Poly-carbonate and light reduction?

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Cactus Kid
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Poly-carbonate and light reduction?

Post by Cactus Kid »

I have double glazed my new greenhouse by putting 4mm twin wall Poly-carbonate sheets up on the inside of the glass. This has had the desired effect of holding in the heat and reducing heating costs.

Should I remove some or all of it during the summer as I have heard it reduces the amount of light the plants receive?
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BryanW
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Re: Poly-carbonate and light reduction?

Post by BryanW »

Ralph M is the expert on this subject
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Cidermanrolls
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Re: Poly-carbonate and light reduction?

Post by Cidermanrolls »

I leave mine up.
I cannot quantify the loss of light impact.
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Re: Poly-carbonate and light reduction?

Post by Cactus Kid »

Thanks to those for replying so far.

A quick search online gives varying answers with light reduction from 4% to 20%, so not very helpful!
Joined Havering branch 2006, although have been growing on and off since the age of ten!
Have a large balanced collection of both cacti and succulents but enjoy growing cristate, monstrose and generally anything a bit weird!
Mike
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Re: Poly-carbonate and light reduction?

Post by Mike »

It probably depends on the siting of your greenhouse. In a sunny site I doubt it will be a problem. I have internal bubblewrap in the greenhouse. It stays up all year, and doesn't seem to cause any problem.
Mike
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BryanW
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Re: Poly-carbonate and light reduction?

Post by BryanW »

Cactus Kid wrote:Thanks to those for replying so far.

A quick search online gives varying answers with light reduction from 4% to 20%, so not very helpful!
Have you got a Light/Lux meter on a soil moisture/acidity probe lying around?
You can pick up a Lux meter fairly cheap from the usual online market places.
I did a test behind double glazing and I got a very low reading compared with outdoors, I haven't tested polycarb though.
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iann
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Re: Poly-carbonate and light reduction?

Post by iann »

The light reduction compared to plain glass, or even nothing, is negligible. If you're worried, then cleaning would be the place to start. Polycarbonate is actually highly transparent, as good as glass, but the major loss of light comes from reflections at each surface and twinwall has more surfaces.

There are significant differences though. A considerable amount of the light is diffused by twinwall rather than passing directly through. So shade is brighter, but direct sunlight is less bright. Polycarbonate also transmits infrared differently to (typical greenhouse) glass and the interior will heat up less quickly than a glass greenhouse. It also transmits less UV, or rather blocks longer wavelengths of UV than glass, effectively all UV. Scorching (of certain types) is potentially reduced. Very old polycarbonate may begin to yellow and then it would block a lot more light.
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DaveW
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Re: Poly-carbonate and light reduction?

Post by DaveW »

Instead of a Lux meter you can use one of the old photographic light meters (some do measure in Lux too). Take a reading pointing up at the polycarbonate then go outside and take a similar reading of the sky and note the stops difference. You can even use a digital or film cameras light meter in the same way.

https://www.jeffgeerling.com/blogs/jeff ... n-exposure

Really you are only interested in roughly the amount of light either glass or plastic stops compared to the light falling on it outside, not serious Lux measurement. A Lux meter for a one off measurement is therefore expensive overkill. You can even get Lux Apps for Smartphones:-

https://www.dial.de/en/blog/article/lux ... luminance/
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BryanW
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Re: Poly-carbonate and light reduction?

Post by BryanW »

Quite right Dave, I should have kept the word 'Lux' out of the equation, I stand corrected (tu)
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habanerocat
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Re: Poly-carbonate and light reduction?

Post by habanerocat »

A UV grade polycarbonate is available for greenhouses. Generally polycarbonate is as transparent as glass but it's the yellowing that causes it to be less transparent over time. The UV stabilised grade is still only guaranteed for ten years though.
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