Historic windows almost invariably present two specific challenges to home energy efficiency:
1) Air & moisture infiltration/exfiltration
2) UV & thermal transfer
Assuming that sashes are in good shape and weather tight, infiltration/exfiltration and moisture are often still problematic because sashes often don't fit tightly; for example, if the window frame is not completely square, or because of a general lack of sealing between the sash and frame. The window frame and casing often are also in need of sealing. In the worst case, an inadequate drainage plane might result from poor flashing, inadequate sill slope, or an inward leaning wall, causing water accumulation, deterioration, rot, mold, etc.
This thread invites further questions, as well as useful solutions that anyone might've devised, or otherwise observed, in solving these issue. In particular, we are interested primarily in solutions that emphasize preservation and repair, primarily, and replacement with historically accurate components only when repair is not possible. That is, we are generally striving for minimal impact to historic fabric.
(Please note that I will launch a separate thread covering UV & thermal transfer. Thanks!)
Thanks for starting these historic window threads. I am passionate about this topic.
David Clark covers window rebuilds on his post, but I have found massive air leaks around the outside of the window buck on older masonry homes. I remove the trim and spray foam the gap. If you think the drain plane is compromised, you can check this at the same time.
I buy my weatherstripping from Conservation Technology, they are high quality and they carry pretty much anything you need.