Types of Greenhouse Covering Materials

The greenhouse must be covered with as clear a material as possible to provide the maximum amount of sunlight for the plants. This is the primary requisite of the structure. The durability of the covering material, the initial cost, and the maintenance costs then determine the desirability of the material.

Glass for greenhouse

Light transmission through glass is excellent, and for that reason, it has been the primary covering material used through the years. Applying a glass covering is called glazing. The initial cost of glass is reasonable, but it is difficult to install, and the periodic reglazing required is costly. It is subject to considerable breakage in areas prone to hailstorms. Hail damage is such a threat in certain locations that wire netting is installed above the glass to protect it from hail.
Glass is placed on the greenhouse much like shingles on a home—except that the panes are not lapped as much and are held in place by glazing points at the bottom, sides, and top. To make the house watertight, the glass is set in a bedding compound on the bars, and glazing compound is applied along the sides, then painted to complete the seal. The exterior of a glass house needs repainting every 2 to 4 years to maintain a good seal. Loose glazing compound must occasionally be replaced before repainting. If the seal between bars and glass becomes loose, reglazing is necessary. Reglazing involves removing all the glass, cleaning the bars, and relaying the glass. This expensive procedure is essential for optimal growing conditions.
To reduce the need for frequent painting and reglazing, aluminum bar caps are used. These caps protect the seal from weather and reduce maintenance requirements.

Fiberglass for greenhouse

Fiberglass consists of glass fibers embedded in plastic, making it flexible yet somewhat rigid. Commonly corrugated, this form increases rigidity and requires less support than flat panels. Fiberglass is manufactured in panels 134 cm wide, with effective coverage of 120 cm per panel after lapping. Greenhouse roofs without ridge ventilators often use continuous fiberglass panels from ridge to eave. Closures are made with strips that are plane on one side and corrugated on the other, placed between the panels and ridge or gutter.
Fiberglass is translucent, transmitting about 90% as much light as glass. Plant growth under well-maintained fiberglass is comparable to that under glass. However, degradation occurs over time, causing discoloration and surface erosion. The addition of UV stabilizers reduces this degradation. Fiberglass with Tedlar film bonded to its surface maintains good light transmission for years with minimal maintenance. Manufacturers offer warranties of 10 to 20 years, depending on the grade, but the actual useful life is often shorter.

Polyethylene Film for greenhouse

Polyethylene (poly) film is the least expensive greenhouse covering material and needs frequent replacement. Standard poly film degrades rapidly in sunlight, but the 6-mil grade used for greenhouses is more durable, transmitting 80-90% as much light as glass. It is available in widths up to 12.2 meters, with rolls commonly 30.5 meters long—large enough to cover some greenhouses entirely.
Poly film-covered houses, known as “tight houses,” lose less heat by infiltration but may have high relative humidity and limited gas exchange. This can lead to oxygen or carbon dioxide shortages unless adequate air inlets are provided, especially when using unit heaters or carbon dioxide generators. To save heat energy, some greenhouses use a double layer of poly film inflated to create an air space. However, this further reduces light intensity, and the effect must be weighed against the heat-saving benefits