Which polythene?

The thermal anti drip polythene seems to be the best one in the uk... so the websites say! It holds the heat inside the polytunnel for longer and doesn't let condensation drip on to the plants. The suppliers call it clear but it is foggy and you can't see through it really.
I got a 180 micron cover with my polytunnel from premier polytunnels and it has uv barriers in it and the warranty is 5 yrs but they told me it should last 8 to 10 yrs.
I wouldn't go for anything else otherthan thermal.

Thanks Reedy, I'll check that out.

So what is the benefit of foggy rather than clear? Surely the more light that get to the plants the better? I've heard of scorching. Is it something to do with that?

Do UV barriers block UV or simply prevent the material degrading as fast in light? If they do block UV and this is an advantage, why are other covers marketed as UV open?

Blimey, I'm not all that sure! :@ My polytunnel cover is classed as clear but has a foggy/misty finish so you can make out the shapes of plants inside but can't seem them clearly. It still lets plenty of light through though and the info I got from premier polytunnels says the polythene is manufactured so that the light bounces around inside and shaded areas get light too. Aren't the "clear" polythenes just that - clear plastic and nothing special in them to help with growing? The lady at premier told me that the UV barriers have warranty for so many years after which time the polythene will start to go brittle in the sunlight so I guess it could start cracking and splitting and you'll need a new cover. The UV barriers decrease a bit each year I think. But my neighbour has had his polytunnel cover on for 11 years now and it hasn't gone brittle. I hope this helps.

Which Polythene?

I’ve been considering reply to this post for a few days now and so finally here is my “Guide to Polytunnel Polythene”.

Anti-drip/anti-fog: An additive in the polythene makes the condensation form a continuous layer or “sheen” on the inside surface of the polythene rather than condensation droplets. The advantages of this are firstly that you don’t get drops of water dripping on everything in the polytunnel (including yourself). Without the anti-drip additive, droplets form on the inside face of the polythene which reflects a lot of sunlight. You will notice this on a cold, sunny morning when, from certain angles, your polytunnel will look very bright as it reflects the sunlight – a waste of solar energy. Virtually all horticultural polythenes now have this additive but its effect diminishes over time and so it is only effective for the first couple of years.

Clear or Diffused? “Standard” clear and diffused polythenes both allow the whole spectrum of light to pass, from Ultra-Violet to Infra-Red. What a diffused polythene does is “scatter” the light. This has the effect of producing more “even” light levels throughout the polytunnel. Whereas under a clear polythene the plants at the sun side tend to absorb most of the sunlight whilst the plants at the back may be a bit in the shade. From a distance a diffused polythene will look slightly “milky” and the objects inside will appear “vague”. A clear polythene is reasonably clear – more like looking through a slightly dirty window.

Ultra-violet stability: The more UV stable a polythene is, the slower it will degrade. Most horticultural polythenes have a lifespan of 4 -5 years which is not necessarily dependant on the thickness of the polythene. If a polythene is subject to high levels of UV light i.e. in a Mediterranean climate, it will degrade at a faster rate.

Ultra-violet blocking: Not to be confused with UV stability, additives in the polythene filter out or reflect UV light which stops most of the UV light from entering the polytunnel. A lot of fungal diseases such as mildew and botrytis require UV light to spread. Also most flying insects such as aphids require UV light to see. The effect of a UV block is to significantly reduce fungal diseases and aphid infestation.

Far Infra-red blocking: The common term for this is “thermicity” and determines how effective a polythene is at retaining heat. A polythene with high thermicity is ideal for a polytunnel which will be heated as it will retain more heat than an ordinary polythene.

Near Infra-red blocking: Near Infra-red radiation is the solar energy or “heat” from the sun. In the height of summer it is preferable to reduce the amount of heat entering the polytunnel to avoid stressing the plants. A polythene which blocks near Infra-red is the solution and most thermic films will block near Infra-red as well as far Infra-red light.

I hope this clarifies some of the terminology used by polytunnel and polythene suppliers - Or maybe I’ve confused you even more!

Thanks- that's very helpful!

Excellent! Now I understand what the various products are designed to do I will be able to make a well informed choice. Thank you very much.