Permanent cloudiness between two panes means the seal failed and you are looking at moisture trapped inside the insulating glass unit (IGU). A foggy glass window is not a cleaning problem. It is a failed component that has lost most of its R-value, and ignoring it raises your heating bill by 10 to 20 percent on that opening. The good news is that the frame is almost always fine. Three real solutions exist, and the right one depends on the window age, brand, and how many panes have failed.
Why Window Seals Fail
Modern double-pane windows are sealed glass units filled with argon or krypton gas, edged with a butyl or silicone spacer. Sun exposure, temperature swings between 0°F and 110°F, and improper installation flex the seal until it cracks. Once the seal breaks, room air, humidity, and dust enter the gap. Water condenses on the cold side of the glass and you cannot wipe it off because it’s between the panes.
Pella, Andersen, Marvin, and Milgard all warranty their IGUs for 10 to 20 years against seal failure. Builder-grade vinyl windows from the early 2000s typically fail at 8 to 15 years. Check the spacer between the glass panes for a date code stamp before you call anyone.
Option 1: Defogging Service
Defogging companies drill small ports in the glass, suck out the moisture, run a desiccant pass, and reseal the holes with valves. Crystal Clear Window Cleaning, Foggy Window Solutions, and regional outfits offer this for $75 to $150 per window. The catch: the U-factor never returns to original, the argon fill is gone, and the result looks acceptable from inside but the energy savings are gone.
Defogging works if you plan to sell the house within five years or you cannot match the glass. For a long-term fix, replace the IGU.
Option 2: Replace the Insulated Glass Unit Only
If the frame is sound, replacing just the IGU costs $150 to $450 per window installed. Local glass shops measure, fabricate a new sealed unit in 3 to 7 days, and pop it into the existing sash. This restores full energy performance, the argon fill, and the warranty resets on the new glass.
Look for a glass shop that handles your brand. Pella and Andersen sash kits sometimes require proprietary glazing channels. Bring the spacer date code and a photo of the spacer profile when you get the quote. Expect a 4 to 8 week timeline on warranty replacements direct from manufacturers if the window is still covered.
Option 3: Full Window Replacement
If three or more windows in the house are fogged, or the frames show rot, swelling, or operational problems, full replacement is the better math. Insert (pocket) replacements run $450 to $1,200 per window installed. Full-frame replacements with new trim and flashing cost $750 to $1,800. Andersen 400 Series, Pella Lifestyle, Marvin Elevate, and Milgard Tuscany all offer 20-year glass warranties on new units.
Check for ENERGY STAR certification and U-factor below 0.30 for the most northern climate zones. The IECC 2021 sets minimum U-factor requirements, and many local jurisdictions enforce them on permitted replacements.
What Defogging Cannot Fix
If the foggy area covers more than 40 percent of the pane, the glass has internal etching that no defog process restores. Calcium and mineral deposits from condensation can permanently cloud the glass over time. Tempered safety glass in bathroom or stair windows is even more sensitive: surface fluoride etching from cleaning chemicals is irreversible. Replacement is the only fix.
DIY vs Pro Decision
Pulling and reinstalling an IGU is doable on single-hung vinyl windows for a confident DIYer. You need a suction cup glass lifter ($35), a glazing tool ($14), and an exact-match replacement unit. Cost savings: $80 to $200 per window. The risks: cracking the new unit on install, voiding warranties, and water intrusion if the glazing channel reseats incorrectly.
Skip DIY on casement and awning windows where the IGU is part of a structural sash, on wood double-hung windows over 30 years old, and on any window in a second story. Pro labor runs $90 to $150 per hour and includes warranty on the install. The math favors a pro for anything above eye level.
Preventing Future Seal Failures
- Avoid pressure washing windows. The blast pressure forces water past the seal.
- Trim landscape so sprinklers do not soak the glass twice daily.
- Run bathroom and kitchen exhaust fans during showers and cooking to drop indoor humidity below 50 percent in winter.
- Caulk around the exterior trim every 5 to 7 years with high-grade urethane caulk like Sika 1A ($9 per tube).
- Keep storm windows in good repair on older homes to reduce thermal stress on the primary glass.
When Foggy Glass Signals a Bigger Problem
Persistent condensation on the room-side of the glass, not between panes, means your indoor humidity is too high or the window U-factor is too low for your climate. A digital hygrometer ($15) confirms it. Ideal indoor humidity sits between 30 and 50 percent in winter. Mold around the sash points to water infiltration, not just seal failure, and that needs a contractor’s eyes on it before you spend money on glass. Diagnose the room first, the foggy glass window second.
Insurance, Warranty, and Documentation
Most homeowner insurance policies do not cover foggy glass because seal failure is classified as gradual wear and tear, not sudden damage. Storm damage from impact is a different claim and worth filing if a flying branch cracked the seal. Save the original window paperwork from your builder or previous owner. Manufacturers like Andersen, Pella, and Marvin honor original glass warranties for 10 to 20 years and will replace IGUs for $0 to $150 in shipping when the warranty card is on file. Take date-stamped photos of every fogged window before contacting the manufacturer, since claim adjusters use those images to verify coverage and approve replacement units quickly without an on-site visit.
