Just to help in your light calculations, single layer polycarbonate has about a 90% light transmission, double layer polycarbonate about 80-85% and greenhouse grade 6 mil polyethylene about 92% (depending on manufacturer). So, when you subtract the light loss for each layer, you may end up with a less than adequate level of light with multiple coverings.

Polycarbonate is nowhere near 100% PAR transmission. More like 75%.

Here is a link that might be useful: PAR transmission for different glazings

Thanks for the link S.B. However, it seems the Rutgers paper used values for polycarbonate based on products from many years ago (NRAES-3, 1985). More current research on modern polycarb panels shows 94% PAR transmittance for single layer and 83% for double.

Here is the link

Here is a link that might be useful: greenhouse coverings

My twinwall 8mm polycarb is 80%. 6 mil from Home depot is in the 70%-ish range. 6 mil greenhouse film like Tufflite is 92% IIRC.

So a HFGH twinwall is ~80% and then a HD 6 mil painters plastic gives you not enough light, Tufflite-type GH film gets you right at the minimum transmissivity.

Dan

Dan (sorry brandond to get off track)- What figures are you using to determine the "right" light minimums? CR

.8 * .7 = ~.56, which is too low for that temp and photoperiod, probably looking for ~.65 minimum for any growth in winter. Feel free to point out my error(s).

Dan

CR, thanks for the reference, it has been bookmarked, and I stand corrected.

Dan, reference please? I recall reading somewhere that citrus, as an example, can't use anywhere near all of the PAR they receive in full sun, that they were photosynthetically maxed out somewhere under 50%. Maybe comparing apples to oranges, I don't know. Educate me.

Many years ago in GH Ops undergrad class, we were taught 'rule of third' where in winter you could drop light 1/3 and still get growth as long as you could heat, and as photoperiod goes up don't take covers off more than 1/3 increase in lux. That was many years ago, and my texts on my shelf are vague as to actual numbers (including, surprisingly, Ronald's Greenhouse Gardening), so I'd be happy to be corrected.

Dan

At least for tomato, cucumber, pepper, optimal yields are achieved at about 50-60% of full sun, summer, outside light. However, only about 50-70% of sunlight actually reaches plants in a typical greenhouse. Fall and winter outside light is below optimal in about half of the USA states, and less than 25% of the light needed for optimal production inside a greenhouse.
There are many related references, but the one below gives some of the numbers.

Here is a link that might be useful: greenhouse light requirements

I don't see how that ppt either supports or negates the topic of sunlight light available in winter through materials and effect on plants, and subsequent topic of ~% light transmission for plant effects. Perhaps I missed it but it looks like the topic of ppt is supplemental lighting in commercial GH ops.

Dan

In reference to the question/comment from S.B., many crops do only utilize 50% of full outside sunlight. The powerpoint, specifically page 4, shows a 30+ requirement for the crops mentioned (other refs. use 23 "units").
Since full outside summer sunlight is 58 "units", these crops only use approx. 50%, supporting the question/comment. The effects of additional GH covering materials has already been addressed in the first comment.