The appeal of glass: Choosing the right greenhouse glazing for light, durability and plant health

Glass was the traditional greenhouse glazing material until plastic materials became available. Its advantages over plastic still make it a good choice for many crops. Improvements such as tempering, light diffusion and coatings have kept it competitive. Advantages include:

• Durability: A life of 40 years is common, with little deterioration.
• High light transmission: Greater than 90%, depending on the thickness.
• Low thermal expansion: This reduces the problem of failed edge seals.
• Non-combustible: Decreases fire potential and insurance.
• Ultraviolet light protection: This reduces some insect and disease problems.
• Low maintenance: Longer life means less replacement than plastic material.

The greatest disadvantage to glass glazing is that it has a high heat loss as compared to air-inflated poly, double-wall polycarbonate or acrylic material. This can be easily overcome in most greenhouses by installing one or more retractable energy/shade screens. Glass is also heavier than plastic glazing and requires a stronger, more expensive frame.

Several types of glass are common. Considering light is generally the limiting factor in plant growth, glass selection is made to get the highest light level.

Float glass: This is common in older greenhouses. Due to its limited strength, pane width is limited to about 24 inches. Overlapping of the panes on the greenhouse creates gaps that increase heat loss. Float glass is available for replacement.

Annealed glass: Annealed glass is float glass that is cooled slowly in the manufacturing process. This increases the strength. It can be cut easily.

High strength glass: To increase the strength more, annealed glass is reheated and cooled rapidly. This doubles the strength.

Low iron glass: All glass contains some impurities. Reducing the amount of iron increases light transmission by a few percentage points.

Tempered glass: This is the most common glass used today. It is available in sheet sizes as large as 6 feet wide. It is about four times stronger than float glass. When it breaks, it forms corn-sized pieces rather than long shards. It is usually installed on the greenhouse with rubber gaskets rather than a bead of putty. Tempered glass is made by heating annealed glass to a high temperature and then quenching it with a blast of cool air. This creates internal stress, with the outer layers in compression and the inner layers in tension.

Double pane glass: Although saving about a third on heating costs, double pane glass is not common for commercial greenhouses due to problems with moisture leakage between the two layers and the additional weight on the frame.

Other greenhouse glass options

Coatings can be added to affect plant performance. They are usually applied by spraying or vacuum deposit.

Anti-reflective: Acid etching, sputtering and soft-gel coatings can increase light transmission as much as 5% by reducing low angle reflection of the sun’s rays.

Hydrophilic coating (anti-condensate): This is like the coating available on plastic glazing and reduces the surface tension of the moisture, keeping it in a thin film rather than droplets. This coating can also reduce dirt and dust collection and prevent algae and fungal growth.

Safety films: These can be added to reduce injury should the glass break.

Stippled glass: Typically available as tempered glass, this is used to diffuse light transmission and increase light transmissivity at low sun angles. Diffused light will penetrate deeper and more uniformly into the plant leaf surfaces. Diffused light can also reduce scorching, container temperature, fungal spores and insect propagation. Scattered light can also change the balance between red and far-red light. Typically, 3- or 4-millimeter thickness is used.

The glazing you select can affect plant production and greenhouse operations for many years. It is usually best to choose one with the highest light transmission to get maximum plant growth and then add inside or outside shading to reduce summer heat.