Wharf, for years I grew perennials in two 160' double poly houses. One had 1 clear and 1 white sheet, and the other was glazed with 2 clear poly sheets. I liked growing in the clear house more than the white house in winter and just the opposite in summer.
The plants stretch slightly more in the white house, but not enough to worry about, hot temps were worse for stretching than the reduced light from the white poly. The thumbnail is my cold frame and has mostly white polycarbonate glazing on top and clear on the sides. I germinate my seedlings and take cuttings in this house, then grow on to the final transplant size in this house.
The stealth the white provides for our growing, is more than worth it for the small amount of stretch difference between the two coverings. If you have more questions pm me, I will be glad to share my info. I looked up the difference of the light transmission between the clear and white poly sheets and it was 88% clear, 70% white, the polycarbonate panels would probably be similar.
Edit: Can't get the pics to upload, will try to add later.
Edited by pointswest, 23 February 2012 - 06:22 PM.