I have an old farm house with some updates. The first floor has double-pane windows (15+ years old), and the upstairs has old single-pane windows. We replaced 11 windows with Energy Star-rated windows. I reviewed my energy use, and found that it did not decrease at all. Could I have been sold bogus Energy-Star windows?
Evaluating any energy improvement in a home is difficult. You need to compare energy use based on similar heating degree days. Degree days take into account the dramatic variations in temperatures. But you also need to consider wind, interior temperatures, heating plant performance, air leaks, ventilation, use of fireplaces, and other variables.
New, Energy-Star rated windows with thermally insulated glass will normally perform better than old wood windows. If they are rated and certified, the window should not be an issue. But on-site installation and air sealing is a key detail affecting the window and wall system performance.
If your old single glazed windows were tight fitting and you had tight fitting wood storms, the actual change in the heat transfer through the new windows will not have a large effect on your energy usage. The replacements just don’t save much money if the original windows were in pretty good shape.
If you were given some type of guarantee or promise from the window installation company, I suggest you contact them with your concerns and read all the claims and fine print.
In my mind, the biggest reason to replace old damaged wood windows is lower maintenance, ease of operation, appearance, comfort, and finally energy savings.