Repair the window when the frame is sound and the problem is a part — glass, cord, balance, putty, weatherstrip, a few inches of sill. Replace it when the frame itself has failed — rot through the jambs, a rusted steel casement, an opening settled out of square — or when the window is a 1950s–60s metal unit that cannot be made to perform. In a City historic district, the standards say repair first, and the approved replacement list puts a sash kit in your existing frame ahead of a new window. The in-home sales appointment is not built to give that answer; this page is.
The decision tree
- Is the glass the problem? Fogged double-pane: replace the insulated glass unit, keep the window ($220–$550). Cracked single pane: reglaze ($120–$260). → Repair.
- Is it a moving part? Broken sash cord or chain, failed spring balance, loose lock, sash that will not stay up. → Repair ($90–$220).
- Is it stuck or drafty but intact? Painted shut, missing weatherstrip, loose meeting rail. → Repair ($80–$350).
- Is there rot? Probe the sill nosing and the bottom of each jamb with a screwdriver. Soft for the first inch at the nosing → repair by epoxy consolidation or a dutchman ($200–$600). Soft into the jambs, or a sill you can push a finger through → replace, full-frame.
- Is the frame metal? 1950s–60s steel casements and hollow aluminum sliders: they cannot be repaired into a thermally decent window. → Replace, full-frame.
- Is the opening square? Measure the diagonals. More than about three-eighths of an inch difference and the sash will bind forever. → Replace, full-frame.
- Is it a 1990s–2000s vinyl window? Fogged glass with a straight frame → glass unit replacement. Fogged glass with a warped frame or cracked welds → replace, insert.
- Do you want a step change in performance from single-pane? Restoration plus a good storm window gets an original double-hung close on air leakage and part of the way on U-factor. If you want the full modern number → replace — but weigh the historic-district rules first.
Why St. Louis leans toward repair more than most cities
Because the original windows are better than the replacements most people are offered. The City (57.9% pre-1940) and the inner ring were built with weight-and-pulley double-hung sash in old-growth wood: tight grain, rot-resistant, designed to be taken apart. A sash that has survived a century with its paint kept up will survive another with new cords, fresh glazing and weatherstripping, and it will look like the house. A mid-grade vinyl insert in the same opening loses an inch and a half of glass and will be the window that needs replacing in twenty-five years.
The City's historic districts write this into the rules. Soulard's standard begins: "existing window repaired, or replacement that duplicates original." The Cultural Resources Office's approved list ranks all-wood sash replacement kits — which keep your frame — first. Skinker DeBaliviere requires "special windows" to be retained or repaired. If your street-facing windows are original, the path of least cost and least review is restoration. See the district rules.
Where repair stops making sense
- The 1950s–60s belt. Florissant, Ferguson, Hazelwood, Affton, Crestwood, Lemay: the steel casements and aluminum sliders of that era are not worth repairing. Replacement, full-frame, is the right call and the biggest comfort gain available in those houses.
- Settled flats. Many City four-families have openings that have moved; a repaired sash still binds. Full-frame resets the geometry.
- Rot past the nosing. Once the jambs are gone, consolidant is throwing money at a frame that will keep failing.
- Storm-broken frames. A frame bent by hail or debris is a replacement and usually a claim; see storm damage window replacement.
What each costs, side by side
| Window | Repair path | Replace path |
|---|---|---|
| 1920s wood double-hung, sound frame, broken cords, loose putty | Cords + reglaze + weatherstrip: $300–$650 | Vinyl insert $700–$1,200 (brick); historic replica $1,500–$2,800 |
| 1998 vinyl double-hung, fogged glass, straight frame | IGU replacement $220–$550 | Vinyl insert $550–$950 |
| 1962 steel casement | — | Full-frame $900–$1,900 |
| 1910 wood double-hung, sill rotted into the jambs | — | Full-frame $1,000–$2,800 by material |
Full ranges in the cost guide.
The lead rule, either way
On a pre-1978 house, a repair that disturbs more than six square feet of painted surface in a room (a full sash removal and reglaze often does) falls under the EPA's RRP rule; a sash-cord swap usually does not. Replacement always does — window replacement is written out of the minor-repair exemption. See lead-safe window replacement.
Frequently asked questions
Can fogged glass be "cleared" without replacing it? Drilling-and-venting services exist; they remove the fog, not the failed seal, and the unit no longer insulates as designed. Replace the glass unit.
Can old wood sash take double-pane glass? Some can be routed for a thin insulated unit; it is a case-by-case job and the sash must be thick enough. Often a good low-e storm window is the better answer.
Is a storm window worth it on an original window? Yes — an exterior or interior storm cuts air leakage dramatically and roughly halves the heat loss through the glass. In a historic district the storm's material and finish are regulated; see the district rules.
Will a repaired window ever be as efficient as a new one? On air leakage, nearly. On U-factor, a restored single-pane with a storm sits around 0.45–0.50 against 0.25–0.28 for a new double-pane low-e unit. Which matters more depends on the window, the room and the house.
Next step
Window repair is the service for the repair path; window replacement and full-frame replacement for the other. Related: the complete St. Louis guide.