Radiant barrier only works when there is an air space and a ventilated attic. Your project, being living space in an a actual attic area, would not be able to utilize a radiant barrier. The only way to get the code required insulation for a residential space is to lose some headroom and use spray foam to get the required R value. Otherwise, that is going to be one HOT living space that even AC won't help much. And it'll cost a pretty penny to try to cool as well. Ditto 30 pound roofing felt. You've posted about quite a few issues with this build that do not meed residential code standards. I'm curious as to why you wouldn't want to at least meat the minimum required standards for safety, comfort, and energy efficiency. Especially when there are so many opportunities to design something that exceeds code standards and performs so much better for safety, comfort, and energy efficiency.

Yes agreed. Radiant barriers are generally only cost effective in hot climate (possibly mixed climate) with traditional vented roofs. Open cell spray foam on roof deck (other side of roof structrual sheathing) would mean unvented roof thus not compatible with radiant barrier. Now if you sprayed the OC foam to drywall ceiling plane and kept vented attic it might still make sense but probably not likely depending on if you sprayed to code minimum levels. If you spray to code minimums and stick to best practices of keeping ducts out of that space then absolutely dont waste money or effort on radiant barrier.

Check Clemson and SCSU web sites as well as other land grant university web sites in states with hot humid climates for advice in local building problems. They publish lots of research and bulletins. My memory is that radiant barriers were popular for a time, but have become less so. The concept has merit. It is impossible to pull enough air through an attic and have much effect because most of the heat gain in the house ceiling is heat from the hot roof deck re-radiated to the other surfaces in the attic. Radiant barriers, theoretically, help with this. I think that what caused them to go out of favor is that they get dirty very quickly and that makes them ineffective. You should, however, confirm this with unbiased sources. If you need a new roof, consider a cool metal roof. Consider hiring a home energy rater (BPI or Resnet) to identify low-hanging fruit for energy (and comfort) improvements. Often, there are local government or utility programs to pay for the evaluation or discount the cost. They know local problems and solutions appropriate for your climate.

It would be redundant and might make it difficult to safely move around in the attic.