Regd. glazing U value to be considered as per ASHRAE 90.1 2010

First of all make sure to ultimately model the whole assembly U-Value taking into account the frame effect, not just the U-vlaue of the glazing. There are a couple of ways to proceed. You could use the more conservative value. Conservative meaning the one that produces the worst energy use for the proposed case. Depending upon your project type, climate and other factors this could be either one of the values so you might need to model it both ways. Another alternative would be to split the difference between the two. If doing that I might try to do some sort of weighted average that was respresentative of the local climate (i.e. is it a hot or cold climate).

How you account for the frame effect depends on the software you use. We use eQUEST and one has the option of using a whole assembly U-value (usually obtained from the window manufacturer, not the glazing manufacturer) or letting the software calculate the whole assembly by modeling the glazing separately from the framing. The effect of the framing depends a great deal on the materials used and the quality of the thermal break if applicable. I would not use a weighted average for this. See also 90.1-2010 Appendix A8. You often cannot tell if a higher U-value is worse or not as it depends on many factors, for example is the building internal or external load dominant? So you are in a relatively hot climate. In that case I would weigh the U-value closer to the summer U-value than the winter. Typically the difference is not all that much so let's say that the summer is U0.26 and the winter is U0.29. For a hot climate I might use something close to U0.27, for a cold climate I might use something close to U0.28. You could do a weighted average based on the ratio of cooling to heating degree days for your location as one possibility. There are probably other ways to estimate this too.