More efficient cladding on polytunnels helps stave off increases in fuel costs

Smart films: enable active management of ambient light and temperature in polytunnels  Image; BPI.VISQUEEN
Smart films: enable active management of ambient light and temperature in polytunnels Image; BPI.VISQUEEN

It is freezing cold and you want to turn the heating up but, with margins continually squeezed, most growers are looking for ways to reduce bills and still produce a quality product. The constant threat of higher fuel costs has spurred many to undertake a thorough energy audit, with some then rethinking their heating systems.

Recent research reports suggest the type of cladding using on polytunnels and plastics-clad greenhouses should also be a priority consideration. Huge amounts of money could be saved on heating costs and thermal screens, while greater frost protection can be assured for unheated crops.

The dramatic effect some claddings can have on inside temperature and heating cost reduction has been highlighted by the use of SunMaster Super Thermic tunnel covers at New Place Nursery. The trials, recorded by freelance consultant David Hutchinson, were conducted at IPPS president Mike Norris's West Sussex site during the very cold part of winter 2009-10.

Using Easylog data loggers supplied by Dove Associates, temperature levels were studied in a glasshouse that had a thermal screen and in a single-span polytunnel covered by SunMaster Super Thermic cladding, with no thermal screen. Both structures had under-floor hot water heating because both were used for propagation.

The data loggers were placed in the air, approximately 2m above floor level and below the thermal screen in the glasshouse. In February, the same temperatures were recorded in the glasshouse and the tunnel. Further data shows the temperature at crop level to be consistently 0.5 degsC warmer than the temperature 2m above the crop, both under the Super Thermic cladding and in the glasshouse with thermal screens.

"For growers without expensive glasshouses and thermal screens, SunMaster Super Thermic has the potential to reduce their carbon footprint and save vast amounts on heating bills," says XL Horticulture general manager Les Lane. It is calculated that energy reductions of as much as 30 per cent can be achieved over a non-thermic film.

The film is manufactured by Plastika Kritis using a new technology developed by its own master batch company and is not available for general sale to other polythene manufacturers. The firm has its own wind farm plus a factory covered with photovoltaic panels and, despite polythene production being a power-hungry process, a surplus of electricity is generated for feeding into the national grid. XL Horticulture is the only UK importer supplying films made by Plastika Kritis.

In addition to the heat gain in winter, growers' crops can also benefit from the temperature-reducing properties of the diffused film in the summer months. "SunMaster Super Thermic is typically 10 per cent cooler on a hot summer's day than a clear film," confirms Lane.

The new film has anti-drip additive, giving it the same properties as AF films, but also has true anti-fog additive to reduce the number of days a fog occurs in the tunnel. Lane explains: "The thing about thermic films is they keep the moisture in the atmosphere - and that can spread disease. We've used an additive that halves the amount of time you would get a fog in the tunnel." This anti-fog additive is another formula exclusive to Plastics Kritis.

Impressed with the results of SunMaster Super Thermic film in the New Place Nursery trials, XL Horticulture has now changed its Sterilite cladding to the super thermic formula. In addition, Sterilite cladding benefits from a reduction in thickness - a factor that means the new formulation has not added to the price.

"Sterilite was thermic, but it was normal thermicity. Now it is more thermic and it also contains a co-polymer additive that allows the film to be made thinner than other films. Sterilite is now 600-gauge or 150mu thick, but the additive also increases the film's tear strength, making it massively strong," says Lane. "By reducing the thickness down to 150mu, we keep the Sterilite at the same price."

As a typical example, using grower prices current for December 2010, the popular size of 9.2m-wide Sterilite and SunMaster Super Thermic is £7.04 per linear metre. This compares to XL's price of £6 per linear metre for 9.2m-wide clear polythene.

Sterilite has a five-year guarantee, although Lane suggests that growers can typically expect eight or more years from it. Some of the original Sterilite launched back in 1997 is still used on tunnels at a nursery in Herefordshire.

Both Sterilite and SunMaster Super Thermic have the same anti-fog and anti-drip properties. They are also both diffused to produce a more even growth pattern, which is an important factor in winter months because diffused films are capable of deflecting sunlight into the structure, whereas a glasshouse or clear tunnel cladding is more likely to reflect it.

The difference between Sterilite and the new Super Thermic films is that Sterilite blocks ultraviolet light, giving control of flying pests and fungal diseases, whereas SunMaster Super Thermic is UV-open and results in a stockier crop with a thicker cell wall, improved rooting and, most importantly, enhanced flower and fruit colour.

Either can be used where crops require heat. Use Super Thermic on crops where reds and blues are important, including cut flowers and red-leaf salad crops, and for over-wintering nursery stock where frost protection is needed.

Extra frost protection can also be given to crops inside tunnels by using CosyTex, another XL Horticulture product. It can be used as a tunnel cladding but is currently enjoying high sales to growers of vegetables, soft fruits and other crops inside tunnels. Made from knitted thermal polythene, this film reportedly gives good frost protection to about 2 degsC. It allows irrigation water to pass through but moist air to escape.

As films and claddings are made smarter, the technology can be used by growers to improve quality and reduce inputs - be that pesticides, growth regulators or, indeed, heat. Wrap them up and keep them warm this winter.


Polytunnels and plastics houses are not cheap. Add to that the cost of annual insurance, which may not always cover you for everything, and you need to be sure that you are getting a structure that will stand up to the elements.

Yet we are hearing reports from up and down the country that the snowfall of late November/early December has caused tunnels and houses to collapse. One nursery estimates the damage to have cost £100,000. It seems the polythene can take it, but the steel structures have simply buckled under the weight of snow.

Loose snow can weigh 160kg/cu m. It would be reasonable to suggest that polytunnels should be designed and constructed to take at least 100kg loading. It would appear that many are not.

It has been suggested that the inclusion of crop bars in polyhouses increases strength, preventing the structure collapsing inward or out. Perhaps what we really need is a standard.

- Email your experiences of snow loading on polythene structures to

NEWS FROM bpi.visqueen

As part of a £7m investment, bpi.visqueen's Ardeer factory has been enhanced with new machinery and latest technology. Much of the information regarding the new line remains a closely-guarded secret, but the firm has revealed that the facility includes a five-layer wide-width film line, believed to be among the widest and most advanced in the world.

It can produce films up to 22m wide and as such it will augment the manufacturer's track record of supplying wide-width films for horticultural applications. It can co-extrude films comprising of five individual layers, helping bpi.visqueen to develop new products and advance its range of "smart" films that enable the active management of ambient light and temperature in a variety of climatic conditions.

The manufacturer will be able to tailor each layer to specific performance characteristics to aid growers looking to improve crop quality and yield. The company continues its research into plant science with programmes running in partnership with the University of Lancaster and the University of Reading to better understand what various crops, often grown in very different environments, need to thrive.

In addition, the line's advanced nature provides bpi.visqueen with precision control over film width and thickness as well as polymer blend, dosing and any additives employed in the film's production. The new line means films will be produced with greater durability and strength, thanks to their five-layer construction, but also improved flexibility in terms of film folding and the avoidance of film weak spots. Customers can expect full product traceability covering all aspects of the film, right down to the additives used.

- Greenock-based British Polythene Industries, Europe's largest recycler of polythene film, has been named as winner of the Best Green Large Company category at the 2010 Scottish Green Awards ceremony.

The business was recognised for a host of green practices, including the first "fully closed loop" farm plastics recycling initiative, its use of a natural bio-filtration system that uses mussel shells to control VOC emissions and its efforts to place environmental concerns at the heart of its business activities.

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