People who "replace" their windows in a 1920's era home aren't actually replacing their windows. At most, they're addressing a mere 25% of the whole window.w.
Why is this?
Every window, historic or modern, has three vital components. They are the sash, the frame, and the mechanics. The sash is the component with the glass, the frame (largest component) holds the sash, and the mechanics tie everything together. The window replacement industry’s standard practice for a historic home is to remove the sash from the frame to insert a condensed set of sashes into your existing window assembly. What's left is a window within a window, stacked like little Russian dolls.
Why does this matter? It matters because your existing window frame makes up at least 75% of your window – and is integral to the structural fabric of your home. Know this - modern window companies don't work on the window frame itself. They switch out the sash and leave the frame alone. This means they ignore it, under the guise of "replacement." This leaves the home's entire protective envelope at catastrophic risk. (And don’t even get us started on the lifespan of the insert!)
Maybe think of it like rewiring only 25% of your home?
And leaving the other 75% knob and tube…
This illustrates the parts that make up your whole window. See the 'Top and Bottom Sash'? That's the only part that gets replaced...