Calculation of edge U-factor

[J. Ray]

I'm trying to gain a deeper understanding of the calculations behind WINDOW/THERM and how different components affect the overall performance of the assembly.  I understand the area-weighted approach taken to calculating overall unit U-factor, SHGC, and VT, as outlined by section 4.4 of the THERM 6.6 / WINDOW 6.3 NFRC Simulation Manual.  I also have observed that THERM files will return a value for Uedge, the edge-of-glass U-factor.

What I don't understand is how this value is calculated.  I've looked through the TARCOG and CONRAD documentation from the website, and did not find any mention of how Uedge is determined.  What is the formula for calculating Uedge, and what factors affect the output?