Below is an excerpt from The Year Round Solar Greenhouse. For more on choosing a glazing material for a greenhouse, plus costs and tips for installing glazing materials, please see the book.

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:

Rigid Plastics

Polycarbonate

Polycarbonate is probably the most common material today for backyard and commercial year-round greenhouses, due to it’s many advantages:

polycarbonate greenhouse roof
Ceres polycarbonate greenhouse roof

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.

greenhouse polycarbonate options
Greenhouse Polycarbonate options for Polygal materials

The most common products are a 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

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 poly- carbonate 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

Fiberglass is made by embedding shards of glass fibers into a 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.

ETFE

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.

ETFE glazing on a greenhouseFilm Plastic

Polyethylene

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:

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

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

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:

Disadvantages of glass:

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.

glass view windows in a school greenhouse
glass view windows in a school greenhouse

For more information on choosing and installing glazing in your year round greenhouse contact us directly.