If you used insulating foam, which is opaque, solving the transparency problem, you won't get any light in. IF you only used it on the side panels, leaving the top without insulation, you would loose almost all insulation value. But that's the only way to get light in and plants have to have light.
There are several manufacturers of polycarbonate paneling that allows sufficient light in but you can't really see through. My lean-to is made of a material called "Solexx" and is sold by Greenhousecatalog.com. It comes in rolls and you simply cut it to length. It is not very expensive and is double walled, giving you some insulation value. I used the 3.5 mil paneling, but if I had it to do over, I would use the 5.0 mil material. It is semi-transparent, allowing about 80% light and you certainly can't see what's inside the greenhouse. I have now completely covered my two greenhouses with 15 mil bubble wrap, and it comes in transparent and semi-transparent material. I used both, wanting more light transmission in some areas, less in others. But my usage of the bubble wrap is an experiment and only time will tell whether I get the insulation value that I hoped for. I will give the results after winter has passed.
