There are many different options today for homeowners when choosing to upgrade their old inefficient windows to new high performance energy saving replacement windows. Most replacement grade windows are going to come standard issue low e glass and argon gas. Different manufactures attach different names for this glass but they essentially perform in the same way.
The task these replacement windows perform is to reflect the heat in both directions allowing radiant heat outside to be reflected away in the summer months and when heating your home in the winter it helps keep that heat from radiating out. Air infiltration can also contribute to heat gain-loss and most new windows will have low air infiltration ratings providing additional comfort and energy savings.
With all glass being relatively the same throughout all replacement windows available today, your choice will come down to budget and what material is used to construct the frame in the manufacturing process. Some the most popular window frame material choices are composite windows, fiberglass windows, aluminum windows, and vinyl windows. All of these have advantages and disadvantages and costs will vary depending on the material as well as the manufacturer you choose.
You will find that the majority of companies offer a lifetime warranty on the mechanics and glazing of the window. Replacement grade windows are custom made to fit into your existing opening and a qualified contractor will be able retrofit the new windows without any modification or mess. Before you decide on a contractor make sure that they inform you of all the options you have available to you.