How about the p2000 foam insulation: http://www.p2insulation.com/

For 1 inch, installed with seams properly taped and an air space like
you have, you can get close to R-40. Yes... 1 inch... r-40. It's
expensive but great for limited space. A number of the timber framers
around here are starting to use it since it's so useful for limited
space areas. The 1 inch of p2000 gives better r-value than the regular
rigid foam they used to use. I understand it's been used commercially
for some time and is just now getting onto the residential market. From
the literature it seems it gets it's high r-value by addressing
radiative and conductive heat along with a vapour barrier all at the
same time.

- Scott