U values have no place for most of us in window rating. U values are
the mathematical inverse of R values. So an R value of 2 equals a U
value of .05 or 1/2. I use R values they are a logical comparison
instead of throwing in U values as a comparitor. U value to R for most
is like metric with feet, or scones to Decibels, confusing and unessary.
And what is that 1/ R comparitor you posted, its wrong.. So is air
infiltration as an R factor. R or U just convert it, its measured the
same.