Glazing Your Greenhouse: 5 Ways to Cover Up

A greenhouse glaze refers to the transparent material that makes up the walls and roof of a greenhouse and allows light to reach the plants within. When setting up your first greenhouse, one of the major decisions you’ll make is which glazing material to use. Here’s a look at five materials commonly used on hobby greenhouses.

Polyethylene Film

Polyethylene film, sometimes referred to as painter’s plastic, is one of the most popular glazing options. Like its name suggests, polyethylene film is a thin, pliable material that can be customized to fit various sizes. Polyethylene film is extremely popular for large hoop houses and temporary cold frames.

The two largest advantages of polyethylene film are its low cost and its versatility. Polyethylene film is by far the least expensive greenhouse glazing option, and it can be cut or folded easily, which makes it a great candidate for custom greenhouse projects. It is easy to lift or roll up a section on a hoop house wall for better ventilation on hot summer days, and it is lightweight compared to other glazing options, which makes it a popular choice for remote locations.

On the down side is its relatively short life span. Unless it is some sort of specially treated film, the average life span of polyethylene film is 1-4 years. From an environmental standpoint, this is bad news because the used-up film will most likely end up in a landfill, where it will slowly break down over the next few hundred years.

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Another disadvantage is that it has virtually no R-value. The R-value refers to how well the material insulates. Polyethylene film will be a more expensive option during the winter months, when heating a greenhouse may be necessary. Some growers add an additional layer of polyethylene film during the winter months to help increase its thermal properties.

A single layer of polyethylene film has an R-value of around .85, while a double layer has an R-value of around 1.5. Although both of these values are low, adding a second layer can have a significant impact on heating costs during the winter months. However, the second layer also reduces light transmission, meaning less light will reach the plants. A single layer of polyethylene film has a light transmission of 80-85%, while a double layer is lowered to 72-77%.

Polycarbonate Plastic

Another popular glazing product, polycarbonate plastic, is much stronger than polyethylene film, can withstand more abuse and has a much longer life span. Most single-wall polycarbonate plastics have a life span of 5-10 years. Polycarbonate plastic’s advantages include its high light transmittance, high impact resistance and relatively low cost.

Unfortunately, when compared to polyethylene film, the only real advantages are its impact resistance and longevity, as its light transmission (86-91%) and its R-value are similar to polyethylene film. Heating a greenhouse covered with polycarbonate plastic will cost about the same as if you used polyethylene film. It should also be noted that polycarbonate plastic scratches easily.

Scratches in the plastic will reduce light transmission and cannot be repaired. Most growers on a budget avoid polycarbonate plastic because they can get similar performance at a reduced price with polyethylene film. Those with larger budgets who care more about aesthetics and efficiency tend to choose a multi-wall polycarbonate or glass glaze.

Multi-wall Polycarbonate

The glazing option that offers the best all-around value for greenhouse growers is multi-wall polycarbonate, as it is durable, easily customizable and offers an increased insulation value. The higher R-value is due to the air channel created between the sheets of plastic. Although the R-value of multi-wall polycarbonate will vary depending on the thickness of the material, it usually ranges from 1.5-2.5. The most common thicknesses are 8 mm, 10 mm and 16 mm.

Another advantage of multi-wall polycarbonate is its longevity, with most types having at least a 15-year life span. The light transmission of multi-wall polycarbonate ranges from 75-82% and varies depending on the thickness of the material.

Read also: Optimizing Climate in Your Greenhouse

Multi-wall polycarbonate has a unique advantage due to its multi-walled construction: it’s a natural light diffuser, which is an advantage because it automatically reduces the likelihood of hot spots and ensures all plants inside receive an even amount of light.

Multi-wall polycarbonate’s higher energy efficiency compared to other plastics, combined with its customizability and longevity, make this glazing option the most popular choice on this list. It has also become the glazing material of choice for retrofitting or updating existing greenhouses.

Single-pane Glass

Glass has been used as a glaze since the beginning of greenhouses. Many older-style greenhouses outfitted with glass had odd, overlapping panels that led to a series of problems, including heat loss, condensation and humidity issues, but modern glass greenhouses have addressed many of these issues and have specialized, sealed-glass designs.

The two largest advantages of a glass greenhouse are aesthetics­—they look awesome—and longevity. Glass greenhouses are designed to last at least 25 years, which is significantly longer than any other glazing option. The biggest disadvantages are the high initial cost and low impact resistance.

Even thick glass panels can be cracked or broken, which can significantly impact the performance of the greenhouse. Single-pane glass offers a comparable R-value (.90) to polyethylene film, so energy efficiency is not an advantage. In fact, growers who live in northern climates and wish to grow year-round should avoid single-pane glass as a glazing option.

Read also: Build It Right: Determining Greenhouse Design by Climate

Although single-pane glass has a low R-rating, it offers the highest light transmission rating of any glazing option, with a whopping 88-92%. This advantage can become a disadvantage, as you may need to install shade cloth to avoid hot spots that could damage plants. Single-pane glass greenhouses best suit growers who are concerned about the appearance of their greenhouse, and growers who aren’t planning on growing year-round.

Double-pane Glass

Double-pane glass is the most expensive glazing option available. However, as the old saying goes, you get what you pay for. Double-pane glass is the nicest-looking and one of the most energy-efficient options available. Although its high cost deters many people, any grower looking for an aesthetically pleasing structure they can grow in year-round should consider a double-pane glass greenhouse. Double-pane glass has a higher R-value (around 1.5) and it is built to last at least 25 years.

The biggest disadvantages of double-pane glass are its high cost, low impact resistance and degree of difficulty to install. Double-pane glass has a lower light transmission than single pane glass, around 75-80%. It is not practical in all applications, but double-pane glass is still considered the best of the best when it comes to glazing options.

The greenhouse glaze you choose will have a significant impact on the performance of your greenhouse. Geographical location, your budget and the types of crops you want to grow are all things that must be considered before investing in a greenhouse and choosing a glazing material.

For some growers, the insulation value of the glazing material may be the most important consideration. For others, light transmission or aesthetics may be the highest priority.

All in all, there are many different glazing options because there are many different applications for greenhouses. With a close examination of the advantages and disadvantages of each, you can determine which material will best suit your particular application.