Reflectix is rated differently for different applications.http://www.reflectixinc.comhttp://www.reflectixinc.com/images/uploads/allpdfs/f41%20foil%20bubble%20lit%207%2004.pdf
and using the application for a metal building, (which probably approximates a bus), and considering I only have a 1/4" dead air space, I figure as a WAG that the R value is probably around 1. However it also works as a reflective barrier system which also helps.
However... I don't really believe all the hype that I've seen from these kinds of companies. I saw a few that touted the R value as "Up tp r-25" for foil-bubble-foil.
Reflectix is available at Lowe's and Home Depot and many other home centers.
The blue foam is available from Lowe,s and other places. It is a Dow Building systems product with an R-3 rating per 1/2"http://www.lowes.com/lowes/lkn?action=productDetail&productId=15348-46086-15348&lpage=none
I cut the foam to fit between the ribs, using 3 layers. I also filled all the cracks and seams with canned spray foam insulation. The foil goes over the ribs, (which I covered with felt as a conduction barrier), and the edges of the foil are sealed together with aluminum foil HVAC tape.
Hopefully all of this gives me at least an honest R-9 rating. It seems to help quite a bit anyway.. we can heat the bus with 2 1500 watt space heaters and be in shirt sleeves down to about 20° if the wind isn't blowing and the nose isn't faced into the wind.. that part isn't done yet.
In the summer time, if we are in the shade, our two window A/C,s keep it comfortable inside up to about 95°. If we had roof air or mini splits, I don't think we'd have a problem cooling at over 100°