The standard advice to homeowners with old windows is to replace them, usually framed around energy savings. It is worth examining that advice, because for a certain kind of window it is frequently wrong.
What original windows actually are
Windows in houses built before roughly the mid-twentieth century were typically made from old-growth timber: dense, slow-grown, and considerably more durable than the fast-grown softwood available now.
They were also built to be repaired. Individual components are replaceable, joints can be taken apart, glazing can be redone, and sash cords can be renewed. A window designed this way has no fixed service life.
This is the fundamental asymmetry. A modern sealed unit has a finite life determined by its seal, typically measured in decades. A well-maintained wood sash window has already lasted a century and can last another.
What actually fails on them
Old windows rarely fail structurally. What fails is everything around the glass.
Glazing putty hardens, cracks, and falls out, which lets water sit against the wood and begins rot in the lower rail.
Weatherstripping, where it existed at all, degrades, and this is the main source of draughts.
Sash cords break, so windows will not stay open, and people stop operating them.
Paint fails, exposing timber.
Hardware seizes or is painted over.
All of these are repairable, and the repair is well understood.
The performance question
The honest comparison is not a bare single-glazed sash against a modern double-glazed unit. That comparison favours replacement obviously.
The realistic comparison is a restored window with proper weatherstripping and a storm window or interior secondary glazing against a modern replacement unit.
That comparison is much closer, and the restored assembly frequently performs competitively.
Air leakage is where most of the loss occurs in an old window, not through the glass. Addressing the leakage addresses most of the problem.
The economics over time
Replacement is generally cheaper per window in the short term, particularly across a whole house.
Restoration costs more initially and produces something with an indefinite life. Replacement produces something that will itself need replacing.
Where the calculation lands depends heavily on how many windows there are, their condition, and how long you intend to own the house.
The character question
On a period building, original windows are a substantial part of what the house looks like.
Old glass has visible irregularity that modern float glass does not. Sash proportions, muntin profiles, and the depth of the reveal all contribute to the appearance of the facade.
Replacement units, even good ones designed to look traditional, generally read differently, and the difference is noticeable on a house with strong character.
Where a house sits in a historic district, there may be formal requirements governing what is permitted, and those should be established before any decision.
When replacement is the right answer
Some old windows are genuinely finished.
Extensive rot through frames and sashes, where the material remaining is not worth building on. Windows that have been so badly altered that little original fabric survives. Aluminium replacement units from the mid-century, which have no character to preserve and perform poorly.
And practical constraints count. If restoring twenty windows is not affordable and the house is uncomfortable, replacement is a reasonable decision rather than a failure.
A middle route
Many households restore the windows on prominent elevations and replace those on secondary ones.
Others restore progressively over several years, which spreads cost and keeps original fabric.
Storm windows, whether exterior or interior, are a comparatively inexpensive intervention that addresses most of the performance gap without touching the original.
Getting a balanced assessment
The difficulty is that most people offering an opinion sell one solution or the other.
Where a window’s glazing has failed but the sash and frame are sound, a glass specialist such as Window Replacement NH can advise on what can be done within the existing sash, which is a narrower and more factual question than restore-or-replace.
For the broader decision, an assessment from someone who does restoration work, alongside a replacement quote, gives you both sides rather than one.