Here are the numbers: Infra red (ie. radiant) is as happy going up as down. The ratio of heat up to down generally is the inverse of the ration of R-value up vs. down. For example, if you have an R-value of 2.5 up (not unusual for carpeting) and an R-value of 25 down (not unusual with R-19 insulation combined with the R-value of sheet rock and other assembly contributions to overall R-value) you will have 91% of your heat going up and 9% going down.

What the 9% down does is slightly skew the control of the lower zone. For example if the lower zone is calling for 25 BTUH/sf, you might actually be getting 27.5 or so. This may in fact be a trivial overshoot factor depending on the differentials between the upper and lower zones but it is a factor.

I am not suggesting that we should spend money willy nilly on insulation just to make us feel good, but in general, more insulation is better than less and the cost of going from R-13 to R-19 or even R-30 is relatively trivial compared to overall construction cost, and it does provide more precise zone by zone control.

I only mentioned sound control as but one more mitigating factor in justifying the relatively small marginal cost increase. And yes, I did insulate my interior walls for the same reason and my home is exquisitely quiet as a result. I realize not everybody wants their home this quiet but one of the main reasons I love radiant is in fact that it is quiet and so high levels of interior insulation further that advantage.

All I'm saying is the difference (by your method) between an R-19 and an R-30 is 3%. So, and extra 3% is going down into a space I'm already heating. Big deal.

Admittingly, I don't know the price difference, but I can't see spending an extra dime to prevent 3% infiltration into an area I'm already paying to heat.