So I have an interesting question that's I'd like to hear from you on. I have a tapered basement wall that has been covered with a waterproof membrane. I plan on attaching 1-2" rigid foam insulation, and then building a framing wall which I'll fill with batt insulation. What this leaves me with is a very interesting triangular gap between the two layers of insulation. My guess would be around 2" at the top.
Edit: Basement is 3/4 below grade. Climate is Toronto, Ontario, so high humidity in Summer and gets down to -40 in Winter.