Steve Adams
Fogged double pane windows are one of those repairs where nearly every article you’ll find online steers you toward the wrong fix. They’ll tell you to replace the whole window, quote you $400-$800 per unit, and send you off to find a contractor. Some of those articles are written by window companies. Funny how that works.
Here’s the actual situation: the fog means the seal failed, and the argon or krypton gas between the panes escaped. Now moisture is getting in and condensing on the inside glass surfaces where you can’t wipe it. The window still blocks drafts, still holds together structurally. What you’ve lost is the insulating gas layer and the clear view. Those are two different problems, and which one you actually care about determines what you should do.
What’s Really Broken (And What Isn’t)
The glass unit inside a double pane window is called an IGU, insulated glass unit. It’s two panes bonded together at the edges with a spacer and sealed with butyl tape and secondary sealant. When that seal goes, the insulation value drops. On a window filled with argon, you lose roughly the difference between an R-3 and an R-2 value. Real, but not catastrophic. The window still beats single-pane by a wide margin.
What a failed seal doesn’t do: it doesn’t make the window structurally unsafe, it doesn’t cause air infiltration (that’s the frame seal, not the IGU seal), and it doesn’t mean the whole window unit is shot. People conflate these constantly.
The fog itself is mineral deposits. Water vapor comes in, moisture evaporates, leaves calcium and silica residue on the inner glass surface. Early on, fogging looks like haze. Later it looks like someone smeared the inside with a thin layer of dried shampoo. It won’t go away on its own.
Your Three Real Options, Ranked
| Option | Cost (Glass/Service) | Labor Time | Restores Clarity | Restores Gas Fill | Best For |
|---|---|---|---|---|---|
| IGU Replacement | $75-$200 | ~45 min-1 hour | Yes | Yes | Most situations; standard vinyl/aluminum frames |
| Defogging Service | $75-$150 | N/A | Yes | No | Aesthetics priority; mild climates |
| Full Window Replacement | $350-$600+ | Installation labor | Yes | Yes | Rotted frame, unusual size, hardware failure, 20+ years old |
Replace just the IGU. This is the right answer for most situations. A glass shop orders a replacement IGU cut to your exact dimensions, you or they pop out the old unit, drop in the new one, re-glaze or re-bead, done. Cost is typically $75-$200 for the glass depending on size and glass type, plus an hour of labor if you hire it out. For a standard 24x36 double pane unit, I’ve paid $90 for the glass and done the swap myself in 45 minutes. This restores full clarity and the insulating gas layer.
The catch: you need a window where the frame and the IGU are separate components. Most vinyl and aluminum windows work this way. Some older wood windows have the glass set directly into a routed channel and re-glazed with compound, which is a different process. Look at the edge of your glass. If you see a rubber or vinyl bead running around the perimeter, the IGU is likely replaceable. If you see putty or glazing compound painted over, you’re looking at a traditional glazed sash.
The defogging service. A handful of companies will drill small holes through one pane, inject a cleaning solution, blow it dry, then plug the holes and install a vent plug to prevent future pressure buildup. Companies like RestorationGlass and local equivalents charge $75-$150 per window. It removes the haze and can look remarkably good. What it doesn’t do is restore the gas fill or re-seal the unit to original spec. You’re trading clarity for insulation value. If aesthetics are your main concern and you’re not in a brutal climate, this is a legitimate option, and I say that even though my first instinct was to dismiss it. Done properly, results hold for years.
Full window replacement. Right call when: the frame is rotted or damaged, the window is an unusual size that makes IGU ordering expensive, the operation hardware is failing anyway, or the window is older than 20 years and you’re planning to upgrade. A new Andersen 400 series double-hung runs $350-$600 for the unit, then installation labor on top. Don’t let anyone sell you full replacement just because the IGU fogged.
How to Replace an IGU Yourself
This is the kind of job that sounds harder than it is. You need a utility knife, a putty knife or stiff plastic scraper, a rubber mallet, and basic glazing supplies. Some windows use glazing beads (the removable plastic or vinyl strips); others use a compression fit. Check yours before ordering glass.
Start by measuring the IGU, not the frame opening. Measure the actual glass unit: height, width, and thickness. The thickness on a standard double pane is usually 5/8" or 3/4". If you’re not sure, a glass shop can measure it. Order from a local glass shop, not online. Local shops can cut to exact dimensions, will call you if something looks off, and you can return it if the measurement is wrong. Getting glass shipped is a whole thing.
Remove the window sash if you can. Most double-hungs let you tilt the sash in and pop it out of the frame, which makes working flat on a table dramatically easier. Lay it on a padded surface, glass side up.
Pry off the glazing beads starting at a corner. They’re usually snapped in and will flex out with a firm push from a putty knife. Work around all four sides. Set the beads aside in order. The IGU will now lift straight out. Some units also have a setting block (a small rubber pad) at the bottom edge supporting the glass weight. Keep it, you’ll reuse it or replace with one from the glass shop.
Clean the channel. Remove old glazing tape residue, dry it completely. Apply new glazing tape or glazing compound around the channel perimeter per your glass shop’s recommendation. Set the new IGU on the setting block, press it gently into the channel, and snap the beads back starting at the corners. Don’t force anything. If a bead won’t seat, check that the glass is fully seated first.
Reinstall the sash. Test operation. You’re done.
One thing I’d tell you from my own experience: the first IGU swap I ever watched, the homeowner cracked the new unit trying to muscle a bead back on while the glass was slightly cocked in the frame. Patience on that last step.
The DIY Line
Don’t attempt IGU replacement on windows above the first floor without staging or a helper. Working off a ladder with a piece of glass is genuinely dangerous, and the glass alone is an awkward load. On upper-story windows, hire it out. The labor isn’t expensive, and it’s not the place to prove a point.
Also, if your window is under a manufacturer warranty (some vinyl windows carry 20-year warranties on the IGU seal specifically), call the manufacturer before you touch anything. A failed IGU seal on a window that’s only eight years old might be a warranty replacement. I’ve seen homeowners void their warranty by drilling defogging holes before they even checked.
Skylights are a separate conversation entirely. The access, angle, and waterproofing considerations make those a pro job in almost every case.
What It Won’t Fix
Sources
If the fogging only appears on the outside surface of the glass on cold mornings, that’s not a failed seal. That’s exterior condensation, which means your windows are actually performing well, keeping the glass surface cold relative to the outdoor dew point. It’ll burn off when the sun hits it. Nothing to repair.
Interior condensation on the room-side surface, same thing, usually means your indoor humidity is high, not that the window is failing. A failed seal fogs the inside face of the inner pane or the inside face of the outer pane, the surfaces you physically cannot touch. If you can wipe the moisture off, the seal is fine.
Photo: Rino Adamo via Pexels
