Greenhouse ?

14 years ago

Hey everyone. I recently applied a layer of 6 mil thick plastic to the inside of my greenhouse on 2 sides. I have a 10x10 house that has an insulated north wall. I have polycarbonate on the east, south,and on the lower slope of the west roof. Heres my question? The polycarbonate is clear and allows 100 percent light transmission. The plastic allows probably 50 t0 60 percent light transmission. When things start heating up in there in the spring, should I pull that plastic off allowing more light, which means more heat. Or would it be beneficial in terms of allowing some light but not too much keeping it at a more moderate range, and not allowing soring temps in there once summer hits. I know it seems as a double edge sword. The other option I have if with time is placing a second layer of polycarbonate on the inside, that has a 4 in air gap inbetween. Again this would allow 100 percent light transmisson but would also have more R value since its double layered. thanks

Comments (12)

oakhill (zone 9A, Calif.)
14 years ago
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.
stressbaby
14 years ago
Polycarbonate is nowhere near 100% PAR transmission. More like 75%.

Here is a link that might be useful: PAR transmission for different glazings
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oakhill (zone 9A, Calif.)
14 years ago
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.
oakhill (zone 9A, Calif.)
14 years ago
Here is the link

Here is a link that might be useful: greenhouse coverings
Dan _Staley (5b Sunset 2B AHS 7)
14 years ago
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
oakhill (zone 9A, Calif.)
14 years ago
Dan (sorry brandond to get off track)- What figures are you using to determine the "right" light minimums? CR
Dan _Staley (5b Sunset 2B AHS 7)
14 years ago
.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
stressbaby
14 years ago
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.
Dan _Staley (5b Sunset 2B AHS 7)
14 years ago
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
oakhill (zone 9A, Calif.)
14 years ago
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
Dan _Staley (5b Sunset 2B AHS 7)
14 years ago
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
oakhill (zone 9A, Calif.)
14 years ago
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.