Window replacement covers two different jobs depending on the problem. For a cracked or broken single pane, you can replace just the glass in the existing frame — a $20–50 repair that takes about an hour. For failed double-pane seals, rotted frames, or a full upgrade, you’re installing a new window unit either as an insert into the existing frame or a full frame-out replacement. This guide covers both scenarios so you can handle whatever your windows need.
What You’ll Need
| Tools | Materials |
|---|---|
| Utility knife | Replacement glass or window unit |
| Putty knife and glazing tool | Glazing compound (for single-pane) |
| Pry bar and hammer | Glazier’s points (for single-pane) |
| Drill/driver | Exterior window caulk |
| Level | Flashing tape (for full replacements) |
| Measuring tape | Shims |
| Safety glasses and thick gloves | Expanding foam (minimal expansion) |
| Work gloves (cut-resistant) | Matching exterior paint or trim |
Safety and Precautions
- Wear cut-resistant gloves and safety glasses when handling broken glass. Glass shards break into irregular shapes with razor-sharp edges. Thick leather gloves are the minimum; cut-resistant work gloves are better. Wrap broken glass in newspaper or cardboard before disposal — never place loose broken glass in bags where it can cut through.
- Support window sashes before removing hardware. Window sashes can fall suddenly when springs or balances are released. Have a helper or use temporary bracing to prevent a sash from dropping unexpectedly.
- Secure the opening if work spans more than one day. An open window opening is a weather vulnerability and a security risk. Cover the opening with plywood or heavy plastic sheeting and tape the edges if the work won’t be completed in the same session.
Step 1: Diagnose the Problem
Match the Repair to the Actual Issue
| Symptom | Cause | Solution |
|---|---|---|
| Cracked or broken single pane | Impact damage | Replace the glass pane (Section 2) |
| Foggy or cloudy between double-pane glass | Failed thermal seal | Replace the IGU (insulated glass unit) or full window |
| Drafts around the sash | Failed weatherstripping or worn seals | Replace weatherstripping on the sash |
| Rotted or damaged frame | Water infiltration over time | Insert replacement window or full frame replacement |
| Hard to open/close | Swollen sash, broken balance, or damaged track | Plane the sash, replace balance, or repair track |
Step 2: Replace a Single Pane of Glass
The Fast, Affordable Fix for Broken Single-Pane Windows
- Working from inside if possible — safer for the initial removal. Use a utility knife to score around the old glazing compound (the putty that holds the glass in the frame).
- Wearing thick gloves, carefully remove the broken glass pieces. Start with loose pieces, then work out any remaining shards by pressing them inward to break the bond.
- Remove the old glazier’s points (small metal triangles embedded in the frame that hold the glass) using pliers or a flathead screwdriver. Remove all old glazing compound — a stiff putty knife and heat gun (on a low setting) make this easier.
- Measure the opening: height and width from rabbet to rabbet (the groove the glass sits in). The replacement glass should be cut 1/8 inch smaller in both dimensions to allow for fit and expansion. Take these measurements to a glass shop, or cut your own if you have a glass cutter.
- Apply a thin bed of glazing compound in the rabbet groove before setting the new glass. Press the glass firmly into the bed.
- Drive glazier’s points into the frame every 6–8 inches to hold the glass in place (use a stiff putty knife as a driver).
- Apply a bead of glazing compound over the glass edge and rabbet, angling it at 45 degrees. Smooth with a wetted putty knife. The glazing should be flush or just below the sash wood — glass should still be visible from inside when looking at the sash edge.
- Allow the glazing to skin over (24 hours), then prime and paint the compound and frame to seal it from moisture.
Step 3: Replace a Double-Pane Insulated Glass Unit (IGU)
For Fogged or Failed Thermal Seals — No Full Frame Removal Needed
Modern double-pane windows have an insulated glass unit (IGU) — the sealed glass assembly — that can often be replaced without replacing the entire window frame or sash.
- Measure the IGU: remove the interior stop bead (the trim strip that holds the glass unit against the frame). Carefully slide the IGU out, supporting it from both sides as it comes free.
- Take the old IGU to a glass supplier. They can fabricate an exact replacement. Provide the width, height, thickness (typically 5/8 inch or 3/4 inch for standard double-pane), and glass type (clear, Low-E, tempered, etc.).
- Set the new IGU in the frame with setting blocks at the bottom corners (small plastic spacers that support the glass and prevent edge contact with the frame).
- Reinstall the stop bead. Apply a bead of interior glazing tape or sealant around the perimeter as specified by the window manufacturer.
- Apply exterior window sealant around the perimeter if required by the window design.
Step 4: Install an Insert Replacement Window
New Window Unit Into an Existing Frame — Weekend Project
An insert replacement window removes the old sashes and glass from inside the existing frame and drops in a new factory-built unit. The exterior trim and frame remain intact — no interior damage to walls or siding.
- Remove the interior stop beads, the lower sash, the parting bead, and the upper sash. Remove the sash weights (in old double-hung windows) and weight pockets.
- Measure the opening: width and height of the existing frame opening. Most replacement window manufacturers offer exact custom sizing.
