What is the light transmission rate of a 2m x 1m polytunnel?
Jul 28, 2026
For most small polytunnels covered with new, clean, single-layer clear polyethylene film, a realistic starting point is about 85% to 90% light transmission. The exact result depends on the film specification, additives, age, cleanliness, condensation, frame shading, and the way the tunnel is positioned. The 2m x 1m footprint does not set the transmission percentage by itself; the cover and the condition of the complete structure matter more.
A new single-layer cover may transmit close to 90% of photosynthetically active radiation under test conditions, while the usable light measured at plant level is often lower once the frame, seams, dirt, droplets, and sun angle are included.
As a supplier of 2m x 1m polytunnels, I often get asked about the light transmission rate of these structures. Understanding this aspect is crucial for anyone looking to make the most out of their polytunnel for plant cultivation. In this blog post, I'll delve into the concept of light transmission rate, factors that influence it, and what it means for your 2m x 1m polytunnel.
What is Light Transmission Rate?
Light transmission rate refers to the percentage of sunlight that can pass through a material, in this case, the covering of the polytunnel. It is a vital characteristic because plants need sunlight for photosynthesis, the process by which they convert light energy into chemical energy to grow and thrive. A higher light transmission rate means more sunlight reaches the plants inside the polytunnel, which can lead to better growth and higher yields.
PAR Versus Total Sunlight
Product data sheets may report total solar transmission, visible-light transmission, or photosynthetically active radiation (PAR) transmission. PAR covers the wavelengths plants use most directly for photosynthesis, so it is the most useful figure when comparing greenhouse films for crop production. University of Massachusetts Extension reports typical PAR transmission of 88% to 91% for UV-stabilized film, while films with infrared, anti-condensate, or diffusion additives can have different ranges. Always compare figures measured with the same method rather than treating every "light transmission" number as identical.
Factors Affecting Light Transmission in a 2m x 1m Polytunnel
- Material of the Covering: The type of material used for the polytunnel covering has a significant impact on light transmission. Commonly used materials include polyethylene plastic, which is popular for its affordability and durability. High - quality polyethylene films can have a light transmission rate ranging from 80% to 90%. However, over time, the material may degrade due to exposure to sunlight, weather conditions, and chemicals, which can reduce its light transmission capabilities.
- Thickness of the Covering: Thicker coverings may block more light compared to thinner ones. While a thicker covering can offer better insulation and protection against the elements, it might also sacrifice some light transmission. For a 2m x 1m polytunnel, a balance needs to be struck between light transmission and the need for protection.
- Dirt and Debris: Accumulation of dirt, dust, leaves, and other debris on the surface of the polytunnel covering can reduce light transmission. Regular cleaning of the covering can help maintain a high light transmission rate. Even a thin layer of dirt can block a significant amount of sunlight from reaching the plants inside.
- Condensation: Condensation on the inside of the polytunnel covering can also affect light transmission. Water droplets can scatter and absorb light, reducing the amount that penetrates to the plants. Proper ventilation can help reduce condensation and maintain a more consistent light environment.
- Angle of Sunlight: The angle at which sunlight hits the polytunnel can influence light transmission. During different times of the day and throughout the year, the sun's angle changes. A well - positioned polytunnel can take advantage of the sun's path to maximize light exposure. For example, orienting the polytunnel in an east - west direction can ensure that plants receive sunlight for a longer period each day.
Single-Layer Film Versus the Complete Polytunnel
A laboratory value for the film is not the same as the average light available across the crop. A single layer usually passes more light than a double layer, but the full structure also creates small losses from hoops, doors, zips, overlaps, and wrinkles. University of Nevada, Reno Extension cites 88% to 92% PAR transmission for new single-layer film and 77% to 85% for double-layer film. For a compact 2m x 1m polytunnel, the frame occupies a slightly larger share of the visible area than it does in a wide commercial tunnel, so plant-level readings may sit below the film's published value.
