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What Kind of Plastic Should I Use for a Greenhouse?

Light diffused inside a greenhouse with plastic covering

Light diffusion is critical for a greenhouse.  To maximize your plant growth, you need to use a greenhouse covering material that offers a nice and even light distribution across your entire greenhouse.

Too much light concentration can burn and negatively affect your plants.  Likewise, uneven light distribution can stunt the growth of plants that aren’t positioned in the right spot. 

So the question is what greenhouse plastic should you use for a greenhouse and what kind of plastic offers the best light distribution for your greenhouse? 

Greenhouse Light Distribution

Light is the engine that drives photosynthesis inside your greenhouse.  The right amount and distribution of light inside a greenhouse leads to steady and consistent plant growth. 

When light shines on a greenhouse panel, the light waves interact with the surface of the material.  The light diffusion effects vary depending on the type of material covering the greenhouse. 

Greenhouse lighting is categorized in three dimensions: duration, quantity, and quality.  By selecting the right greenhouse material, you can influence the light quantity and light quality inside a greenhouse. 

The light duration will be influenced by your greenhouse orientation and the region of the country your greenhouse is located.

Light Transmission 

Light quantity is measured based on the amount of light transmission inside a greenhouse.  Depending on the type of greenhouse material, the light transmission inside a greenhouse will react in one of three ways:

  • Pass directly through the greenhouse material.
  • The greenhouse material reflects the light.
  • The greenhouse material absorbs the light. 

When light is not absorbed or reflected by the greenhouse material, that light quantity becomes available for distribution to plants inside a greenhouse. 

Light transmission of greenhouse plastic can be measured using the ASTM D1003 test.  This test provides a standardized measure to quantify the value of haze and luminous transmittance of transparent plastics.   

The ASTM D1003 test provides a percentage value that measures the relative value of light transmission for a given type of material. 

  • Greenhouse material with a value of greater than 30% is considered light diffusing material.
  • The higher percentage value indicates higher relative amounts of light diffusion. 

Light Diffusion

Light diffusion represents the distribution of light inside a greenhouse.  When light hits greenhouse material, there are two effects:

  • The light may transmit directly through the greenhouse material.
  • The light may scatter after hitting the greenhouse material. 

The relative amount of light that scatters after going through greenhouse material represents light diffusion.  Relative measurements of light diffusion are described as partially or fully diffuse material. 

Light that is diffuse will bend around corners and fully cover the space inside a greenhouse.  Diffuse light is also less intense and safer for plants than direct sunlight.  Better light diffusion inside a greenhouse provides many benefits including: 

  • Higher rates of plant growth
  • Better light coverage for all plants inside a greenhouse
  • Reduces the risk of plants burning from excessive direct sunlight 

Impact of Ultraviolet Light Transmission

Excessive ultraviolet light causes skin damage in people.  Plants react in a very similar manner when exposed to excessive ultraviolet light from the sun.  Too much ultraviolet light directly impairs plant growth.

As a result, it is very important for greenhouse material to include material designed to reduce or reflect ultraviolet light inside a greenhouse.

The type of light being transmitted is measured in nanometers (nm).  Light transmitted at less than 400 nanometers is ultraviolet light.  Light transmitted between 400 and 780 nanometers represents ideal growing conditions for plants.  This is called photosynthetically  active radiation (PAR).

For growing greenhouse plants you want greenhouse material that emphasizes the following ranges:

  • Maximizes light transmission within 400 to 780 nanometers to optimize plant growth.
  • Minimizes light transmissions less than 400 nanometers which represents harmful ultraviolet light that can damage plants and inhibit growth.

Greenhouse covering material can reduce the amount of ultraviolet light transmission by adding ultraviolet additives to the material during the manufacturing process. 

Evaluating Greenhouse Plastic Covering for Light Transmission 

When you compare greenhouse plastic covering options for a home or commercial greenhouse, making sure to select material with optimal light transmission qualities will make a major difference for plant growth potential.

Using the Criteria we discussed in the article, we will now measure and evaluate some of the popular greenhouse plastic options.

Solexx Greenhouse Rolls

 Solexx Greenhouse Rolls used to cover a greenhouse structure

Solexx bulk rolls and greenhouse panels are designed with high density polyethylene material.  The material utilizes ultraviolet light inhibitor during the manufacturing process to limit the amount of ultraviolet light while maximizing the beneficial light transmission in the 400 to 780 nanometer range.

Many people use the Solexx greenhouse panels to re-cover old greenhouses, as part of do it yourself greenhouse projects, or even for large scale commercial projects. 

Solexx greenhouse covering is available in two sizes of thickness:

  • 3.5mm
  • 5mm

Both options offer very strong insulation values, but the 5mm thickness is best suited for colder climates and for commercial greenhouses because of the higher insulation values.

The following list compares Solexx panel insulation values compared with other greenhouse covering materials:

  • Solexx 3.5mm panels: R-Value: 2.10 / U-Value: .48
  • Solexx 5mm panels: R-Value: 2.30 / U-Value: .43
  • Double Pane window: R-Value: 2.0 / U-Value: .50
  • 5mm Double Polyethylene Film: R-Value: 1.50 / U-Value: .67
  • 6mm Single Polyethylene Film: R-Value: .83 / 1.20

Higher R-values and lower U-values indicate better overall insulation values. 

Greenhouse plastic options typically include greenhouse covering made with single polyethylene film or double polyethylene film. 

Solexx greenhouse material has more than double the insulation value of 6mm single polyethylene film and also significantly exceeds 5mm double polyethylene film. 

Higher insulation values results in lower heating costs for Solexx greenhouse material than single or double sized polyethylene film. 

Light Transmission & Diffusion Values

 Light diffusion inside a Solexx greenhouse

Solexx greenhouse panels have the following values based on D1003 ASTM testing: 

3.5mm XP Panels

  • Light Transmittance: 75% to 80%
  • Light Diffusion: 98.4%

The 3.5mm XP bulk panels are available in widths of 49.5 inches with lengths of 400 feet or 900 feet long.

5mm Pro Panels

  • Light Transmittance: 75% to 80%
  • Light Diffusion: 100%

The Solexx 5mm Pro bulk rolls are available in larger widths of 65 inches and 77.5 inches wide. The 5mm Pro bulk rolls are available in lengths of 300 feet or 900 feet long.

Overall Light Quality and Value

Solexx greenhouse panels offer tremendous value in terms of both superior light transmission, but also light diffusion.  The panels are manufactured with UV resistant material and provide significant insulation values to minimize heating costs.

For more information, see our greenhouse parts and supplies.