So, what's my jersey for then? And how does another layer, doing the same job, not make you warmer, not cooler? I know different things work for different people, but the math doesn't work here.

I have always just gotten a jersey that fits right (tight) and that keeps sweat from building up. I never have sweat dripping around on my torso -- it's wicked and evaporated. I see no way that a base layer could possibly help. Maybe more people are wearing jerseys too loose to use for this purpose?

Note that I ride in a very warm climate that goes from humid to moderate, and I've raced many times over 105F.