Hi all,

I have a question about how to accurately model U-factors in windows. As thermal resistance (or I guess heat transfer coefficient as we're talking about U-factors, not R-values) varies with temperature, a window will have a different U-factors in typical summer and winter conditions (I believe the environmental conditions are specified by NFRC).

My question is: given a window with a U-0.19 in winter and U-0.20 in summer, what is the best way to model this? Is it best to average the two numbers? Or pick the winter U-factor as it will be more accurate while modeling winter months (where the difference between inside and outside temperature can be on the order of 70?F as compared to the temperature difference in summer that is only 30?F or so)?

If a window is imported from, WINDOW5 does eQUEST actually calculate the U-factor at every temperature/temperature difference? And if so, is it worth trying to create this file to import to eQUEST?

I'm using eQUEST, but maybe this topic applies to all energy modeling software (and should be sent around the Building-Sim List Serv).

I appreciate any and all thoughts.

Kind regards,
Alex Krickx