I have replaced all windows in my house with a supposedly decent quality window from a local lumberyard. We did 1 or 2 rooms a year for several years. All were Low E with Argon. The Low E reduces heat from sun light. It is a clear coating between the double panes. The Argon adds to the R value, (insulates). At about the 9 year mark some windows are developing a pattern that looks like a screen between the panes. The lumber yard calls it seal failure. I don't think it is seal failure, there doesn't seem to be moisture between the panes. I think it is the Low E coating peeling off. I would be hesitant to buy windows with Low E coating at this point. My windows have a 10 year warranty and they have replaced several but I'm sure some will go bad after warranty is over. I guess I should bust one of the old ones to see if it is the Low E coating coming off.