Notice a hazy fog hovering between the panes of your double-hung that no amount of Windex can wipe away? That milky film is moisture trapped inside the insulated glass unit, and it means the window seal broken failure has already let the argon or krypton gas escape. The good news: you have several repair paths, and only one of them requires gutting the frame.
What Actually Fails Inside an Insulated Glass Unit
Modern windows use an IGU, or insulated glass unit, built from two or three panes of glass separated by a spacer bar filled with desiccant. A polyisobutylene primary seal and a secondary silicone or polysulfide seal lock the perimeter. When that perimeter sealant degrades from UV exposure, thermal cycling, or trapped water at the sill, the insulating gas leaks and outside air rushes in carrying humidity.
The desiccant absorbs the first round of moisture, which is why early-stage failures are invisible. Once that bead is saturated, condensation forms on the inside surfaces of the glass and the haze becomes permanent. Most IGUs last 15 to 25 years, though south-facing windows in Phoenix or Houston can fail in under 10.
How to Confirm the Seal Is Actually Broken
Run your finger across the inside pane on a cold morning. If it’s dry but the glass still looks foggy, the moisture is between the panes and the seal has failed. Shine a flashlight at a low angle along the perimeter spacer. Streaks of mineral deposits, rainbow-colored sheen, or visible water droplets all confirm the diagnosis. Compare with an adjacent identical window for reference.
Energy bills tell the other half of the story. A failed argon fill drops the U-factor from roughly 0.30 back toward 0.50, which adds 15 to 25 percent to the heating and cooling load on that opening. If three or more windows on the same wall have failed simultaneously, the manufacturer’s batch likely had a sealant defect.
Repair Options and Real-World Costs
You have three legitimate paths, and the right one depends on the age of the frame and the warranty status.
- IGU-only replacement: A glazier removes the sash, pops out the failed glass unit, and installs a new factory-built IGU into the existing frame. Cost runs $175 to $475 per window for standard sizes, $500 to $900 for low-E triple-pane or specialty shapes.
- Defog service: A technician drills two small holes, injects a cleaning solution, runs a dehumidifying valve, and seals the holes. Pricing is $75 to $200 per window. It restores clarity but does not restore the gas fill or U-factor.
- Full window replacement: Required when the frame is rotted, the sash channels are gouged, or the unit predates the 1990s. Vinyl replacement windows from Pella, Andersen 100 Series, or Milgard Tuscany run $550 to $1,400 installed per opening.
Manufacturer Warranties Worth Checking First
Before paying out of pocket, dig out the original purchase paperwork. Andersen offers a 20-year glass warranty, Pella covers IGUs for 20 years on most lines, Marvin runs 20 years, and Milgard’s full lifetime warranty transfers to subsequent homeowners on many series. Jeld-Wen runs 20 years on the glass seal. Even if the original buyer was the previous owner, many of these warranties travel with the house.
You’ll need the manufacturer’s stamp etched into the spacer bar between the panes. It usually shows the date code, plant location, and model identifier. Photograph it before calling, because the rep will ask. Warranty replacements typically cover the glass unit itself but not labor, leaving you a $75 to $150 install fee.
DIY IGU Replacement: When It Makes Sense
Replacing the glass yourself is feasible on aluminum or wood frames with removable glazing stops. Measure the existing IGU with calipers for thickness, then width and height to the nearest 1/16 inch. Order from a local glass shop for $90 to $220 depending on size and coating. Most shops turn around custom orders in 7 to 14 days.
Pry off the interior glazing stops with a stiff putty knife, lift out the old unit with suction cups, clean the rabbet, and lay a fresh bead of butyl glazing tape. Set the new IGU, replace the stops, and caulk the perimeter with a paintable acrylic-latex. Budget two hours per window the first time, one hour each after that. Fiberglass frames like those from Marvin Integrity are not DIY-friendly because the stops are factory-bonded.
Prevention That Actually Buys You Years
Most premature seal failures trace to standing water at the sill. Clear weep holes annually with a plastic pick, especially on vinyl frames where landscape mulch tends to clog them. Recaulk the exterior perimeter with a high-grade polyurethane like Sikaflex 1a or OSI Quad Max every 5 to 7 years.
Install or maintain proper drip cap flashing above each window. Check storm window venting too. A trapped microclimate behind a poorly vented storm panel accelerates seal degradation by 30 to 50 percent according to NFRC field studies. South and west exposures benefit from low-E films or exterior shading, both of which cut UV load on the sealant.
When Replacement Beats Repair
If the frame shows soft spots from a screwdriver probe, if the sash sticks or won’t latch, or if more than half the windows in the house have already failed, full replacement makes financial sense. The IRC and most state energy codes require ENERGY STAR rated units in the climate zone. Expect $8,000 to $18,000 for a 10-window vinyl replacement project, or $20,000 to $45,000 for fiberglass Andersen 400 Series or Marvin Elevate.
Pull a building permit even for like-for-like replacement in most jurisdictions, particularly if the opening is in a bedroom, where egress sizing applies. A qualified installer registered under the EPA’s Renovation, Repair and Painting rule is required if the home was built before 1978.
