Window Seal Repair: A Complete Guide for Homeowners

The fog you see between two panes of an insulated window is not condensation you can wipe away. It is moisture trapped inside a sealed gas chamber that has lost its perimeter seal, and once it appears, the window has lost most of its insulating value. Effective window seal repair ranges from a $40 DIY caulk job on the exterior frame to a $300 professional defogging service to full insulated glass unit replacement. Knowing which fix matches your problem saves you from either paying too much for a new window or wasting money on a repair that will fail again in 18 months.

Two Different Failures Get Called “Seal Failure”

The first failure is the perimeter caulk and weatherstripping around the window frame, where it meets the wall. This seal blocks air and water infiltration around the outside of the window. Cracked caulk lets drafts in, drives up your heating bill, and can leak water into the wall cavity.

The second failure is the internal seal of the insulated glass unit (IGU), the spacer and sealant that separates the two or three glass panes and holds in argon or krypton gas. When that internal seal fails, moist air enters the cavity, condenses on the cold inner surface, and creates the permanent fogging or mineral haze that no amount of cleaning will remove.

Diagnose Which Seal Has Failed

Look at the window from outside on a cold morning. If you see fog, condensation, or rainbow streaks between the panes that you cannot wipe off from either side, the IGU seal has failed. If you feel a draft along the edge of the frame or see daylight where caulk used to be, the perimeter seal needs work. Both can fail on the same window.

Press a sheet of paper against the closed window frame and try to pull it out. If it slides out easily, the weatherstripping has compressed past its useful life. New strips run $8 to $15 per window and install in about 20 minutes.

Repairing the Perimeter Seal

This is the easiest DIY fix in the entire window category. Tools needed are a utility knife, a 5-in-1 painters tool, a caulk gun, and a tube of high-quality polyurethane or silicone exterior caulk. OSI Quad and DAP DynaFlex 920 both run about $9 a tube and last 20 plus years.

Cut out the old caulk down to bare substrate, vacuum out debris, mask both edges with painters tape, lay a continuous 3/8-inch bead, and tool it smooth with a wet finger or a rounded plastic tool. Pull the tape immediately while the caulk is wet. Total time per window is 30 to 45 minutes; total cost runs about $12 in materials.

Repairing the Internal IGU Seal

This is harder, more expensive, and rarely a permanent fix. Two approaches exist.

The first is professional defogging, where a contractor drills two small holes in the glass, flushes the cavity with a cleaning solution, vacuums it dry, and installs micro vents to allow continued moisture exchange. Cost is $80 to $150 per window. The result clears the fog but does not restore the insulating gas, so the R-value never returns to the original spec. Many contractors no longer offer defogging because customer satisfaction is mediocre.

The second is full IGU replacement, where the glass unit alone is replaced inside the existing frame. A new IGU runs $150 to $400 per window depending on size and gas fill, plus $75 to $150 in labor. The original frame, sash, and hardware all stay; only the glass changes. This restores the window to like-new performance.

When Replacement Makes More Sense Than Repair

Three conditions favor full window replacement over IGU repair. First, if the window frame itself is rotted, warped, or has failed weep holes, replacing only the glass leaves underlying problems untouched. Second, if the existing windows are single-pane or builder-grade aluminum with no thermal break, the energy savings from a modern unit pay back the higher cost in 8 to 12 years. Third, if more than half the windows in the house show seal failure, bulk replacement gets you 15 to 25 percent volume pricing from most installers.

Standard double-hung replacement windows run $450 to $1,200 each installed for vinyl, $700 to $1,500 for fiberglass, and $900 to $2,500 for premium wood-clad units.

What Causes Seals to Fail Early

The average IGU seal lasts 15 to 25 years. Several conditions shorten that lifespan dramatically. Direct south or west exposure with no overhang exposes the seal to UV and thermal cycling that accelerates degradation. Pressure washing the window edges forces water past the seal. Painting over the weep holes blocks drainage and traps water against the spacer. Settling foundations rack the frame and tear the corner welds.

Cheap windows installed in the 1990s and early 2000s used a single-seal organic spacer that failed at 8 to 12 years; modern dual-seal warm-edge spacers extend that to 25 plus years and come with better warranties.

Warranty Matters More Than Most People Realize

Most major window manufacturers warranty the IGU against seal failure for 10 to 20 years; a few premium brands offer lifetime IGU warranties. If your house is less than 15 years old, pull the original purchase paperwork or check with the previous owner before paying for repair. A free warranty IGU replacement from the manufacturer beats a $300 defogging job every time. Andersen, Pella, Marvin, and Milgard all maintain online warranty registration lookups.

DIY IGU Replacement: Possible but Risky

For wood and aluminum-clad windows with removable glazing stops, replacing the IGU yourself is feasible if you can lift the unit (a 36×60 IGU weighs 60 to 80 pounds). Order a measured replacement from a local glass shop for $150 to $350, pry off the inside glazing stops carefully, lift the old unit out, set the new one in fresh glazing tape or butyl, and reinstall the stops. Vinyl windows often use snap-in glazing beads that can crack on removal; for those, hire a pro.

Prevention Extends Seal Life

Inspect exterior caulk annually each spring. Keep weep holes clear of paint, debris, and insect nests. Clean glass with mild soap and a microfiber cloth instead of pressure washing. Trim shrubs back at least 12 inches from the window to allow airflow that dries the seal after rain. These five-minute tasks add 5 to 10 years to the life of every window in the house.