The debate over restoration versus replacement is often presented as a straight forward comparison but it is far from such. It gets reduced to one fact: your old house is drafty and cold and the windows are the culprit. Seemingly, the only solution is to replace the windows and eliminate the inefficiency. While this is partially true, it turns out to be only a small fraction of the consideration that needs to be given to the issue.
The whole truth is that your old house is made of real (not composite) materials and it wants to breathe, contract and expand. All the materials are so much closer to their natural sources than any modern materials and, therefore, want to act more like they did in those states. That means following the seasons according to what the season brings. In so doing, they employ the bend-don’t-break pattern that keeps a tree lasting for decades and even centuries in it’s natural state. And these are not the trees so quickly grown and processed for modern lumber but trees that had decades and centuries to slowly grow in strength and durability. This translates into windows that have that capability. These kinds of trees barely exist anymore so even the finest replacement wood window pales in comparison to the quality of wood used to construct the existing windows in a historic home. As with most all things that we process from nature to serve our purposes, there is a requirement on our side to attend to the needs of the material in order to allow it to continue serving us. In a day when “maintenance-free” is what builders are selling, this has become a foreign idea but the proof for the superiority of these historic windows is in their staying power as the idea of maintaining ones house was moving from the social conscience over many decades.
In conjunction with the superior material that historic windows are made out of, the craftsmanship by which they were built is, again, unlike anything being manufactured today. Joinery refers to the method by which the pieces of a sash are fitted together. In historic windows, this method is such that, should a part of your sash fail, the whole thing can be pulled apart, the failed part replaced and the rest of the sash reassembled with the bulk of the window material remaining the same. A modern window is typically constructed in such a specialized way that, if a part should fail, it is almost always necessary to scrap the whole thing and get a new one. This single difference highlights a philosophical shift in how we started to view the homeowners relationship with his or her house. This doesn't even go into the nuanced ways that these historic sashes were constructed with a balance in mind that considered weather-resistance, maintainability, and longevity.
Both of the above reasons are deeply important though they might not come into a pragmatic discussion about why you should restore rather than replace your historic windows. The issue remains that you still feel a draft when you are by your historic windows and that makes it seem that your house is cold for that reason. Not untrue but not close to the major reason why your house is cold. Most historic houses are cold because they lack insulation in the walls and ceiling. You lose over half the efficiency of your house due to this. Your windows end up being closer to 10-15% of your overall loss. Multiple studies (linked below) show that simply putting a storm window over the top of your windows will drop this percentage to levels equivalent to that of a new insulated glass unit. Moreover, most of these studies go on to say that the energy savings you might see from a replacement window will never come close to recovering the great expense you will incur paying for these new windows. And, again, if your house’s walls and roof are not insulated, the difference will almost be imperceptible on your energy bill.
The other pragmatic argument for keeping your historic windows is the aesthetics. The scale, design and placement of the windows in a historic house define it as much, if not more, than any other architectural detail on it. The sensitivity to this fact has diminished in years since houses were constructed on a large scale in a “style” but the design to these historic homes is so strong that when original elements remain it is abundantly apparent to even the casual observer. Even high-quality wood replacement windows will show subtle differences in scale to the original on top of having the above mentioned maintenance issues. This aesthetic issue ties into the greater look of a historic neighborhood and the responsibility that an owner of a historic home has to it.
All of these reasons amount to a more nuanced approach to the repair versus restore discussion than is commonly presented. Because we have introduced HVAC to our houses it makes it necessary to have the discussion but, even so, with the more interactive nature of a historic house a balance between historical integrity and comfort can be struck by keeping the original widows in good shape and supplementing them for efficiency sake, if deemed necessary.