- Install the new insert unit: apply caulk around the perimeter of the frame opening, slide the new window unit in, and shim until it’s level and plumb. Check that the window operates smoothly (opens, closes, and locks) before finalizing.
- Secure the unit with screws through the side jamb channels into the existing frame.
- Fill the space between the new unit and the existing frame with minimal-expansion foam, or stuff with fiberglass batt insulation.
- Replace the interior stop beads and trim. Caulk interior and exterior joints and paint.
Step 5: Full Window Frame Replacement
When the Frame Itself Is Rotted or Damaged
Full replacement involves removing the window, exterior trim, and the entire frame — down to the rough opening. This is a larger project, but necessary when the frame or rough sill is rotted.
- Remove interior trim (casing). Cut the exterior caulk line with a utility knife. Pry the exterior casing/brick mold away from the siding.
- Remove any nails holding the window fin or frame to the rough framing. Slide the old window out of the opening.
- Inspect the rough sill and jack studs for rot or damage. Repair any rot with epoxy wood filler or replace the framing member before proceeding.
- Install flashing tape on the sill and up the sides of the rough opening. This is the moisture barrier — it must be continuous with no gaps.
- Set the new window in the opening, check plumb and level, and shim as needed. Drive nails or screws through the window fin into the framing.
- Apply flashing tape over the window fins, starting at the sides and finishing with a piece across the top. The top flashing laps over the side flashing so water runs down and over, not behind.
- Apply exterior caulk around the perimeter, reinstall exterior trim, and caulk all joints. Reinstall interior casing and trim.
Pro Tips and Common Mistakes
- Always prime and paint glazing compound — it cracks if left raw. Unpainted glazing compound dries out and cracks within months, allowing water in behind the glass. Prime within 24 hours of application and apply at least one finish coat.
- Flash before you install, not after. Flashing tape must be installed before the window goes in, not applied as an afterthought around the outside. Proper flashing sequence (sill first, sides, then top cap) is critical for water management.
- Don’t fully foam the gap in insert replacements. Minimal-expansion foam is the right product — standard expanding foam exerts enough pressure to warp window jambs, preventing the window from opening and closing correctly.
- Measure the rough opening, not the old window. The rough opening is always larger than the window unit. Measure the R.O. and order a window sized to fit it. Windows are measured by rough opening width x height.
Troubleshooting
| Problem | Likely Cause | Fix |
|---|---|---|
| New single-pane glass cracks after install | Glass cut too big, no expansion gap | Replace with glass 1/8 inch smaller each dimension |
| Insert window hard to operate after install | Over-expanded foam warped the jamb | Remove foam; repack with fiberglass batt insulation |
| Condensation between new double-pane glass | Defective IGU seal (warranty issue) | Contact manufacturer; replace under warranty if within coverage period |
| Drafts after full replacement | Poor flashing or incomplete caulking | Check exterior caulk; apply flashing tape to any uncovered gaps |
| Window won’t stay open | Broken or weak window balance | Replace window balance on both sides (spring or spiral type) |
Frequently Asked Questions
Can I replace just the glass in a double-pane window?
Yes — the insulated glass unit (IGU) in a double-pane window can usually be replaced without touching the frame. You remove the interior stop bead, slide out the old unit, and install a factory-matched replacement. A glass shop can fabricate the exact replacement from your measurements. This is significantly cheaper than full window replacement.
How do I know if my double-pane window needs replacing?
The most common sign is persistent fogginess or moisture between the panes — this means the thermal seal has failed and the argon or krypton gas fill has been replaced by humid air. This can’t be cleaned out; the IGU needs replacement. If the frame is solid and the window hardware works, just replacing the IGU is the right approach.
How much does window glass replacement cost?
Single pane glass cut at a glass shop typically costs $5–15 depending on size. An IGU (double-pane replacement unit) runs $60–150 depending on size and glass type. A full window unit (insert replacement style) runs $150–500 per window depending on size, type, and brand. Labor to install runs $100–300 per window if hiring out.
Can I replace a window myself without building permits?
In most US jurisdictions, replacing a window with a same-size window in an existing opening does not require a building permit (it’s considered like-for-like replacement). Changing a window size, adding a new window opening, or installing egress-required windows typically does require a permit. Check with your local building department if you’re unsure about your specific project.
How do I glaze a window properly?
Apply a thin bed of glazing compound in the rabbet groove, set the glass, press in glazier’s points every 6–8 inches, then roll a rope of compound and press it against the glass and frame at 45 degrees. Smooth it with a wet putty knife to create a clean beveled bead. Prime and paint within 24 hours to prevent cracking.
Conclusion
Window replacement ranges from a simple 1-hour glass pane swap to a full-weekend frame replacement project, and choosing the right level of repair for the actual problem saves time and money. For a broken single pane, the glazing process is a genuinely accessible DIY job. For failed double-pane seals, IGU replacement is the most cost-effective solution short of full replacement. For rotted or damaged frames, proper flashing during full replacement is the step that determines whether the repair lasts or fails again in five years.
Once your windows are sealed, consider checking the door weatherproofing as well — our guide on how to seal and weatherproof a door covers the complementary exterior sealing work. And if you’re also dealing with window frame oxidation alongside the glass repair, our guide on how to clean aluminum window frames covers the frame restoration process.
