The roofing is 3.5mm Solexx double walled polyethylene. It has an R-value of 2.1, 77% light transmission, 100% light diffusion, and only costs $1.55 per square foot. It is by far the most cost effective glazing solution that I've come across. It has a 10 year warranty, lasts 10-20 years, is as insulative as triple walled poly-carbonate, yet is only a fraction the price. Furthermore it can be lapped, meaning you don't need to buy H-channel and U-trim (an added expense of poly-carbonate). And it comes as a roll, reducing the cost of shipping and making it easier to handle and store. You have to order it through one of their dealers but it ships quickly and cheaply. I am one of their dealers and am happy to offer a discount for all the Permies out there, just let me know your on Permies at the time of order.