Links
Ten top reasons to repair and restore wood windows
A few links to energy savings and comparison information along with discussions on sustainability
http://www.dahp.wa.gov/sites/default/files/Windows%20Energy%20Efficiency%20Facts%20and%20Myths.pdf
Here is some information from the U.S. secretary of the interior on rehabilitating and preserving historic windows.
https://www.nps.gov/tps/standards/rehabilitation/rehab/windows01.htm
https://www.nps.gov/tps/how-to-preserve/briefs/9-wooden-windows.htm