Controlling Heat Transfer
most vinyl replacement windows are manufactured today to reduce heat loss trough the glass and the replacement window itself. Some do a better job than others.
all replacement windows under the right conditions will produce condensation. Think of a pot of boiling water below a kitchen window, obviously the window will develop condensation. The idea here is for the replacement window to not produce condensation under normal heating and cooling conditions. Why is this important? One, if the replacement window is developing condensation it means it's not effectively controlling heat transfer. And two, moisture collecting on replacement windows usually collects (if there is enough of it) and drips down the replacement window and usually to a wood surface - not a good thing.
Blocking Harmful UV Light
LoE glass can block 40 to 70% of solar heat and UV like tinted glass, but without sacrificing daylight. This is one of the most important reasons for buying replacement windows.
Maximizing Natural Light
The Visible Light Transmission (VTL) of replacement windows indicates the percent or fraction of available visible light transmitted through a window. A higher VLT means more visible light gets through.
This is typically achieved with window tint or tinted glass. Replacement windows can be ordered with a tint option. Another option is installing the window tint after the windows are installed with a tinted film, then if you change your mind the tint may be removed.
Another item to consider is a windows STC Rating which indicates a window's Sound Transmission Loss. The higher the STC the better. A Replacement Windows average STC Rating range of 25 to 50. Most humans can only detect noise level differences in increments of 4 or more, so the difference between a STC 33 and a STC 35 is not significant.