Building Greenhouse - Polycarbonate choices driving me crazy

[orchidfan]

I'd like to build an energy efficient lean-to greenhouse - however, the higher the mm for the clear polycarbonate the lower the light transmission numbers. I have a mix of cattleyas, laelias, mixes of same, zygopetalums, dendrobiums (mix), miltoniopsis/miltonias, phals, paphs and a few cymbidiums. 32mm has a light trans of 59%, 20mm @ 62%, 25mm @ 61%. R value vs. Light transmission is the issue. For the product I'm looking at - For 32 mm, I'd get an R value of 3.57 with light trans of 59%. For 16mm with a 4 M wall type, I'd get 2.7 R value with light trans of 67%. For 16mm, triple wall type, I'd get 72% light trans and R value of 2.5. I just don't know what light trans the plants need - and I know that with my selection of orchids it will vary. Any one have any suggestions or experience about which light transmission level to go with? Help!!!