In vegetable production, a rule of thumb states that 1% more light equals 1% more potential production. The greenhouse covering therefore should allow sunlight to enter the greenhouse as much as possible for a maximum potential photosynthesis. Just like every constructive element in the greenhouse, glass is evaluated carefully for its light translucency and its return on investment, if not more.
Types of greenhouse glass
Non-tempered float glass
The typical glass that is used for glass greenhouses is the horticultural float glass. Float glass, also known as annealed glass is manufactured by allowing the molten glass to cool slowly in a controlled environment. Once produced, the large float glass sheets are cut to the required size (dependant on the aluminium system) and can then be subjected to further manufacturing processes, such as tempering.
If broken, float glass will shatter into sharp fragments. Therefore, for safety reasons, this is no longer used on the roof of greenhouse structures. It makes the installation, glazing, process unnecessarily dangerous. The properties it allows for cutting are useful in the sides, where details often must be cut out. Compared to tempered glass, float glass is relatively inexpensive. DutchGreenhouses® chooses not to use non-tempered glass in the first layers of glass in the walls, to mitigate risks if accidents occur.
Tempered, toughened and safety glass
Tempered glass, toughened glass and safety glass are different names for the same type of glass. In greenhouses, tempered glass is used on the roof and sometimes in side walls.
Tempered glass is created through a heat tempering technique, making it four to five times more durable and structurally stable than its annealed counterpart. Additionally, it offers enhanced safety features; when shattered, it fragments into small, relatively harmless pieces rather than dangerous, sharp shards. This unique shattering behavior is due to the tension established between the surface and the core of the glass sheet, a result of the varying cooling speeds employed during its production.
The so-called “low-iron” glass is more transparent than regular glass. Another way of allowing more light pass through the glass, is by applying AR-coatings. These permanent anti-reflection coatings reduce the reflection levels of sunlight on the glass, which results in more light in the greenhouse. This is mostly done with tempered greenhouse glass, when further treated.
Do you want to know more about greenhouse glass?
Do you want to know more about greenhouse glass?
Diffuse greenhouse glass
Diffuse light is light that is scattered and distributed evenly in all directions. Having this diffuse property on the roof of a greenhouses, limits hard shades. Reducing shaded areas is desired for a homogenous cultivation. Especially with crops that grow vertically (tomatoes, peppers and cucumber) diffuse light has a positive effect as light can penetrate further into the crop. The lowest leafs can also take in sunlight, increasing photosynthesis and thus, crop growth. Crops can also handle diffuse light better and more efficiently than direct sunlight.
The level of light diffusion is quantified by a specific number known as the haze factor. This number represents the proportion of incoming light, coming in at a perpendicular angle, that deviates more than 2.5° from a straight path. For instance, if 60% of the direct light is redirected, this would correspond to a haze factor of 60%.
There are two different ways to create diffuse glass for use in greenhouses: chemical etching and creating a structure into the glass with a pressure roll. In both cases, light diffusion costs a percentage of total light translucency. For most greenhouses, especially vegetable greenhouses, this is undesirable. AR coatings can compensate for the loss of light during diffusion. AR, anti-reflection coatings, limits the reflection properties, which increase translucency. AR coatings can be applied on both sides of the greenhouse glass sheets. The drawback is that it is costly, but also vulnerable for cleaning agents.
Choosing diffuse glass in greenhouses creates the effect all year round, for a longer period. Throughout the season the diffuse properties might not be desired. When reselling the greenhouse, or changing costs, the diffuse character of the roof glass cannot be changed. In order to be able to deal flexibly with changing market conditions, options such as temporary diffuse coatings and diffuse screen cloths should always be considered.
Greenhouse glass can be categorized into two glass types: non-tempered glass and tempered glass. These can be low-iron for more light translucency, or just as regular float glass. For safety reasons, non-tempered glass can only be used in side-walls. Tempered glass gives options for diffuse glass and AR-coating possibilities, to compensate for the loss of light due to diffusion.
The special property of diffuse glass to scatter light, ensures a more uniform distribution of sunlight across the plants, reducing shadows and hotspots. More even light distribution allows for better photosynthesis, which can enhance plant growth and productivity. Additionally, diffuse glass can help in reducing stress on plants caused by direct sunlight, making it particularly useful in regions or seasons with high light intensity. Overall, the use of diffuse glass contributes to higher yields and better quality crops.
Although diffuse glass has many benefits, the disadvantages also need to be considered:
- Costs (tempered diffuse glass is more expensive than regular tempered glass)
- Loss of light (AR coatings can compensate the loss of light transmission, but add onto the costs of the diffuse glass)
- Loss of flexibility (tempered diffuse glass will limit the flexibility in switching crops, reselling and anticipation between seasons)
The Haze-factor is the level of light diffusion, quantified into an index number. The Haze-factor equals the percentage of light that is scattered above an angle of 2,5 degrees.