Condensation Between the Panes: What It Means and What It Costs
Short version: fog you can wipe off is humidity; fog you can’t reach is a failed seal, because moisture has gotten inside the sealed glass unit itself. It won’t clear up on its own — but it also doesn’t automatically mean new windows. Sound frame, one bad unit: a made-to-measure glass or sash swap fixes it for a fraction of replacement cost. Failing frames or several foggy windows at once: replace the whole unit and do the install details right. Here’s how to tell which side of that line you’re on.
Why the fog you can't wipe off means a failed seal
A modern double-pane window isn't two pieces of glass in a frame — it's a sealed sandwich called an insulated glass unit, or IGU. The two panes are bonded to a spacer around the edge, the cavity is typically filled with argon gas for insulation, and a desiccant inside the spacer keeps the trapped air bone-dry. That dryness is the whole trick: dry air between the panes means nothing to condense, no matter how cold the glass gets.
When the edge seal fails — and every seal eventually can, from years of the glass expanding and contracting, sun exposure, or a manufacturing defect — humid outside air starts pumping in and out of the cavity with every temperature swing. The desiccant absorbs moisture until it's saturated, and then the water has nowhere to go. That's the morning you notice fog between the panes, on surfaces no towel can reach.
It usually starts as haze that comes and goes with the weather, then progresses to permanent fog and, eventually, white mineral streaks etched onto the inside of the glass. One clarification worth making: condensation on the room side of the glass, or on the outside on a cold clear morning, is normal humidity behavior — not a failure. It's only the moisture sealed inside the unit that means the seal is gone.
Bring us the age of the windows and a count of how many are fogging, and we can usually tell you before anyone climbs a ladder whether you're looking at a glass swap or a replacement conversation. One failed unit is a part; a pattern of them is a timeline.
When a glass or sash swap works — and when it doesn't
Here's the honest part of this article: a failed seal does not automatically mean you need new windows. What failed is the glass unit, and if the rest of the window is in good shape, the glass unit is a replaceable part.
An IGU or sash replacement makes sense when the frame is sound, the window operates smoothly, and you're otherwise happy with it. The new sealed unit is made to measure — every opening is measured individually, because nominal sizes and actual sizes rarely agree — and swapped into your existing sash or frame. You get clear glass and restored insulation for far less than the cost of a full window. It's also worth checking your paperwork first: some manufacturers cover seal failure for the original owner. Milgard's limited warranty, for example, has historically covered glass seal failure for the original purchaser — if your windows qualify, the fix may cost you little or nothing.
Whole-unit replacement is the better answer when the seal failure is a symptom rather than the whole story: frames that are warped, rotted, or chronically drafty; sashes that no longer lock tight; single-pane or early double-pane units with poor glass all around; or several windows failing within a few seasons of each other, which usually means the rest are on the same clock. Putting new glass into a failing frame is new tires on a bent axle — and a proper replacement also means the install details get redone right, with sill pans where the assembly calls for them and flashing tied into the wall.
What it costs, and how to decide without overpaying
Full window replacement typically runs $900–$2,500 per window installed, depending on size, brand, frame material, and how the window goes into the wall. A glass-only or sash swap generally costs a fraction of that — which is exactly why it's the right call when the frame deserves it, and the wrong call when the frame doesn't.
A few rules of thumb for deciding. Count the failures: one foggy unit in a house of otherwise healthy windows leans repair; three or four across different walls leans replacement, because seal failures cluster by age. Check the frames: press the sill and lower corners, inside and out — softness, rot, or gaps mean the wall assembly is the real project. Weigh the age: putting new glass in a 25-year-old builder-grade window buys clarity but not performance, since the frame still leaks air the way it always has.
And consider timing. If a re-side is anywhere on your horizon, it's worth knowing that replacing windows during a re-side typically runs 25%+ less than doing them as a standalone project, because the wall is already open and the trim and flashing work overlaps. A foggy pane or two might reasonably wait for that project; a failed window in a wall you're not touching for a decade shouldn't.
