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.

I can't speak to the efficacy of the barrier itself - Solargary may know more about that. But I'd point out that electricity is almost certain to increase in cost as other fuels also increase. Your payback rate is likely to accelerate in the coming years. And if you have any "green" inclinations at all, for you it may just be the "right thing to do." Hope this helps!

Thanks. I actually tried the radiant barrier paint additive. I added it to an "oops" gallon of paint that I saw at a big box store. I applied 2 coats to the inside of the garage door and it did seem to help the garage. Prior to application at 5:00 on a 100 degree day, the garage was over 120. After the application the garage was less than 110 on two separate 100 degree days at 5:00. I'm not sure if that will translate to a meaningful energy savings, but it will definately pay for itself since it was only $8 for the additive and $5 for the paint. I dont believe I would use this on walls that are already insulated, but I'm considering using it on the underside of the attic as well. For anyone interested, I bought from a company called Kool Coat. I was very skeptical as the product arrived in a ziplock bag with a 1 page word document of instructions. I'm sure that its a 1 man operation who buys in bulk and sells smaller quantities on the internet. I'm sure most companies selling these additives buy from 1 or 2 suppliers, I just went on price. I'd definately do this project again. Hope this helps anyone considering it.

Oakland has relatively moderate weather as the OP knows. It's neither very hot in the summer nor very cold in the winter. I think the minimum insulation standard here is R30, about 10 inches of fiberglass batts, as an example. Having a bit more is fine but there's a fall off in the cost/benefit relationship as you add more so to say "lots of insulation" isn't necessarily usefl. . The OP hasn't said what kind of structure he's in but there are a lot of older homes in Oakland. Air leakage may be a greater problem than insulation. It's another thing to consider. Short of the expensive task of replacing single pane windows (which are very common), easy tasks for a DIY to start with in the air leakage department are to seal plumbing and electrical openings in walls and to replace worn or inadequate weatherstripping around external doors and also windows.