Hi, I've gotten quotes for replacing the windows in my house. One company sells Slocomb 477 double-pane windows with a U-Factor of .22 on double-hungs. They get that by applying both a hard and soft Low E coating on their windows. Is that a bad technique to get a low U-Factor? Most double-panes have about .26 but only use soft coat Low E. Thanks for any help and opinions!

Generally, but not always, the hard coats are used for high solar heat gain coefficient (SHGC) windows, and soft coats are used for low SHGC windows.  Higher SHGC are obtained with some increase in U-factor due to increased IR radiation losses, the same effect that gives increased solar heating due to solar IR energy.  You did not say if you are looking for low or high SHGC.  Also, visual transmission (VT) may sometimes be reduced in an effort to get a very low U-factor.

Thanks for your reply! I was most interested in the U-Factor. I did wonder if using a hard coat and soft coat on the same window at the same time, on different panes in the window of course, is a good idea. Do any other window makers do this? Any thoughts if it makes sense? Thanks!

Be sure to check the CR (Condensation Resistance) number on that window with the surface 4 low-e. They can have a slight drop in CR number and if you have an issue with condensation, you will have more potential to do so in this case as well.

Where is the home located? Tuning the window elevations and glass packages is best done in accordance with exposure and climate. Unfortunately, most window companies sell what they sell based on the deal they get or the promotion.

FYI...they are still sputter coats vs a true hard coat low-e (in most cases) but they get rid of the silver and thus make them suitable for exposed interiors.