How to find a very low U-value replacement window for a bathroom?
I pushed the big box salesman to (a) give me the U-values for the windows I was considering, and (b) play with the parameters in the ordering software, to see what added features did to the U-value.  I found out that Pella 350 with foam insulation in the frame, and triple pane, got us down to 0.17.  Which is what I want for the bathroom.

I was not able to get much in the way of detailed information from the Pella salesman.

I bought the big box (Lowe's) Pella windows.