The curtains have evolved over the years. The changes often came about as a result of a customer reaching out to me. I have had so many people contact me. " Hey Sue, can you do something like this? " or " Do you think you can change that?" Sprinter people really use their vans and know what they need. I always listen, and try to come up with a way to achieve what they want.
One of the significant changes was the addition of Temptrol. This came about when a customer sent me a couple of breathable, washable emergency bivvys he had purchased at REI. He asked me to cut them apart and use them as lining for his Sprinter curtains. I was familiar with this item. I had done quite a bit of winter hiking in the White Mountains in NH. I always carry one in my pack. It made sense to me. If it could save a life on Mt. Washington, it should be useful as a curtain in a Sprinter. Reflect the heat out or hold the heat in. But as I was carefully ripping out stitches in the bivvy so as to have a piece large enough to cover the slider window, I thought to myself, " There has to be an easier way to do this." Then I discovered Innovative Insulation and Temptrol. Aha! 
I could purchase this same material by the yard on a bolt. It works fantastic as a reflective barrier in the curtains. As a plus, I can use the scraps to line my mittens and boots when I winter hike in the Whites.