Thermal Glazings

The best kept secret. The simple truth about infrared Thermal Greenhouse Coverings

Infrared Films were invented in 1972 to diffuse light for faster, fuller, even crop growth. The light diffusion feature of IR films averages leaf temperature and decreases stress. The diffusers raise the thermal value and slow the loss of heat in the evening. Infrared Films were not developed to save fuel, but they do. As a Bonus, the energy savings pays for the film.

A grower’s business day starts when the sun comes up and shines through the covering put between the sun and the money (plants) being grown.  It’s the start of the day and the beginning of profits being grown. The choice of the covering is the least expensive and one of the most important decisions a grower can make.  It dictates how the only FREE resource, the SUN, will be used. There is nothing in a greenhouse that can make more money than optimizing the free resources of the sun.  When growers look at the difference between the cost of normal film and 4energy–efficient film they find that saving nickels on a covering is actually losing dollars. The tools to maximize your business day exist.  It’s your choice.

How did it all start?

In 1972 inSpain, film producers put diffusing particles in polyethylene films to diffuse the low winter light with the hope that the even distribution of the light would produce faster, fuller, more even crop growth approaching spring.  It worked and they also found that the unheated greenhouse stayed warmer longer into the evening.  For 17 years, manufacturers paid royalties to produce patented “infrared Films”.  The benefits, including growth and energy savings, were proven 12 years before the technology came to the United Statesin 1984.  Had the film been presented to theUS growers with more emphasis on growth, instead of energy savings, it may have been accepted more rapidly.  In 1984, Rutgers University did experiments that proved enhanced plant response. Chrysanthemum flower diameter increased from 9cm to 14cm and the development of color pigment in Dark Red Annette Hegg poinsettias increased 88%. This information was somewhat overlooked in the hunt to survive the 84 energy crises. Now we are having another “energy crisis”.  IR films still make plants grow and as a bonus, growers realize free film due to the energy savings.

Do all Coverings have a “thermal value” and slow the loss of heat at night?

Yes– All light transmitting coverings have a “thermal value” so they resist the loss of absorbed radiation, thus slowing the loss of heat in the evening. Coverings that slow heat loss to less than 20% (on a spectrum analysis graph) are technically referred to as Infrared, IRE or Thermal films. Some “non-Thermal” films slow heat loss to as little as 30%, while others to 60%. It is up to you to ask for every film’s thermal value and use the information to make an informed choice.

How does IRE film save Energy and how much can I expect to save?

IRE films slow the loss of Re-radiation just like the insulation in your attic. You can expect to save a minimum of 20% to 40% on heating costs by using one layer of IR film in a double poly installation. Two layers of IRA film don’t offer significant savings over the use of one layer.

Each wavelength of solar radiation adds to heat gain during the day in differing amounts. The solar radiation can be split into 3 types: UV, PAR, and NIR.

The UV absorbers present in the film for long life expectation block much of the UV (100 to 380 nanometer wavelengths).  Full blocking is necessary to prevent rose petal blackening.  UV blockage to 380nm inhibits fungal sporulation when combined with temperatures above 62 F.  UV bleaches color on some plants and enhances the color of others.  UV is also a key factor in the development of aroma.  It causes sunburn, but has NO HEAT.  That is why you can get sunburn on a cloudy day at the beach and never feel the heat.

PAR or Photosynthetic Active Radiation (380 to 780 nm) is necessary for photosynthesis.  Blue is used less than the red and it also has less “heat” in it.  The further you go towards and past the Red, the more “heat” the wavelengths contain.

Infrared, IR and long wave infrared LIR (780 and up) are the part of the solar radiation that gives the most heat.  During the day, all wavelengths add heat to the greenhouse.  In the evening, when the greenhouse is cooling down, the absorbed radiation is re-emitted or pulled off the earth.  The absorbed radiation is re-emitted in just one area of the IR spectrum, the Longwave IR range, around the 1000 nm range.  Normal polyethylene lets more LIR through than thermal films so the greenhouse cools down faster.  Thermal films contain additives that absorb and slow the heat loss.

It’s that simple.

Is an IR film house hotter in the daytime?

NO. You would think so but once you realize that the wavelengths responsible for heat gain differ from those responsible for heat loss it is obvious that a thermal film is actually great in warm climates because of the diffusion factor.  IR films only slow down re-emitted energy, which is pulled off the earth when the sun goes down.  The universe can pull energy off the earth easier on clear sky nights than on cloudy nights.  During the day all radiation goes through the film and is absorbed by objects in the house.  In an IR house during the day, re-radiation wavelengths are absorbed and reflected back into the greenhouse more than non – IR houses.  This does not show up in warmer temperatures because it is masked by the massive amount of energy coming into the house.  It would be like trying to notice an additional 55 gallons of water going over Niagara Falls. At night, when there is not energy coming in and only energy going out, the IR films ability to slow re-radiation is relevant and shows up in saved fuel and more money in your pocket.

