I'm the newest member of the HWH Insulaters' Club.
Home Depot sells a kit claiming R13 from ~3" of vinyl-backed fiberglass. I think it was $15. Not sure how that compares to the Kraft-backed stuff a few aisles over.
The blanket fit perfectly on my HWH, which is more barrel-shaped than most. It's a gas HWH, so I left a gap around its base for combustion air, didn't cover the top, and made a cutaway for the pilot light access and the controls.
I think my HWH already had well above average insulation, but now it's that much better.