Size, Frame Shadow, and Orientation
The small footprint does not automatically reduce the film's transmission rate. However, it can make local shade more noticeable because plants sit close to the hoops and end panels. Keep the tunnel away from walls, trees, fences, and sheds that cast long shadows. Orientation should be chosen for the site and season: an east-west ridge can favour winter solar gain at some latitudes, while a north-south arrangement may distribute light more evenly through the day. Observe the site before installation instead of relying on one rule for every garden.
Average Light Transmission Rate for a 2m x 1m Polytunnel
Based on industry standards and our experience as a 2m x 1m polytunnel supplier, the average light transmission rate for a new, clean, and properly installed polyethylene - covered 2m x 1m polytunnel is around 85%. This means that approximately 85% of the sunlight that hits the covering is able to pass through and reach the plants inside. However, as mentioned earlier, this rate can decrease over time due to various factors such as material degradation, dirt accumulation, and condensation.
For a clean, correctly tensioned, single-layer polyethylene cover, "around 85%" is a sensible practical estimate for the complete small polytunnel. Use the manufacturer's tested PAR figure when an exact product value is required.
How to Measure Light Transmission in Your Polytunnel
You can check the actual light transmission rate with a PAR meter or a reliable light meter. Take one reading outside the tunnel in unobstructed light and a second reading inside at crop height. Make the readings within a few seconds of each other, keep the sensor level, and avoid moving cloud. Calculate the result as: inside reading divided by outside reading, multiplied by 100. For example, an inside reading of 765 and an outside reading of 900 gives 85% transmission.
A Simple Field Check
Repeat the test near the centre, both ends, and the sides, then average the results. Measure at the same time on a clear day when comparing performance after cleaning or after replacing the cover. A phone light sensor can show broad changes, but it is not a dependable substitute for a calibrated PAR meter. The goal is not only to confirm the film specification; it is to find shaded or dirty areas that may need attention.
Measure before and after cleaning. A clear improvement in the inside-to-outside ratio shows that maintenance has restored useful light, even when you do not know the film's original factory value.
Importance of Light Transmission for Plant Growth
Plants rely on sunlight to produce food through photosynthesis. Different plants have different light requirements, but in general, a consistent supply of adequate light is essential for healthy growth. In a 2m x 1m polytunnel, a high light transmission rate can help ensure that plants receive enough light to develop strong stems, lush foliage, and abundant flowers or fruits.
For example, leafy greens like lettuce and spinach require a relatively high amount of light for photosynthesis. In a polytunnel with a good light transmission rate, these plants can grow quickly and produce high - quality leaves. On the other hand, some flowering plants may need a specific amount of light to initiate blooming. If the light transmission rate is too low, the plants may not flower properly or may have a reduced lifespan.
How to Maintain High Light Transmission
Clean the outside with water, a soft cloth, and a cleaner approved by the film supplier. Avoid abrasive brushes, solvents, and concentrated chemicals that can scratch or weaken polyethylene. Remove leaves from the roof, keep nearby branches from rubbing the cover, and retension loose film so deep folds do not hold dirt or water. Inside the tunnel, ventilation helps limit persistent droplets. University of Alaska Fairbanks Cooperative Extension notes that condensation can limit the transmission of light and solar radiation. Inspect the cover at the start of each growing season and replace it when it becomes cloudy, brittle, badly scratched, or difficult to clean.
Our Product Offerings and Light Transmission
At our company, we take great care in selecting the materials for our 2m x 1m polytunnels to ensure optimal light transmission. Our polytunnels are covered with high - grade polyethylene films that are designed to have a high initial light transmission rate and to maintain it over time. We also provide guidance on proper installation, maintenance, and cleaning to help our customers get the most out of their polytunnels.
In addition to our 2m x 1m polytunnels, we offer a wide range of other polytunnel products. You can check out our transparent polytunnel, mini garden polytunnel, outdoor greenhouse, and small tunnel greenhouse. Each of these products is designed with the same attention to detail and quality to provide the best growing environment for your plants.