Our nation’s fixation on R-value for our insulation is ridiculous.  Large scale testing has proven that an air barrier is far more effective than any R number.  Take this one experiment- where a duplex had half the attic insulated with R-20 Foam and the other half with R-30 batts.

When the snow came, the roof with R-30 batt had no snow, and the R-20 foam side had snow.

That means the higher R-value allowed more heat to escape because it didn’t have an air barrier.

Stop the air flow, stop the heat and money loss.  Period.