Do all IR films diffuse light?

NO. There are several ways to increase a films thermal value.

Mineral Fillers – Mineral fillers are small particles that are places in one or more layers of the film to absorb and reflect re-radiation back into the greenhouse.  These particles were first used with the intent to diffuse light, not to save fuel, as the houses at that time, in Spain, were not heated.  The original monolayer Mineral filler IR films made in the US were weaker than normal films. This is because you have to put more non-polymer mineral fillers in a one-layer film to make it Thermal.  With the recent onset of three-layer US extrusion (present in Europe for 10 years), the IR films became much stronger because they contain 2/3 less non-polyethylene mineral fillers.  Manufacturers can put the mineral filler additive in only one of 3 layers and get the effect they need with fewer additives.  This results in a stronger film.  Recently Klerk’s Plastic Products Mfg. patented the use of light reflecting Nacreous Opalescent Pigments in greenhouse films to alter the light spectrum to reduce heat.  These pigments are excellent at shattering light.

EVA – Most greenhouse films contain a percentage of ethyl vinyl acetate (EVA).  This co-polymer additive does not have light diffusion qualities.  High amounts of EVA increase a films thermal value to the point where it is considered a thermal film.  EVA is lower melt polymer that ad softness to film.  However, the higher the percent of EVA, the more the film will creep or stretch.  To offset this, European film producers (where these types of film are more common for single layer applications) concentrate the EVA in one layer of a 3-layer film.  This allows enough EVA to be added and still maintain the mechanical properties needed for stability.  In the US, films are generally designed for inflated roofs and use low amounts of EVA with mineral fillers to achieve the thermal value.

Can diffusion allow a plant to thrive in higher heat conditions?

Yes– Diffusion averages leaf temperatures. When the top of the plant is under less stress, it can handle higher air temperatures than the same crop with direct light.  With the addition of UV block to 380nm and heat reflection, plants can thrive in light quantities that are far higher than under normal film.  One prominent Pennsylvania grower was able to eliminate the use of shade paint when they changed from using clear, direct light film to diffusing IR film.  After 2 years of trials they confirmed that although the temperature was no different under the IR house in the gutter connected house, the plants did not need shade.

What influence does condensation have on heat loss?

Condensations on coverings absorb heat from the air and by conduction, transmit the heat through the covering. Heat consumption and thus the heat loss is increased due to condensation. This effect is much more obvious on single layer covering applications.  Condensations on coverings also reflect up to 15% of the valuable early morning light.  Water needs to be eliminated from the greenhouse roof to get the light, or money into the greenhouse where diffusion can get more of it into more leaves.

How does diffusion promote faster fuller even growth?

Imagine that you could bring the sun down and toll it around individual plants.  Suddenly you could get more light to more leaves with less heat and intensity stress to any one leaf.  Each leaf would receive a correct spoon-feeding of PAR light.  By giving the optimal amount of light to each leaf, the leaf’s capacity to manufacture food would be maximized.  This is a key attribute of an IR, light diffusing film.  Tall crops or crowded houses with hanging plant benefit dramatically from diffusion. Bedding plants and any plant that originated low in the canopy thrive in a diffused environment.  The least to benefit would be vegetable transplants.  Often, direct light produces a stiffer, harder plant that is more acclimated for direct transplanting.

Is there any research on the benefits of diffused lighting?

According to research at the University of Oregon, diffused light has the advantage of getting more light down into plant foliage canopies.  This is an advantage to plants, which grow upright; where the newly emerging leaves end up shading the older leaves below them.  Since the older leaves originally emerged and developed for maximum food production under highlight conditions, the shading caused by the newly emerging leaves reduces their food production.  The benefit of diffused light is that it provides more light to the lower (older) leaves and thus increases their food production.  The upper, more sensitive young leaves get less intense light resulting in less burn and stress, which allows plants to tolerate higher air temperatures.

Since 1972, when IR films were first developed, there has been significant research on the effects of light diffusion on plant productivity, concluding that diffusion averages leaf temperatures by evenly distributed energy resulting in more photosynthesis, causing earlier and fuller development of healthier, disease resistant crops.  In total, the plant gets lighter under a diffused film than under a transparent film, which can stimulate production of biomass by 20% or more.

Article for Greenhouse Business
Q & A by Jim Ralles, formerly of Klerk’s Plastics