polycarbonate product is almost always worth it financially if your desire is to grow year-round and you live in a moderate to harsh climate. At Ceres, we typically choose something in the middle – using a triple-wall 16 mm product that has a light transmission value of 77% and an R-value of 2.4 – for most year-round greenhouses.
You can find polycarbonate through distributors – often those specializing in greenhouse plastics and coverings. The largest brands are Polygal and Lexan. Their websites can also connect you with distributors in your area.
Acrylic and fiberglass were the main rigid plastic glazings used before polycarbonate dropped in cost and improved in performance. Commonly known by its trade name “Plexiglas,” acrylic is similar to polycarbonate in many ways. It can come in a multi-walled form, making it a good choice for roof and wall applications. It can also be bent over a shallowly curved frame. Acrylic is slightly less impact resistant than polycarbonate—it will shatter more easily—but is still very strong as a material. (Acrylic has 17 times the impact resistance of glass. Polycarbonate has 250 times the impact resistance of glass.) Because acrylic and polycarbonate are so similar in regards to performance, we recommend making a decision based on distributors and costs in your area. It is likely that polycarbonate will be more widely available and cheaper to procure.
Fiberglass is made by embedding shards of glass fibers into plastic resin. Most varieties are opaque; it’s used in storage tanks, sports helmets, boat hulls, etc. Translucent varieties (typically a milky color) can be used for greenhouse glazing; however, they have lower light- transmission. Be aware that it is also flammable, and it often has a rough texture that can trap dirt and further reduce your light transmission. Though fiberglass used to be common in greenhouses, today it has mostly been replaced with polycarbonate.
This is a relatively new product that presents many of the same advantages as polycarbonate – lightweight, good insulation and light transmission, hail resistant and long life-spans. It also has some further advantages – thicker materials can be curved over bent frames, as shown in the aquaponics greenhouse below. Currently, the primary challenge with ETFE is sourcing. As of this writing, there are only a handful of suppliers in the US. Investigate to see whether it is an economical product to be shipped to your year-round greenhouse.
Polyethylene is film plastic is extremely common in the commercial greenhouse industry, primarily because of its low-cost (usually only cents per sq. ft.) It is often rolled over hoop houses or other three-season structures. Though cheap, polyethylene has disadvantages from a performance perspective:
- Short life-span – the material will degrade under UV rays and can easily be torn off in wind or snow, and get damaged in hail. Typically products last 2-4 years in harsh climates.
- No insulation – as a thin plastic, polyethylene provides some crop protection, but on its own does not do much to retain heat on cold nights. For that reason, growers often use two layers of the material and a blower to create an air gap in between. The air gap provides some insulation. Alternatively, some growers – notably Eliot Coleman in Maine – use row covers in their polyethylene greenhouses to provide additional insulation, as shown in the photo below.
Considering those points, we consider polyethylene a good material for season extension, and growing in mild climates. Alternatively, you can grow cold tolerant crops in harsh climates. Due to their short lifespan and lack of insulation, we typically don’t recommend them for a year-round greenhouse in a harsh climate.
SolaWrap – previously called PolyKeder is a higher quality film glazing. It has an R-value of 1.7 per inch, making it less insulating than many polycarbonate products, but much more insulating than polyethylene. It is also much longer-lasting than polyethylene, making it a good choice for curved greenhouses (domes or arched greenhouses) that want to grow year-round in moderate to harsh climates.
Glass is the outlier of the choices above, which are all made out of plastic. Compared to plastic, glass presents a number of pros and cons.
Advantages of glass:
- Transparent – Glass windows provide clear view windows. Installed in vertical walls, this can make a year-round greenhouse feel much more open, more like a sunroom than small enclosed box. Many growers also choose them for their aesthetic, which can blend easily with a home.
- Insulation – There are a huge range of glass windows, and many have very high insulation ratings. In addition to having high R-values glass windows can also be sealed very well to the greenhouse frame. Many plastics in contrast experience high levels of thermal expansion. This causes air leaks, which reduce the overall efficiency of the greenhouse.
- High light transmittance – Most glass windows have very high light transmittance, over 80%. We recommend selecting clear glass windows, over any tinted varieties that will dramatically reduce light penetration. Low-emissivity (low-e) windows are a slightly more complicated category we discuss further in The Year-Round Solar Greenhouse.
- Moderate cost- Windows are cost-competitive with a triple-wall polycarbonate, though still vastly more expensive than polyethylene film and single-layer rigid plastics.
Disadvantages of glass:
- Weight – Glass is dramatically heavier than plastics. That, in turn, makes it harder to install in roof applications. The greenhouse roof requires much sturdier framing, which increases cost.
- Can break- The ultimate disadvantage of glass is simply that it can easily break under hail or falling objects. This makes them hard to use in roof applications. If your greenhouse requires a building permit, glass skylights or windows in the roof need to be tempered, which makes them far more expensive, often exorbitantly so.
Given those pros and cons, we recommend using glass strategically – taking advantage of its use as view windows, and using it in vertical applications of the greenhouse walls. It is more challenging to use in a roof application.
For more information on choosing and installing glazing in your year-round greenhouse, see The Year-Round Solar Greenhouse or shoot us any questions at info at ceresgs.com.