In the past, greenhouse roof light transmittance has always been measured when the roof was dry. But a greenhouse roof spends half the year wet from condensation, and the type of condensation determines how much light passes through the glass. Now there is a measurement protocol that can quantify that. An anti-condensation coating is an option for existing greenhouses and Dutch tomato grower Paul van Paassen is the first grower to try it out.

Measuring the light transmittance of greenhouse roof materials accurately is essential for estimating how much natural light the crop can use. At the beginning of this century, ‘measuring accurately’ still meant measuring light entering the greenhouse perpendicularly. But this did not give the grower an exact picture because most of the light does not enter perpendicularly but at an angle. In that case, two things change: the path of the incident light is longer and when light enters at an angle, more is reflected off the roof. On top of that, when the weather is cloudy the light comes from all directions, so measurements need to be taken from all possible angles.

Hemispherical transmittance

Measuring hemispherical transmittance is standard practice these days. But that doesn’t provide a reliable picture for the grower either, says Gert-Jan Swinkels, light measurement expert at Wageningen University & Research in the Netherlands. There is generally a difference between the transmittance of wet glass and dry glass. The type of condensation on the material also makes a big difference. Droplets reduce transmission, whereas a water film can actually increase it. “In the winter months, the roof of a greenhouse in which vegetables are grown is wet almost 100% of the time. That is precisely the time when the amount of light is the limiting factor for good production. The transmittance of the greenhouse roof when wet should therefore be an important consideration in the choice of material for anyone building a new greenhouse,” Swinkels says.

Erratic measurements

In 2010, a team led by Cecilia Stanghellini took a closer look at the effects of condensation. To do so they built a small greenhouse with a replaceable roof in a climate chamber, where they were able to study condensation in perfectly controlled conditions. The result was surprising. On average across all the materials tested, the loss of light through a roof with condensation was as much as 9% compared with a dry roof. In practice this number started to take on a life of its own as there were big differences between the various materials. For ordinary greenhouse glass, the reduction in light was 5%. Diffuse glass let in almost the same amount of light in dry and wet conditions.
This research demonstrated that it makes a big difference whether the roof is wet or dry. Furthermore, model calculations revealed that greenhouse roofs spend half the year wet to some degree.
The research was useful for raising awareness, but the method involving a trial greenhouse in a climate chamber is clearly too cumbersome for measuring transmittance in practice. There is also another problem, Swinkels says: “Two years ago we measured the amount of PAR light in the Winterlight greenhouse under a dry and wet diffuse roof, using PAR sensors. Over a longer period, the trends were fairly accurate but the measurements were erratic; for example, we saw a progression across the whole day, even in cloudy weather. That told us that these measurements are really too unreliable to use as the basis for decisions and take too long.”

Condensation meter

The conclusion was that a standardised measurement protocol was needed which could ultimately become a standard (or could be incorporated into NEN 2675). So they built a special device which does not yet have an official name but is provisionally known as a “condensation meter”. Swinkels and his colleagues used this device to optimise the measurement method.
“We concluded that we can use this device to accurately measure transmission through dry and wet materials at an adjustable angle, i.e. the slope of the greenhouse roof. Because you can adjust the slope, condensation will drain off and you get a realistic picture of what happens in a real greenhouse. The measurements are reasonably stable but because a lot of factors affect condensation, repetitions sometimes produce slightly different results. For this reason we are not classifying the effect of condensation in percentage terms at present but in classes, such as negative, very negative, positive and very positive,” says Swinkels.

Glass more hydrophobic

The protocol was developed using six samples: four diffuse materials and two clear glass types. “In the case of the diffuse materials, transmittance in wet conditions increased by 0 to 1.5%, while with clear glass we saw a decrease of 1 to 3%,” he says. A slightly different magnitude from the five percent seen in the Stanghellini project, therefore. But Swinkels has an explanation for that: “We noticed that new glass is more hydrophobic – water-repellent – so it produces larger droplets. Glass that has been on a greenhouse for many years and has been frequently cleaned becomes more hydrophilic, so you tend to get a water film on it.”
The device is already in operation. Swinkels hopes that awareness of the importance of wet transmittance will grow and that manufacturers will start paying more attention to the condensation behaviour of their materials.

Existing greenhouses

Understanding the condensation properties of the roof comes too late in the day for existing greenhouses. It had already been suggested in the 2010 report that an anti-droplet coating on existing greenhouses would be a good way of preventing light loss caused by condensation droplets. Mardenkro has developed just such a coating for glass under the brand name AntiCondens, a further development of a formula that has been used on polytunnels for many years.
Paul van Paassen in Bleiswijk is the first Dutch grower to use it. He grows cluster tomatoes (Merlice) on coco substrate on 2.2 hectares. “It was pretty damp last year and the windows stayed wet for a long time. My crop advisor Willem Valstar of Stargrow Consultancy suggested trying out the new coating. We trialled it on six windows in May. The results were impressive so I put in an order after the crop had finished,” he says.

World of difference

Then it was a matter of waiting for a nice dry day once the crop had ended, as the coating has to be applied on a clean, dry roof and then left to dry for a while. Because Van Paassen had to turn up the heating quite high to get rid of the vapours from the new paint job, the coating also dried straight away. The coating was applied with a spray cart with the nozzles close to the glass. It is left there for a season. If the windows are cleaned with fluoride, the coating will be removed as well.
Van Paassen is delighted with the result. “It makes a world of difference. It looks completely different too. The condensation is still there but you can’t see it at all. It is visibly lighter inside the greenhouse now, although it is difficult to say what impact that will have on production and quality.”
A patch of roof was accidentally missed in the middle row. You can still see condensation droplets there. Mardenkro measured the light incidence at this point with a PAR sensor and compared it with the light incidence under the coating. The difference is striking: on three consecutive days in May, the coating improved the daily light sum by 6 to 7% (see figure).

Drying time

Van Paassen uses a form of Next Generation Growing which involves using more screens than in the past but activating the plants properly in the morning. He never had much problem with mildew anyway so he can’t say whether the coating reduces disease pressure. “In theory, it should be easier to dry the air with heat. What’s more, the water film also flows into the condensation gutter more easily,” account manager Paul van Gils adds.
The greenhouse should also be heating up a little faster in the morning as more light is entering. “But you can’t measure that,” the grower says. “But I do think it works and I get the feeling it has saved me money. I will be ordering some more at the end of the year, but I do have to make sure I apply it at the right time as I can’t gauge whether it is still worthwhile if I have to heat up the greenhouse specially just to dry the glass first.”
Van Gils recognises that the biggest stumbling block at the moment is the need for drying time once the product is applied. This won’t be a problem for growers who switch crops during the warm months. The product is not phytotoxic in the form in which it is applied, although there are problems in roses if the buds get wet. Special equipment is needed to apply the product over a crop, however.


Existing measurement protocols for greenhouse roof materials show the light transmittance of a dry roof. But the roof is covered with condensation half the time and the type of condensation has a significant impact on how much light passes through. Now a measurement protocol has been developed for wet greenhouse roofs at an angle. A tomato grower is the first to gain experience with an anti-condensation coating for glass. There is clearly more light in his greenhouse, although the need for the roof to be dry during application is still an obstacle.

Text: Tijs Kierkels. Images: Tijs Kierkels and Wageningen University & Research.