There are no R ratings for sun shade material, I would think you would be aware of that. They are rated as to the percentage of sunlight being blocked up to 100%.
Shade fabric is not made to reflect anything, it reduces or blocks the penetration of sunlight depending on the percentage desired, along with reducing the amount of heat being transferred due to radiation.
I have used Reflectix as insulation for many years for diferent things, but putting it on all the glass must make it very much like being inside a submarine. If that is to your liking, go for it. But if you would like to reduce the heat penetration and also see out, shade material mounted on the exterior is the answer.
