Growers know that a diffuse roof – with or without an anti-reflective coating – offers added value compared with traditional greenhouse glass. However, it is not so simple to decide which one to choose. Researcher Silke Hemming lists all the relevant aspects. An underestimated factor is measuring samples taken from good representative spots for new large-scale building projects.
In the Dutch and Belgian horticultural sector, growers building new greenhouses opt almost unanimously for a roof made of diffuse glass. Also, depending on the light requirement of their crops the glass panes in the roof are often covered in one, two – and with double glass – even three or four AR-coatings. Although concepts such as haze factor, hemispheric transmission and UV-transmission are well established, researcher Silke Hemming of Wageningen UR Greenhouse Horticulture, notes that in practise there is still much uncertainty and confusion. The recent introduction of the new term F-scatter doesn’t make it any easier. What should growers pay attention to in order to make an objective decision?
Anyone considering the installation of a new greenhouse roof should firstly decide how much and what type of light he would like in the greenhouse: What is the optimum amount of light distribution? Is a lower light transmission acceptable or, by using anti-reflective coatings, do you want to increase this as much as possible? What sort of UV-transmission should the roof have? You can also take into account the insulation aspect because that determines to a large extent whether you need to think about single or double glass.
“Take time and care when making a wish list because this will determine the shortlist of options and the eventual choice of glass,” says Hemming. “What I also tell growers and advisors to do is properly determine the optical characteristics of the glazing and compare them with each other in the right way. That means on the basis of a representative sample. I’m amazed that people often only measure one or two panes of glass for projects that will cover several hectares.”
Variation and sampling
During the production of the glass, variation can occur that affects the transmission properties; a brand of glass can change over the years; one production batch can differ from the next batch; and it’s even possible to have variation within one glass pane. These differences are usually small, but can – depending on the quality control and any treatments after production such as structural treatments to the surface and coatings – become larger.
Hemming: “Sometimes we can measure differences of up to ten per cent in haze between batches and even between different places in one pane. This is not necessarily a problem; even when there is variation within a delivery, the product can still be good.”
She finds it risky to determine the optical properties of glass, that is going to cover several hectares, based on measurements taken on a single pane at a visually representative point. This value can by coincidence be good but it can also happen to be bad. “Therefore ensure that a good sample is taken based on the guidelines in the ISSO-publication 88 Quality Requirements for horticultural greenhouses,” stresses the researcher.
Haze and F-scatter
Growers as well as advisors have many questions about the haze factor. The haze factor expresses how much light is scattered. Nevertheless types of glass with the same haze factor can sometimes differ in the way in which light is scattered. This can be either wide or narrow. In principle, light penetrates the crop better when there is a wider scattering of light compared with a very narrow scattering but that can be associated with slightly more light loss. A new term, the F-scatter, was devised to explain the way in which light is scattered: The higher the F-scatter of the glass the wider the light is scattered.
To give the glass the required diffuse properties, a number of structural treatments are possible. Also the surface of the glass can be modified on one or two sides – for example by rolling or etching – to give a more or less rough structure (regular or irregular) in order to scatter the incoming light. The researcher cannot say if a certain structure, such as a matt-matt or pyramid, would be the preferred option. “Compare the optical properties measured, the hemispheric light transmission, haze factor and F-scatter of the materials with each other and see if these match your requirements, she advises.
There are also many types of coatings. It is wise to know their impact on the hemispheric transmission. An anti-reflective coating increases light transmission in dry conditions by 2 to 4% per side. “In addition coatings can change the incoming light spectrum,” says the researcher from Wageningen. “This applies both to the PAR-spectrum as well as the transmission of Near Infra Red heat radiation. Both of them can affect growth and the temperature in the greenhouse.”
The textured layer on the glass, which is often placed on the inside of the roof to limit the amount of dirt build-up, can affect the condensation behaviour of the roof. This applies too to any anti-reflective coating, which when two sides have been structurally treated is applied over the top.
“From autumn to spring, there is almost continuous condensation on the inside of the roof,” says Hemming. “It’s a cheap way to dehumidify the greenhouse, but it can also lead to loss of light. Good condensation properties are important because in the winter there is a need for extra transmission and less need for light scattering.”
Structure and coating together determine the nature of the condensation layer. If the combination is good, an even thin condensation layer occurs that can lead to 5% gain in light compared with a dry pane of glass. If the combination is poor water droplets occur which cause light loss: Undesired droplets occur on wet standard float glass that can cause 5% loss of light.
The researcher notes that the effects of wet glass and condensation, measured using the current methods, are only indicative because the glass lies horizontally on the measuring unit. That differs from an actual greenhouse in which the glass on the roof is always sloping. The results of these measurements can therefore differ somewhat from practice, but they do provide a reasonable indication for comparing one material with another.
“We still have a project running in which we comprehensively focus on the condensation behaviour in the lab and in practise. In time we can say more about this,” says Hemming.
In addition to the technical properties of greenhouse roofs growers should also look at the long-term aspects, such as constant quality with respect to extra orders placed due to glass breakage. “In this context ask what kind of standardisation suppliers can offer,” recommends Hemming.
During and after the construction of the greenhouse, glass must also be properly maintained to prevent glass corrosion and to maintain the optical properties. Last but not least the grower should ask the glass manufacturer and/or supplier about the best way to clean the roof, both inside and outside. After all, a wrong treatment can have nasty consequences, among other things on the properties or lifespan of coatings.
To come to the right decision about choice of glass growers should make a list of their requirements and objectively have samples of glass measured on all the relevant aspects. The importance of a good and sufficiently large sample is often completely underestimated. Also, condensation behaviour deserves attention. New research results on this will be published soon.
Text: Jan van Staalduinen. Photos: Wageningen UR Greenhouse Horticulture