How do you determine the u-factor of a window assembly? I understand that the u-factor is reciprocal of the sum of resistance (U=1/Σr) for wall assembles, but I’m unable to find any relevant arithmetic for windows. I understand that heat flow rates vary between the center of glass and frame, and that u-value of windows is representative of a whole system, but is there not an arithmetic means to determine the u-value at the center of the window? For example, let’s say I have a ¼” double pane glass with a ½” of argon gas between them (1” total thickness). What is the u-value of the glass at the center? Do I use U=1/Σr? Is that correct?