Windows fail in four distinct ways, and the sales conversation tends to treat them as one. They are not, and three of the four are repairs.
1. Fogging between the panes — replace the glass, not the window
The perimeter seal has failed, moisture is inside the airspace, and the coating is oxidising. The window will never be clear again and no defogging service restores it — drilling and venting removes the haze and leaves you with an uninsulated double pane.
But the frame, the sash and the hardware are usually fine. In most frames the sealed unit alone can be replaced, at a small fraction of the cost of a new window.
Two things decide whether it is worth it: whether the sash is designed to be reglazed (most are; some fully welded vinyl units are not), and whether the manufacturer or a local glass shop can supply a matching unit. A glass shop can build one to size for any frame that opens.
This is the single most common reason people call about windows, and it is the one where replacement is least often necessary.
2. Sash will not stay up, will not lock, drags — replace the hardware
Balances, operators, hinges and rollers are wear parts with a ten to twenty-five year life. Nearly all of them are replaceable, and most are inexpensive.
The catch is availability. Balance systems are proprietary, and a manufacturer that closed fifteen years ago supplies nothing. Where the part exists, this is an afternoon and a small bill. Where it does not, and the sash is unusable, it becomes an argument for replacement.
Take a photo of the balance and any part number to a window repair specialist before concluding the part is unobtainable — the aftermarket for common systems is larger than it appears.
3. Draughty — replace the weatherstripping
Air leakage is what people feel, and it is a consumable. Compression seals take a permanent set, foam degrades under UV, and a window that closed tight in 2005 does not now.
Replacing weatherstrip is the cheapest work you can do on a window and it changes comfort more than a step down in U-factor would. On an older wood window, adding modern weatherstrip and a good storm window gets performance close to a basic double-pane at a small fraction of replacement cost.
Check the lock first. Many "draughty" windows simply are not being pulled fully closed because the lock has dropped out of adjustment.
4. Rot at the sill, water in the wall — this one is replacement
Soft wood at the bottom corners, staining below the sill, peeling paint that keeps coming back, a musty smell at the opening. Here the window is not the problem; it is the symptom of water entering the wall, and the damage continues for as long as the path exists.
Fixing it means opening the wall, which means full frame replacement with proper flashing. Patching the visible rot and installing an insert seals the problem inside.
This is the failure that genuinely requires new windows, and it is the one people most often defer.
When several fail together
The arithmetic changes when a house has fogged units, failed balances, tired weatherstrip and out-of-square frames at the same time. Individually each is a repair; together they cost more than replacement and leave you with a twenty-five-year-old window.
The threshold is roughly this: if the repairs on one window approach half the installed cost of a new one, replace it. If they are well under that and the frame is dry and square, repair.
The order to work in
Get a price on the glass units and the hardware before getting a price on new windows. Suppliers of new windows do not quote repairs, so the repair number has to be sought separately — and it is the number that tells you whether the replacement quote is the right conversation at all.