The Lexan corrugated softlite does have 85% light transmission with 100% diffusion. It is also available in 8mm twinwall (100% light diffusion with 79% light transmission) and 16mm triplewall (100% light diffusion and 72% light transmssion). You ask if the 85% light penetration will produce the soft appearance of diffused light. They really don't have anything to do with each other. The 100% light diffusion is the number you should be interested in for the appearance you desire. The sheets do not look like an opaque white sheet does. The best comparison I have come up with is the white sheet is opaque, the opal color sheets are like looking through whole milk and the softlite sheets are like looking through skim milk. Not a perfect definition, but it's the best I have been able to come up with. The diffused light is great for your plants. I would definitely consider using this on your white frame. I think it will complement it nicely. Tammy

we're in the woods in south new york state, not the beach. (but it does sort of look like salt air deposits or corrosion). the vinegar/water solution is interesting, altho i think it would be difficult doing it on a ladder, working overhead -- even though it's only a 1 story house. and after all that, whatever is causing it would probably bring it right back. thanks for your input!

Galvalume is actually sheet steel coated with an aluminum-zinc alloy. What you describe sounds like corrosion, it could be from humidity or possibly poor airflow. If its really a problem try paint or primer made for galvanized metal

Thanks for the extra detail, Verbo. I'll have R10 at the window openings in the winter months, when condensation could be a problem. Windows will be smaller than 2x2 ft, probably closer to 18x18 inches. There will be a vinyl membrane covering both the top and bottom of the decking, so a 1/2 air space with no water permeability. Semco paintable membrane on the inside edge and perimeter of the opening, and dividers that match the profile of the panels will be installed at three places, ridge, middle (below skylight openings), and bottom of the roof area. I'm working on a drawing.